Animals - TV Tropes

• Animals are Eukaryotes. All earth life is made of cells (apart from viruses, if you include them as life), some cells have lots of mini-organs including a nucleus that stores genes, these are Eukaryotes. Eukaryotes also include fungi, algae, plants, many kinds of microbes, but not bacteria, or a group called Archea that resembles bacteria with some chemical differences.
• Animals are muticellular. Like plants, fungi, and some algae, animal bodies are made of lots of cells. These cells specialize, changing shape and other properties to fill specialized roles in the body. Most animals have distinct tissues, which combine similar cells into a larger structure. most with tissues further organize these into organs, and the organs for noticeable organ systems to fill particular roles.
• Animals need food: The word for needing to eat other living organisms, or artificial imitations, is heterotroph. Plants, algae, and some bacteria and Archea are autrophs, they can use nonliving sources of energy such as the sun to power themselves.
• Animals need oxygen: Aerobic is the word for this. Some organisms can break down material with no oxygen to get enough energy, such as yeast that rises bread and makes alcohol. Some animals can do this for short bursts, like your muscles when sprinting, but outside those short periods animals need oxygen for energy.
• Animal cells lack cell walls: Plants, fungi, and many bacteria have a rigid structure surrounding their cells. This provides support in plants and fungi, helping them stand up. Animals lack this, having flexible cells instead. This allows animals to move, which almost no other multicellular organisms can do quickly. Movement is probably the most obvious animal quality for most people.

See Evolution for more.

Officially, biologists use a system called phylogenetic classification: a group includes all descendants of some common ancestor. To classify animals, biologists need to know how animals are related. In practice, the exact ancestor is almost never known, but a tree of relationships between different types of animals can be mostly reconstructed, as described previously animal groups are defined by having a common ancestor implied by relationships between known animals.

Thanks to how evolution works, this system mostly matches common names of animals, groups and/or species with an ancestor in common tend to physically resemble each other. One exception is when one group evolves from within another, such as land vertebrates evolving from fish or bees evolving from wasps, technically “fish” isn’t a formal group and wasps include bees, but a scientists won’t assume a bee stung you while you ate chicken if you say that a wasp stung you while you ate fish. Another possibility is unrelated groups evolving similar characteristics and being lumped together. Outside of animals, algae are like this, three separate groups have evolved but they are similar enough to be called the same thing.

To figure out how life is related, including animals, scientists look for certain types of similarities, preferably ones that are difficult to change through evolution, or are more or less random and don’t affect survival much. Physical structures might use shell or bone structure if the animal has one, or structures of organs where multiple forms can work well. Genetic studies use mutations, often in noncoding DNA or genes with minor effects, to measure the time between splits and similarity between animals; these assume that mutations are more or less random, and when groups split a different collection of mutations will build up over time in each separated group. Animal embryo development can be used as a guide, it often resembles animal evolutionary history, likely because changes in growth and development are a major pathway for physical changes to evolve.

Characteristics that do affect survival a lot and evolve easily will likely adapt quickly to a role or environment, meaning unrelated animals living in a similar way will have them in common while related animals that will live differently will not. The shape of whales, fish, and prehistoric Ichthyosaurs is an example; it is well adapted to moving through water, but other features such as bone structure, whether the creature has gills or lungs, and evolutionary intermediates point to whales and Ichthyosaurs having separate groups of land based ancestors.

Prehistoric animals obviously can’t have genes sampled, but physical characteristics from fossils can be compared. If there are enough fossils of the right animals, ancestry can be directly traced. Fossilization is a chancy process, a dead animal needs to be under the right conditions to successfully fossilize. Having hard body parts helps a lot: vertebrates, molluscs, brachiopods, coral are well understood thanks to shell, bone, tooth, and coral structure fossils, many soft bodied and/or very small animals have nothing or next to nothing fossilized.

The first animal fossils are disputed, possible candidates range from around 570 to around 760 million years ago. Some studies based on other evidence suggest they existed around a billion years ago, some point to a more recent origin, evidence is not clear enough to narrow this down. Multicellular life in general was developing at this time, algae and fungi were also evolving. The first likely animal fossils look like fixed in place creatures that fed on mats of bacteria on the ocean bottom, possibly with some filter feeding, though whether these "Ediacaran Biota" fossils were actually animals is still not fully settled. Clear animals fossils related to modern groups show up from several million to tens of millions of years later. Sometime around 550-600 million years ago, some animals developed muscular movement. The first fossils of burrows developed around 540 million years ago, around the same time animal diversity increased enormously in the Cambrian Explosion. Most modern animal groups came into existence around this time. UsefulNotes.Prehistoric Life has you covered from here.

Dinosaurs kept mammals in hiding. The Lion is the King of the Jungle. Culture often assumes that predators and large animals dominate over prey and smaller animals, and are overall more powerful and successful. Concepts such as higher and lower animals, or the great chain of being, are also common, where larger, more complex animals and vertebrates are considered better, more advanced, etc. than simpler and smaller ones.

This is not how scientists see it, as Evolution describes. While predators and large animals are individually threatening, collectively prey and smaller animals do quite well for themselves. Animals are usually spread out enough that prey and smaller ones aren’t constantly under attack, as you’ll notice if you go hiking or spending time in a wild area. As a result, most smaller animals and prey spend most of their time going about their business and sometimes running or hiding from something dangerous. Lions, Tigers, and Bears might be the big predators in modern environments, but plenty of smaller cats, lizards, birds, spiders, worms, etc. are living out their lives quite successfully. Likely this is true of dinosaurs and mammals, mesozoic mammals were filling similar roles to racoons, rats, squirrels, etc. that live in equivalent areas today.

As for higher and lower animals, The evolution link describes opinion well. All animals have had to face a similar set of challenges in living in an environment, finding food, etc., so anyone survives that is well evolved and adapted to their role: Sponges and jellyfish have adapted just as much as cuttlefish, bees, or whales. Complexity involves tradeoffs, more energy, more difficult growth and development are the big ones, so a simpler and less active animal may well work better in certain ways of living. Vertebrates scientifically are seen as just one group of animals, so invertebrate makes as much sense in classification as nonarthropod or isn'tmollusc or other-than-cnidarian, though on sites like this one invertebrate gets some use as vertebrates are more represented in pop culture and society in general. If a type of animals has lasted a long time, it means they were very well adapted for their environment, and that said environment has also lasted a long time, so “primitive” animals are those which ended up with a long lasting, useful shape in an environment that hasn't changed too much over that time. "Backward" evolution, where an animal ancestor gained something and lost it again has happened often: Flightless ants and ostriches had ancestors who evolved and lost flying, whale and sea turtle ancestors moved from sea to land and back, slug and squid ancestors gained and lost shells, flatworm ancestors gained and lost many internal organs.

Animals are just living their lives, not pursuing any larger goals, but population, diversity in species or in roles a group fills, geographic spread, amount of time a species exists before going extinct, are all used as measures of success.

Colors, shapes, sizes, what they eat, how they act, lots of things make animals different. Some characteristics are described in sections below.

Like a lot of human built machines, animal characteristics are tradeoffs, with survival/success based on how well a combination of qualities fits together. Most commonly, the tradeoff is size and greater nutrition needs, any extra tooth, brain, poison, general size, etc. takes energy to grow and maintain, meaning the animal needs to get more resources to have it. Animal evolution could be thought of as a random, not purposely designed engineering challenge, and animals often face similar issues as human machines with roughly similar results. Flying animals use airfoils and produce a lot of power for their size with metabolism and flight muscles, similar to airfoil using flying machines with high power to weight engines. The streamlined fish shape isn’t too far off from a submarine or ship form, and waterjet vs. propeller is similar to squid jet vs. fish tail (fast but power hungry vs. usually slower but more efficient). Some animal characteristics have inspired machine designs, such as neural net programs based roughly on the brains of animals, and lots of unsteady state motions, especially fluid flow (think eel or snake swimming or insect flying) are used by animals but not understood well, they may have future applications if research can better understand them.

Deep Sea, Desert, Deciduous trees, …uh..dirt? …running out of D’s. Where an animal lives affects everything else about it. Life lives in a very narrow band near the earth’s surface,from the dirt/seafloor below ocean trenches and some deep mines, to flying high in the atmosphere, with much of it in the oceans or close to ground level. This narrow band has very diverse environments, however. Where to live is also affected by size, a microscopic animal will find plenty of space in someone else's body or a tiny hole, for example, which to a large animal would just be a piece of the environment.

Animals are usually well adapted to one environment that they live in long term, but short trips to others can be common, such as land animals swimming or ground dwelling animals climbing. These can be a useful part of an animal’s lifestyle if they regularly lives near a border area, where animals might use different environments for different purposes (such as seals mating on land while catching food in water). How strongly the boundary restricts an animal depends on the animal’s adaptations, and how similar the environments are.\\
Some qualities of environments, and how animals might need to adapt:

* Water vs. Land: Water and air are very different materials, so very few animals can live in both. The differences affect almost everything an animal does: breathing water and air are very different, movement is different, sensory information transmits differently, temperatures change less in water, are among many major effects.

* Saltwater vs Freshwater: You know how you shouldn’t drink too much seawater, because you’ll lose more water getting rid of the salt than you took in? Animals living in water must handle this all the time. A concentration difference between water inside and outside an animal means water will either diffuse in or out, and an animal will need to either expel extra water, or extra salts or other material to keep their internal fluids the right amount of concentrated. Animals usually are adapted with appropriate systems for a certain salinity range, go outside and the animal dies from too much or too little water.

* Richer vs. Poorer environments; All animals need to eat something, and eventually that eating leads to other life forms that get energy and nutrients from the external environment. To have more life in an area, an environment needs mineral nutrients, a source of energy (almost always sunlight), water, and avoid extremes that can kill most life. Environments with all these will be very productive and support a lot of life, environments missing something a lot less.

Shallow water, rainforests, and weird deep sea hydrothermal systems are examples of highly productive areas. All have nutrients from soil/the ground or the sea floor, the first two have lots of sunlight for energy while hydrothermal vents have heat and heat driven chemistry, rainforests have lots of rain while the other areas are in water. Deserts and most of the ocean are examples of poorer environments. Deserts are missing water, the deep ocean has little sunlight and relies on falling material from above for energy (apart from hydrothermal vents and some other seeps), and the open ocean surface has few mineral nutrients, being far away from any rocks that they come from.

Richer environments can support more of all kinds of life, including animals. This greater number allows for lots of different species to maintain good sized breeding populations, meaning more species variety. However, this also means more competition from greater amounts of animals nearby. Poorer environments are often slower paced, animals need to conserve limited resources and adapt to low populations, or be able to quickly move between resources that do exist.

* Complex terrain: Grasslands, open ocean, deserts are easy to navigate, wide open spaces. Forests, reefs, and rugged terrain like mountains are quite a bit more complicated, with spaces to hide in, roundabout travel routes with obstacles, and changes from location to location. Animals in more complex environments may need unique travel methods, and some way to navigate or otherwise find what they need. More open environments allow for faster movement, and better use of some senses (longer sight lines, easier hearing, and similar).

* Temperature, Wind, Moisture, Dissolved sediments, other environmental properties: Obviously, animals will need to adapt to these. Variability in these conditions much also be adapted to if they change a lot (such as variable weather).

* Larger living things: Some animals live on other animals or plants. Some live Inside other animals, or burrow into plants. Living things for obvious reasons don’t want to be burrowed into, thus will likely have defenses that the smaller animals must work around. They will also need a way to move from host to host, which might require some venturing outside.

Living inside an animal gives good temperature control and a good source of food, almost all animals living in others are parasites. Living on the outside of animals might just be a good attachment point, it also may offer feeding opportunities on blood, skin flakes, other parasites, or such. Living on or in plants may be useful for parasites, or the plant is just an easy surface to travel on or burrow into as with some creatures living in wood.

* Burrowing: Burrowing through sediments takes more effort than swimming or moving through air, and there is little light, plus sound and smells don’t travel that well. In return, underground provides protection from things at the surface, such as temperature changes, bad weather, or other dangerous animals. Sediments contain plant roots and other accumulated debris for nutrients, plus other burrowers, but many burrowers will venture outside to get food. Cave dwelling animals let something else do the burrowing, but still have to handle the lack of light and other underground conditions.

* When to Live: Environments change over time. This could be somewhat random, as in weather, or predictable as in day night cycles or seasons. These changes can create very different conditions to deal with, a cold winter, dark night, or watery high tide is quite different then hot summer, bright daytime, or dry low tide. Other life reacting to these changes means animals must further adapt. This could be behavior changes (being active during one part of a cycle and resting in another part, chasing certain times a year for lots of eating, mating, etc.), moving around to follow favorable conditions (seasonal migrations, or ocean creatures moving deeper and shallower over a day/night cycle.), flexibility (changing diet but otherwise acting similarly), or body changes. Irregular changes like weather can be trickier to adapt to, animals might move or change behavior, but might also have preparations for a range of conditions that could occur, even taking advantage of them.

All animals get energy from other living beings, which means all animals need food. A good meal for an animal gives more nutrition than it costs to get and digest, enough extra to fuel the animal’s other activities, and doesn’t poison the animal.

Animals can’t use large particles of food directly, so it must be digested down to component chemicals. All animals to do this use chemical methods to break down food, and most use physical methods as well, to break food into smaller pieces that are more easily attacked by the chemicals, than absorb the results. Catalytic proteins, enzymes, do much of the work, acidic or basic conditions also help break down a lot of nutrients. Most chemicals life uses are made into long chains of smaller units, digestion breaks down bigger chemicals into these small pieces that can be rearranged to suit the animal, or used for other purposes. Vitamins, minerals, and some other smaller chemicals are absorbable directly once dissolved. Physical methods include biting off small pieces, chewing, or stomach churning, among other methods.

Animals need a few nutrients in common. Proteins perform most functions in a body. Many types of molecules can act as energy sources, though carbohydrates and fats are more common. Animals need all required elements (Nitrogen, Calcium, etc.) supplied in some way, the major elements are common to most, but a few minor ones may vary. Water makes up body fluids, and is needed ot ensure they are properly diluted/concentrated. Interconversions are possible between many different types of molecules, but animals may be limited in what can be converted, and conversions do take energy, so some specific chemicals, like vitamins in humans, are needed. Animal’s ability to process nutrients and their diets will tend to evolve together to a point where harder to get nutrients are conserved, or methods evolve to make them from other inputs, while easier to get nutrients can be used more wastefully.

Very few foods are completely absorbed, not all chemicals in them are required or usable by the body, or possibly harmful. Leftover material that isn’t absorbed becomes dung (poop, feces, scat….toilet words are a common thing), and is expelled in some way.

You’ve probably heard of carnivores, omnivores, herbivores, and maybe scavengers, but there are quite a lot of specialized names for other types of animal diets. Piscivores eat fish, insectivores…guess, detritivores eat random particles or debris, grazers eat plants low to the ground, frugivores go for fruit….It’s a long list. An animal that can use only one or a few types of food is called a specialist, an animal that eats lots of types of food a generalist. Generalist carnivores eat lots of types of meat, generalized herbivores lots of types of plants, generalist with no other word attached implies an Extreme Omnivore.

The interactions of lifeforms eating each other form something called a food web, understanding these interactions is very important for understanding any particular environment. At the start of these food webs will be something that captures energy from the environment. In our world, these are almost always photosynthetic plants or algae capturing energy from sunlight, but decomposing bodies or particles from elsewhere, or chemicals released from hydrothermal vents or seeps can also supply the initial energy to appropriate organisms. As these organism are eaten, and the eaters are eaten, and those are eaten, etc., energy is lost at each step through inefficiencies in energy use and the fact that a lot of food energy is used for day to day activities instead of growing a body. This means that the original energy capturers, like plants, will tend to be most common and/or fast growing, herbivores or equivalent less common and/or faster growing, then predators (or parasites), then predators of predators, etc., with more generalist eater abundance depending on their exact diet. The exact decrease depending on how the organisms in question use energy: greater fraction used for growth vs. other things means less energy loss. A lot of energy goes through decomposition, scavenging, and detritus, with the same loss at different steps occurring.

Here are some types of diets, and what they imply for the animal. Remember that within these descriptions, animals may be more or less specialized (A predator may focus on lots of kinds of prey or just a few or one type, plants eaters may just eat particular leave or lots of green stuff, etc.).

* Predators of similar sized prey: These are the most famous predators, probably. T. rex attacking a Triceratops, crocodiles dragging large animals into the water, sharks attacking seals. On a smaller scale, squid catching fish, spiders catching larger insects, some flatworms preying on earthworms also fit this category, among many others. While successful catches are most commonly shown, in practice most attempts fail, or few attempts are made. A similar sized prey animal is a ‘’lot’’of food, just imagine a person dealing with about 100-150 ib/75 kg of meat. A person needs about 2-4 pounds of meat a day if that’s the only food item, so a carnivorous version of a human could catch something like a deer or large fish roughly every couple weeks, and accounting for inedible parts, still have have enough food for themselves plus feeding a family, or group, or just leaving it for other animals if too difficult to carry, and people need more food for our body size than most other animals. With this much food, lots of missed hunting attempts are perfectly survivable, while prey facing death is under much more pressure to escape.

Eating similar sized prey requires weapons and often speed to catch and kill it, this speed is often a short burst for an ambush. If living alone, such predators need enough storage (a stomach or otherwise) to actually store a good amount of food from their catches. Digestion is meat is relatively easy, but there is often a lot at once, these predators may rest after kills to break down the meal. Meat is heavy on protein and light on carbohydrates, so animals adapt energy requirements accordingly. Some vitamins or similar chemical found in plants are also often missing, so predators will adapt to make or not require as much of them, while minerals such as calcium or sodium are more available with the opposite effect.

* Predators of smaller prey, or other prey that can’t fight back: Think anteaters or chimps eating an insect hive, baleen whales gulping entire schools of fish or krill, starfish attacking clams, birds eating worms, or similar. Obviously, such prey can’t physically fight back effectively, but it may still have other defenses such as shells or poison that the predator has to work around. A big challenge is finding enough food, smaller prey can often hide, and requires much total individuals eaten. Colonies or clusters are good targets for predators. Nutritionally, smaller prey and larger prey are similar.

* Scavenging: Dead meat will start to decompose, which means bacteria and fungi which may cause diseases or be poisonous to the predator if the body has been dead too long. Deaths from other causes are also random, so a scavenger needs to compensate for less stability in food sources.

In practice, much scavenging is done by meat eaters taking advantage of a dead body. Meat is meat, and dead bodies don’t fight back or escape, so meat eaters will take advantage of an easily meal if available and not too decomposed, their digestive systems are often somewhat adapted to handle microbes already. Detritivores below also act as scavengers, dead bodies are just another type of particle or debris they can eat, especially if they can chew or separate smaller parts of a bigger corpse. The most well known pure scavengers are probably vultures and some maggots. Flying allows vultures and adult flies to cover more area and see more, allowing them to find dead bodies more easily. This also reduces the randomness of scavenging, over a wide range some sort of corpse will be available. A highly acidic stomach, and secretions by maggots, deal with microbes.

* Detritivores: Earthworms are no doubt the most well known of these. Eaters of leaf litter, skin flakes, bits of organic material in soil, or other random debris. Some appear to literally eat dirt, though they are eating for nutritious particles in the dirt, not the rocks or sand, which are passed through. These have to handle lots of different types of food, since edible debris comes in all forms, plus deal with bacteria doing the same thing. Detritus as a food source won’t try to fight, and lacks defenses compared to living beings.

* Ocean filter feeding: Sponges, Mussels, Basking and whale sharks are well known. Water can carry particles, some animals can feed on these. Many small living things also float in water currents, such as single celled algae or some very small animals. Some larger animals take advantage of this food source. These animals need a way to filter water to capture the food particles, plus some way to move lots of water through said filter. Filter feeding is a common food source for sessile animals, meaning animals that don’t move. These animals have very low metabolism, only needing energy for very basic functions, so can rely on ocean currents to do most of the work delivering food particles to them. Swimming animals that filter feed obviously need more energy, but the act of swimming will move water through their filtering systems.

* Symbiotic Algae or Bacteria: The closest thing to a Planimal, some animals allow single celled algae into their bodies, these use photosynthesis to produce nutrients which the host animal absorbs. A few animals (like tubeworms in hydrothermal vents) so similar with bacteria that use non light equivalents to photosynthesis using surrounding chemicals or heat. Fungi and Algae doing something similar create lichens. The host protects the algae and supplies it with minerals it needs. Animals that get nutrition this way are typically attached or otherwise use little energy, the amount of sunlight striking more active animals doesn't come close to supplying the power they need.

* Roughage eaters, or grazers, browsers, leaf eaters, and such: Eaters of general plant material, in other words. Horses and cows eating grass, termites eating wood, leaf eating beetles….there’s a lot of these. By biomass, plants are the most abundant organisms on earth, so these organisms have plenty of food to eat. However, outside some specific parts, plants are poorly nutritious, and are hard to digest. The main component of plants, cellulose, can’t be digested by many animals, in human nutrition it is one of the main dietary fiber chemicals that pass through. Herbivores therefore depend on certain types of gut bacteria and fungi to do the hard work of digesting cellulose for them. A lot of herbivores do practice coprophagy (eating their own feces) to reabsorb nutrients otherwise expelled as dung. Plants can’t fight or escape, but can have poisons, and some are physically tough or have spines, needles, or other physical defenses.

Animals that eat roughage might spend more time digesting their meal, to better exploit what is there, or use more intense digestion such as chewing. Some animals can digest cellulose, others rely on bacteria to do the job. Termites and cos are the most well known with such bacteria, since these bacteria produce methane and contribute to greenhouse warming. Large digestive systems will be common, the animal will eat more total food than a similar sized meat eater.

* Fruit, seed, tuber, and other eaters of nutritious plant parts: Think bees and hummingbirds eating nectar, fruit flies, Some parts of plants are much more nutritious than stems and leaves. Places where plants store nutrients, such as tubers, are obviously something an animal would like. Spores and seeds need to carry some starter nutrients to get the new organism going, and need to be light to spread easily, so also contain concentrated nutrients. Flowering plants have evolved to take advantage of animal eating to spread seeds, producing sugar rich nectar and fruit that animals can eat, which also spreads pollen and seeds for the plants producing them, often further than wind can do so, and in a more efficient way. Pollinators moving from flower to flower means more pollen actually fertilizes something, and seeds are spread out further, meaning less self competition and a supply of fertilizing dung to get things started.

An animal specializing in these food sources needs less total mass of food than a roughage eater, but less total food is available and the animal must search for it. These food sources are also limited in certain nutrients, which the animal must compensate for. Flowers and seeds are typically brightly colored and give up unique smells, so good senses for these things are useful.

* Fungi: Fungi aren’t common as a main food source, most fungal growth is spread out filaments called hyphae that are hard to separate and eat and most animals for the the mushroom and mold growth instead, but a lot of animals will eat fungi as part of a larger diet. Instead of cellulose, fungi use something called chitin, which is different to digest. Fungal chemistry is otherwise close to animals and not a lot have hard parts, so digestion is relatively simple to do. Fungi obviously can’t run away, but have a range of toxins, most of you have probably heard of hallucinogenic mushrooms or poisonous mushrooms.

* Body fluids, parasitism: Bloodsuckers, tapeworms and other parasitic worms, aphids eating plant juices, botflies growing in skin. When you think of parasites, this is the food source of most of them. Body fluids are an excellent nutrition source, carrying lots of ready to use nutrients for the victim organism, and packed with cells ready to be broken down for more. However, while parasites don’t kill their hosts outright, they do weaken it, so the host organism will fight back and attempt to remove the parasite. These organisms as a result need to avoid defenses, such as an immune system attack from the host, or attempts to physically remove, maybe kill the parasite. Plant fluid eaters don’t deal with as strong defenses, but may have to handle poisons. Parasites also need a way to get into the animal, piercing skin and other defenses for bloodsuckers and plant sap eaters, or getting completely inside the other organism, which means burrowing in, or entering through an existing orifice like a digestive system or wound, digestive systems in particular are obviously hostile and an animal will need protection to move through it.

Halfway between parasite and predator is something called a parasatoid, where an animal lives inside another one for a long time, but is guaranteed to eventually eats up enough of the host to kill it. Many insects use this method of feeding when larvae.

* Extreme Omnivore: Rats, Crows, household cockroaches, and most famously the species reading this page, whose wide range of possible foods makes the culinary industry and lots of Food Tropes possible. Being a generalist eater has many advantages. From the same territory, more potential food sources means more food available. There is less risk, since if one type of food runs out, others are available. More environments can be lived in, since the animal is likely to find something it can eat no matter where it goes. However, less specialization might mean less ability to handle one particular type of food, though many generalists do just fine anyway. Animals eating lots of different foods will seem to have mix and match body parts from several types of more specialized eaters, plus lots of general purpose adaptations that can be used multiple ways.

Many animals are actually less specialized than you might think. Fruit eaters might pick infested fruit with insects, nutritionally this gives some protein. Large herbivores occasionally will eat small animals. In between full on Omnivore and full on specialist, most animals will eat food from several categories described here: predators will scavenge when they can as described in that bullet point, many plant eaters will happily grab leaves, stem, fruit, or anything else soft enough and available on a plant.

In human culture, breathing, heartbeats, and blood are strongly associated with life, and for good reason. We talk about the bloody cost of war, the breath of life, the beating heart of something as its most important, central part. In medicine, airway, breathing, circulation are the first priorities for emergencies, the loss of any of these is immediately threatening. The same, more or less, is true of most animals, though the organs are different, hypothetical cephalopod CPR or Defibrillators would need to handle three hearts, Insect medics would never use mouth to mouth as they get oxygen from tubes in their body instead of lungs, and water creatures would pump oxygenated water over the gills instead of ventilating lungs.

All animals today require oxygen for energy. Reacting oxygen with food produces a lot more energy for the same amount of material than other plausible reactions. However, oxygen is not very soluble in water, nor does it attach well to other chemicals, so is far harder than food to store in a body, most animals as a result need a continuous supply.

Anything that enters an animal either grows the animal or comes out in some form, unless the animal can store a lifetime of deaweight or even harmful chemical products, it must also remove waste. Some chemicals, like carbon dioxide, are the result of normal reactions. Some wastes are removed to keep an animal's internal conditions in a certain range, such as salt or water if an animal's fluids are too concentrated or dilute. Some are the unwanted breakdown products of other molecules, or chemicals or toxins the body must remove.

Small life forms in water can intake nutrients and expel waste through diffusion. In diffusion, random movement of molecules means concentrations tend to even over time, chemicals will tend to move from areas of high concentration to low concentration. An organism producing waste products will build them up, by consuming nutrients will lower their concentration, the higher waste concentration and lower nutrient concentrations within an organism vs. outside causes material to flow the desired direction. Organisms with membranes can use active transport, pulling material across using energy, but within free internal fluid and in water outside, diffusion controls movement.

Diffusion is faster with higher concentration differences, over smaller distances, and across wider areas, so smaller, flatter, lower metabolism animals can get a lot of what they need this way, many worms, for example, breathe through their skin. In larger, plumper, and more active animals diffusion would move nutrients to them and within than too slowly, armor, thick skins, or other outer structures also interfere with exchange. Many waste products also don’t diffuse away in air. Specialized structures are as a result used to intake nutrients and oxygen, and expel wastes, water and air will be pumped to these structures to help exchange materials, and fluid will be circulated within their bodies to move nutrients and wastes where they are needed.

You’ve probably heard that your lungs or small intestine has the areas of some number of football fields/tennis courts/large sports field of choice. This high surface area comes from lots of folds, it allows a large amount of material to quickly transfer into your body. Breathing organs and intestines in most animals are similar, a structure with lots of surface area, thin membranes to allow material to transfer, and circulating blood or body fluid if the animal has it to take up the nutrients. Other structures include gills for water breathing animals, dividing small tubes in many arthropods which air can diffuse into, or tubes within sponges which water is slowly pumped through and nutrients absorbed from. Some animals use a mix, absorbing some oxygen through skin and the rest through lungs or gills or equivalent.

Breathing in air vs. water presents different challenges, and very few animals can do both. Oxygen’s low solubility in water means that a large amount must be passed over gills, and gills must extract as much oxygen as possible. For some animals, water must be moved over gills to get enough, the trope of sharks needing to swim is an example, though not true of all sharks. Oxygen is much more available in air, to the point where some water living animals like whales and some fish can breathe occasionally and get enough air for long underwater periods, but breathing structures can be sensitive to drying out, and a lack of buoyancy means the structures must be supported some other way. Bring a water breather to air, and gills or other water based structures collapse or dry out, bring an air breather into water, and it cannot absorb enough oxygen and breathing structures are possibly damaged by the water. Creatures that can breathe in both either have two sets of structures, such as some fish with gills and lungs, or gill or tube like structures made of strong materials and protected against drying out. A few fish, and many crabs, use the second method.

Gills have a range of forms, from the fish gill style slabs of feathery material, to what look like antlers or tentacles in some species. The baglike lungs you have are a vertebrate and snail thing, air is circulated in them through body movements or specialized muscles such as your diaphragm. Insects, millipedes, and similar have passive exchange, through body tubes in insects or structures near the legs centipedes and millipedes. Arachnids have structures similar to crabs. Smaller land animals can use skin breathing.

If fluid is circulated inside a body, it can either be extracellular fluid, which for you is what fills blisters, or blood separated into a separated system. Circulation can be accomplished by body movements, a good fraction of blood in your vein is pumped this way, but specialized hearts are common. The complexity of hearts depends on the animals, from fish with a single chamber o insects with a simple tube, to the two gill hearts and body heart in cephalopods, or the four chamber bird, mammal, or crocodilian heart, both these heart arrangements pressurize blood travelling to the breathing organs in addition to blood going to the rest of the body, called double circulation system. Hearts to pump extracellular fluid is possible, insects have a system that pumps fluid through a tube to one end of the body, it then slowly moves to the other end as extracellular fluid.

Some chemicals, such as sugars, salts, or carbon dioxide, can dissolve in water and are carried this way. Insoluble molecules can be emulsified, or carried using proteins. Poorly dissolving oxygen is also carried its way, hemoglobin is the carrier molecule in vertebrates, other types of animals have different molecules, causing different colors of blood or body fluid.

Some waste products can be removed by the same structures that intake oxygen and nutrients: breathing organs in animals remove carbon dioxide, and in water animals gills can remove some waste that easily dissolves and diffuses in water. However, animals may need to actively remove wastes faster than such diffusion would allow, or remove material absorbed from the environment, such as saltwater living animals concentrating and removing extra salt. Land animals of course need to actively remove solid and liquid waste which does not diffuse through air. Thus the kidney or equivalent structure. In many animals, this filters blood or body fluid, keeping only what the animal needs and excluding the rest. In humans among other animals, this waste is mixed with lots of water and leaves as a liquid, animals in drier environments or ancestors in the same often exclude paste of thick liquid instead. This waste can be separate or mixed with digestive leftovers.

You’ve probably heard of Warm blooded and Cold blooded. Warm blooded animals make their own heat, are energetic, and have a higher temperature that shows up on infrared cameras Cold blooded animals are the opposite. Even if you haven’t, on this site you’ve probably seen related tropes: The Cold Blooded Killer who shows little emotion and calmly goes about their business, vs. the Hot-Blooded one who is energetic, emotional, constantly active. These types aren’t too far off from the animal qualities they are based on, though the world being the complicated place it is many animals blur the warm blooded/cold blooded distinction, metabolism, how active animal are, and how they handle temperature and heat have lots of variation.

The word “Metabolism” shows up a number of times in these useful notes. It technically means “chemical reactions occurring in an organism”, but mostly you’ve likely heard it to mean “how much energy a person/organism has”. These end up meaning about the same thing: just about everything life does involves chemical reactions, with energy producing ones used to power energy consuming ones. More energy production means more of these take place and the organism can do more of just about everything (Think not just physical activity, but also thinking, growing, basic body functions, etc.) but also needs to eat more, less energy production means the opposite. Closely associated with metabolism is temperature, heat is generated by energy use, and affects how quickly metabolic reactions occur, in addition to worries about freezing or burning. All lifeforms including animals work best at particular temperature ranges, animals have a number of ways of ensuring their bodies are at a correct temperature.

Most lifeforms use a huge variety of chemical reactions too many to describe here, biology often studies “metabolic pathways” or “metabolic systems” to see how materials are reacted, what reactions are connected, how the presence or certain chemical effects others, etc. They do mostly fall into a few categories. Energy producing reactions react or break down molecules to power everything else. In animals, oxygen reacting with nutrient molecules to carbon dioxide and water (also nitrogen or sulfur containing waste products if proteins are broken down) are the main reactions of this type, generally these happen in a series of small reactions where the nutrient molecules go through several steps and are converted to carbon dioxide and hydrogen ions, the hydrogen ions are then reacted with oxygen to produce most of the energy from the reaction plus water, vaguely similar to what fuel cells do in human technology. Sometimes, some animal tissues might break down larger molecules to smaller ones in a way that releases energy. This produces far less energy per mass of fuel molecule, but can be useful for short term power if oxygen can’t reach the tissue quickly enough for what the tissue is demanding. The resulting energy is used to produce a chemical called ATP. This stores energy in small packets, when energy is needed for something, ATP chemically reacts in ways that use its breakdown to power the process in question.

Some energy goes to nonchemical processes, like muscle contraction, nerve impulses, or pumping molecules that move material across cell membranes. Some is used in the second main type of chemical reaction, building large molecules: small building blocks are put together in various sequences to make proteins, fats, and various special purpose molecules. In animals, the smaller building blocks for such reactions come from digesting food, sometimes with some chemical processing (If you read about essential amino acids, the nonessential ones can be made by your body using such processing of essential ones if not enough comes from food) Some reactions get rid of waste products or dangerous chemicals, or at least process them into something easier to remove and/or less toxic. Such waste products could be ingested, or come from unwanted reactions within an organism. Some nutrients are used in a similar way, turning something an animal eats into a similar molecule the animal actually will use. Digestion breaks down large molecules into smaller ones an animal can absorb more easily and process into what it needs. This usually releases a little energy, but not in a way capturable by the animal.

Many toxins affect animals by interfering with some of these chemical reactions, an animal’s metabolic rate also affects how quickly toxins start to take effect, and how quickly they can be chemically processed if an animal does this.

Almost all of these reactions are performed by types of proteins called enzymes acting as catalysts. Catalysts by definition speed up chemical reactions, they can also change what types of reactions occur. The second property in life forms and human industry, laboratory work, and other chemistry, is used to ensure fewer side reactions occur, which avoids wasting material to produce useless or even harmful products instead of the desired one. Of the remaining reactions, some use different catalysts (such as protein production, which uses structures formed from something called RNA to transfer genetic information and perform the actual protein assembly), some occur freely. Proteins can change structure under different conditions, enzymes included. You’ll have seen this happen if you cook eggs or curdle milk, high temperature for eggs and acidity for milk change structure of certain proteins, which cause hardening of eggs or clumping in milk. These changes are part of the reason life forms have to control internal conditions: get too hot, acidic, alkaline, etc. and some proteins won’t do their job properly.

Temperature control in animals has a couple components: whether an animal keeps a constant temperature or can have a wider range, and whether an animal matches the environment or is hotter. All of these come with tradeoffs: a constant temperature means an animal can optimize to function at that temperature and be more active then an equivalent flexible temperature one, but maintaining constant temperature takes energy, behavior, and/or organs that a flexible temperature animal does not need to have. Higher then environmental temperatures mean a more active animal, but higher temperatures mean more heat loss which has to be compensated for in some way. What makes sense for any particular animal depends on how active its life is/how much energy it needs to produce, how much food is available to generate that energy, and what temperatures it must adapt to in its environment.

The temperature an animal settles at depends on how heat production and heat transfer to or from the environment balance, animals can control their temperature by adjusting these things. All animals generate heat internally, metabolic reactions are never perfectly energy efficient and the losses turn into heat. Internal friction, electrical impulses, and other energy transmission that isn’t stored also ends up as heat. The amount of heat generated internally depends how active an animal is, some animals can directly produce heat by not using produced energy for anything else. Absorbing sunlight also heats an animal, the energy in the light converts to heat when absorbed. Turning liquid water (any liquid, actually, but water is what animals use) to vapor absorbs heat. Animals as a result can heat themselves through activity (physical movement, direct heat generation, shivering, or otherwise) or spending time in sunlight, and land animals can cool themselves by coating in water, panting, sweating, or other ways of getting water to evaporate.

Heat transfer to or from surroundings occurs through radiation (of infrared, at temperatures of interest) and through direct contact (conduction and convection, if you’ve studied your physics). It happens faster the more area is transferring heat, the greater the temperature difference, heat transfer through direct contact happens faster if the materials the heat flows through are more thermally conductive, and if any surrounding fluid is flowing (Meaning windchill and water currents.) Water is a far better heat conductor than air, but tends to maintain a more even temperature, so animals in water will lose or gain heat faster but not have to deal with as large of temperature changes. Underground doesn’t transfer heat as quickly and temperature stays more constant, so caves or deep enough burrows can be used to avoid temperature extremes. Body tissues that slow wind and/or transfer heat slower, such as fat or fuzz, add weight and tissue to grow but greatly reduce heat flow. Shapes with more surface area per size increase heat flow, shapes with less reduce it (So wide and flat transfers heat easily, long a thin a bit worse, rounded the least), individual body parts can be shaped this way and moved to allow faster or slower heat flow.

Internally, unless an animals is heating up from the outside, core areas will be warmer than the surface or farther out parts like appendages. This is because, if temperature is to stay constant, heat generation in each part of a body must be balanced by heat transfer somewhere else, and an animal can only lose heat from its surface, so there must be a change from higher in the middle to lower on the surface for heat to flow. This temperature difference will be undetectable for low metabolism animals producing very little heat, but higher metabolism animals will by noticably warmer in ternally then the surface, particularly in thicker parts of the body, Body fluid (including blood) circulation helps to move heat, animals might send more heat to an area that needs to be heated up, circulate a lot through a high temperature area to cool it, keep heat in the middle instead of surface to reduce losses, or make many other adjustments for proper heat control. Animals with separate blood systems might have incoming and outgoing vessels arranged next to each other, this allows outgoing slightly hotter blood to continuously transfer heat to slightly cooler incoming blood nearby. The overall effect is that The outgoing blood is continuously cooled to close to the incoming blood’s temperature, and little heat is lost. (similar arrangements, called countercurrent heat exchangers, are used in industry to transfer heat well and make the most use of it.)

Aniamsl range in size from microscopic to several tends or aone to two hundred tons. You’ve probably noticed that small animals are rarely just copies of large animals or the other way around, they tend to have different shapes, ways of living, and structures. Individuals within a particular species also can have different sizes, sometimes this is random from individual to individual, sometimes it follows a pattern, such as males and females in species with them being different sizes. (And young animals are never born or hatched full size).

There’s a few general ways size affects what animals are like.

First off, larger animals are physically longer, heavier, and have more stuff. Yes, this is obvious. This means they need to eat more, have more internal organs, can’t fit into smaller spaces but can avoid some obstacles smaller animals can’t, and will be physically stronger, including in fights. Also obvious. Not as obvious is that, combined with food availability, smaller animals can be more specialized and more diverse. Animals need to maintain a large enough breeding population to avoid unlucky population collapses/extinction from disasters and maintain some genetic variety, larger animal’s greater food needs means that they must rely on very abundant food sources, or be flexible enough to eat a wide variety of foods, while smaller animals can specialize more and still have enough food to support a good population. Larger animals will also tend to go for larger food items, collecting lots and lots of small food items would take too much effort to get enough food unless a concentrated source could be found. The greater variety of roles available means that smaller sizes have more species of animals then larger sizes, generally, until the smallest of animal sizes are reached.

Male/Female size differences come down to these obvious factors as well. If fighting for mating opportunities is common, whichever sex does more fighting will tend to be larger, typically males. Eggs require more work to produce then sperm, requiring larger organs to do so and more energy, this can push females to be larger in some species to support these organs.

Another major effect is the Square-Cube Law, there are lots of situations where something that depends on surface area or cross section interacts with something depending on volume. Strength of any tissue (muscle, bone, shell, soft tissue, etc.) depends on cross section, while the mass it has to move or hold up scales by volume. As a result, while larger animals are overall stronger, smaller animals are stronger for their size, take less injury from falls or stretching, and do not need as good a support structure for their body on land (Water buoyancy supports animals much better making this less a concern, but large animals may still need support structures for muscle movements) . Air or water drag scales by surface area, while the energy an animal can produce to move scales by volume, so larger animals tend to move faster, flying animals, meanwhile, will tend to be smaller as lift will scale by surface area while mass scales by volume. (Land animal movement is more complicated, though larger animals tend to be able to move faster also, until opposite scaling effects for injuries and strength come into play) Diffusion and heat transfer occur based on surface area, so larger animals will tend to change temperature more slowly, and won’t absorb nutrients or oxygen or remove wastes or lose heat as quickly through skin, this means larger animals tend to have more specialized organs to fill these functions. Flow through a wider tube produces less friction, but takes longer through a longer tube. This means all else equal, food takes longer to move through a large animal digestive system. Blood circulation or equivalent will be different, larger animals will have separate blood circulation as cellular fluid would lose its nutrients and oxygen over a longer distance, and separate tubes reduce obstacles and friction the blood experiences. Greater pressure differences in different body parts mean large animals need stronger hearts to compensate.

Reaction times can be faster for smaller animals, as nerve impulses have a shorter distance to travel.

The largest animals in the world are mostly filter feeders, predators, plant eaters, or some mix of the previous, these food sources are the only ones that can supply enough food easily. Almost all are in the vertebrate group, a hard skeleton is needed to support land animals and very useful for supporting water animals, all the big invertebrates live in water. Other groups with skeletons need to molt, which slows growth and is dangerous if an animal can’t avoid attackers, or are stationary animals that don’t have the lifestyle to grow large. The largest of these, baleen whales and sauropods if you include prehistoric life, have food sources that benefit from large size: Sauropods eat roughage where a long trip through a digestive system is useful for digesting it, Bealeen whales filter feed, which is more efficient with a big mouth to grab big growths of plankton or schools of fish. Other qualities combine to encourage or allow large size: Sauropods have air filled bones which make them light for their size, and reproduce by laying lots of eggs, allowing fast population growth if they suffer a temporary die off and requiring a lower population. Baleen whales live in the ocean which can support them, and the wide open ocean allows most of the planet to act as a single breeding population for any species.

At the smallest scales are several worms, things called tardigrades, and other random worms and jellies and other assorted groups. These include detritivores mainly, including some bacteria eaters, and some parasites. The animals aren’t large enough to attack anything else. Many lack lungs or hearts or some other internal organs, the animas are small enough that nutrients and waste can diffuse fast enough through the animal to keep them healthy.

Multicellular organisms need some way to hold their cells together in larger structures. Much of the time, networks of long molecules form this structures, in animals, these are proteins such as collagen. If you’ve used gelatin, it comes from partly broken down structural proteins, plant based gelling agents (like agar or pectin) come from equivalent molecules in plants.

The resulting gel like texture works for most animal tissue, some animals use larger water filled structures as well to maintain a shape or provide something for muscles to work against, like a water balloon or other full water bag. There are limits to what water and gel can do, however, so many animals use rigid, hard materials for some body parts, such as skeletons for support, shells, scales, scutes, or similar for protection, and/or teeth and claws and similar for weaponry, eating, and other environment manipulation. These structures could be made of hard biological molecules (like your fingernails, made of something called keratin, or insect shells from chitin) or from minerals that make a rockline structure (Bones or snail shells are examples.) These structures may or may not have some living cells mixed in. If living cells are not in the hard part, it will have a larger portion of whatever material gives it strength, but also can’t grow, heal, or change in any way (Why insects and other arthropods must molt, their shells are pure chitin and have to be replaced completely to allow growth). Including more living cells means the opposite (Your bones are an example of this.) Nonliving hard parts are produced by nearby living body part secreting material, while more alive hard parts can self regenerate and grow. Materials with more minerals in them are usually stronger for their size and weight, but also heavier for a certain size, require lots of specific nutrients (calcium based minerals are most common), and genetics to produce them can be tricky to evolve. Even the most mineralized hard materials usually have a little bit of protein, which helps bind the structure and make it less brittle.

If an animal has a jointed skeleton, joint material needs to be both strong and flexible to allow movement but still support the animal. Densely packed structural proteins usually fill this role. Joints create weaknesses, whether in a fight, in accidents, or from wear over time, but a mix of hard body parts of flexibility is extremely useful for animals that have them. Jointed skeletons have evolved only a single digit number of times, but include most animal species, including all flyers and walkers.

The outer layers of an animal have to interact with its environment, whether protecting from dangers (attackers, sharp objects, chemicals, temperature, etc.), using it for resources (breathing, waste disposal, and similar.), and some senses. Some of these of course involve tradeoffs, thinner skin can breathe more easily and have better senses, but won’t provide much protection or heat insulation, skin that can do more of everything will be more complex with the disadvantages that creates. Coverings built more for protection can use any of the structural pieces described above, from more dense protein networks for thick, tough skin, to hard molecules to mineralized shells or plates.

Skin or equivalent can contain lots of sub organs for various roles. Some cells produce secretions. These may come from the skin as a whole, or specialized glands can push chemicals out through a smaller channel while leaving the remaining skin less permeable. Hard body parts like scales, shells or similar usually use specialized structures for production, this allows for better control of their shape as well. Structures like hair or feathers are similar, these are formed from structural proteins just like harder parts. Controlling heat transfer is often important, insulating material like fluffy feathers or fur, or underlying fat layers slows it down, increased blood flow and structures that increase area speed it up. Pigments are typically produced by special cells and stored in the skin, different colors might be used for camouflage or communication. For bare skinned land animals, pigments also help protect from ultraviolet light damage by absorbing it instead of molecules vulnerable to damage. Color changes (as in the chameleon trope) might involve changing the type of pigments an animal has, or changing the shape or size of the structure containing them, affecting how light interacts with the pigment.

Ask most people what is unique about animals, and movement will be high on the list if not first. Almost all animals can move, even attached in place oceanic ones usually have swimming larvae and/or can move individual body parts for various things.

A few early splitting animals mainly use cilia when moving. These are small hairs sticking out of the main body, by vibrating/wriggling they push against water. Lots of cilia doing this in a coordinated way can cause decently large movement. However, most animals use muscles or similar. In muscle cells, fibers of protein in long bundles are attached to each other. Using energy, the fibers can be made to crawl along each other, causing a pull and creating a force. Attaching these fibers to something else allows the muscle to pull things. No equivalent exists to push, but in animals the body parts muscles work against will be arranged so that a good range of motions is possible. Muscles/groups of muscles typically come in pairs, arranged in a way where one group causes the opposite actions as what it is paired with. A couple common pairings are muscles on opposite sides or a limb, tentacle, or body that bend it in opposite directions, or lengthwise and circular muscles around a cylinder. In the second one, the lengthwise muscles pull to contract the tube, whole the circular ones push on material inside the tube, which squeezes out and extends it. Muscles often work with elastic tissue which can store energy, and return to a starting position if muscles relax.

You’ve probably seen red and white meat, these are examples of how different types of muscle fibers can be adapted for their roles. Relatedly, you may know of strength vs. aerobic exercises, or sprinting vs. long distance running. The first requiring more immediate power that doesn’t have to last long, the second requiring more endurance. Muscles can adapt in various ways for these different roles, such as storing energy producing molecules for a quick boost, or having the ability to process lots of food and oxygen to allow for longer periods of activity,. Muscles contract when stimulated some way, what causes contractions and how they are stimulated changes from muscle to muscle. One example is insect wings that beat several times per nerve signal, which allows faster flapping, another his how your heart generates the signal to beat by itself with the rest of the body controlling only how fast it does so, this keeps the heart beating in as many situations as possible even if something happens elsewhere. Other adaptations include the density of contracting proteins or what sources of energy it can use.

Muscles and cilia get used internally for moving things through a body, to move an entire body they have to push against the outside environment. Whether and how fast to move depends on an animal’s lifestyle, faster movement obviously is helpful for a lot of things, but it uses more energy and/or requires particular body shapes to reduce drag and push against the environment properly. Slow moving or attached animals such as coral or starfish can have a variety of shapes, which makes them great for background scenery, these shapes can be adapted for capturing particles coming from all directions or protection in ways that faster animals can’t do.

The exact shapes needed for fast movement depend heavily on environment, but there are a few general principles. Reduced friction with the environment is important as described above, as is a good way to push off of surroundings. Reduced energy losses within the animal are also important. Every muscle contraction takes energy, each movement of elastic tissue loses less energy but losses are still there. As a result, avoiding body motions not needed for movement is useful. Movement requires energy, so good breathing, circulation, and energy production are important, animals that use raw speed or power at the cost of energy efficiency will in particular need ways to generate lots of energy quickly. Many animal’s movements have developed to assist with breathing and circulation for this reason. Control and balance are needed to keep and animal moving the right direction and oriented correctly, this requires body parts that can change an animal’s direction, and keep it pointed in a particular direction in a particular posture if pushed in an undesired way. Some animal shapes provide natural stability, tending to stay moving in the same direction. Animals with larger nervous systems might use them to more finely control movements and allow more flexible shapes.

Water is dense and viscous, which creates a lot of drag while providing a good material to push against. It also supports animals through buoyancy. Swimming is quite straightforward for many animals to do, but to move quickly, animals need lots of streamlining. Pushing against water can work reasonably well using any moving body part, but is more efficient with wide, flat structures that can produce lots of lift or drag, sort of like human ship propellers or oars. A unique water propulsion method used by cephalopods is to squirt jets of water, by slowly sucking in and quickly squeezing water from a part of their bodies. Like with human built jets vs. propellers, the squid jets are less energy efficient but can hit a higher top speed. The fastest and/or most efficient movers through the water are animals with a stereotypical fish shape, using a wide tail for propulsion and a streamlined main body to reduce drag, other fins for direction control and stable movement, squid have a similar body shape but use fins and jets to move themselves (fins for slower, more efficient swimming, jets for quick bursts of speed). Animal parts are close to, but not exactly equal, to water density, so most swimming animals have some form of buoyancy control (gas sacs, dissolved chemicals to change their body’s density, or similar.

On land, air produces a lot less friction than water, but ground produces a lot more, and an animal must support itself instead of relying on buoyancy. Direction control is easier, animals can push off the ground to change direction and air very weakly pushes heavier land animals around most of the time, but falling over if tall enough is something balance needs to account for. Animals can crawl on the ground by pushing with some sections of their body (Anchored or with more weight on them) while other ones slide or are lifted slightly, but this creates a lot of friction and takes a lot of effort. Faster movement comes from having legs for walking. This moves most of the animal off the ground to reduce friction, legs can rotate over where they touch instead of dragging on the ground which also reduces it. To run faster some gaits lift all legs off the ground completely at points, these ones (like horse galloping or kangaroo hopping)) allow fast movement, but also require higher speeds to work properly, using more energy. Having only small areas of ground contact also can create balance and strength issues, relatively small cross sections have to handle the animal’s weight instead of a large stomach, hard skeletons are useful to provide the strength and stability needed. Lots of legs, like with land arthropods, can ensure 3 or more are touching at any particular point and be naturally stable, but 4 legs means in many gates the animal has just 2 points of contact, and animals with 2 legs obviously face this problem all the time, this requires a good balancing system (meaning senses and part of a brain). Longer stride lengths require longer (therefore heavier) legs which take energy to grow and get started moving, but also mean legs have to be stopped and reversed in direction less which saves energy, more legs also take more stops and starts and can be less efficient for that reason. The fastest animals on land are all.

Climbing is needed if a slope is too steep for an animal to walk or crawl up, the exact steepness depends on how heavy an animal is compared to how much friction it can produce with the surface.

Land animals can obviously swim, some water animals walk or crawl on the sea/lake/river bottom, and some water animals can crawl on land. The big limits are generally breathing and/or support. Body shapes for one environment aren’t generally optimized for another: legs sticking out create friction in water, though they can push against it for slower speed swimming, fins can drag on the ground without providing support or speed. Animals adapted for both environments generally have body parts that can handle both sets of requirements, like legs with webbed feet or hands that fold flat against the body. Many animals of course adapt to mostly move in one environment wi=hile being slower but passable in another. Water bottom crawling and walking are similar to land based versions of travel. Water drag means these travel methods are slower, but some buoyancy is provided, and a bottom animal doesn’t need to control it as finely as a swimmer.

Flying has evolved very few times in animals (insects, birds, bats, pterosaurs, plus some very close prehistoric bird relatives are the only ones known), gliding has evolved a few more times but is still rare. Air obviously can’t support a heavy animal by itself, but generates a lot less friction that ground or water and allows open movement. Almost all flyers and gliders use airfoils of some kind, as with airplane wings, when air moves over the foil it is deflected. The air pushes back on the foil, deflect it downward and the animal is pushed upward, fast enough and enough air deflection and the flyer can stay at whatever height it has or even climb. Airfoils produce the most lift per weight and drag when thin and flat, which you are probably familiar with on flying animals, or kites and airplane wings. Deflecting air in this way does automatically create drag, limited the distance a flyer can travel unless it has a way to increase speed or height again. Most gliding animals just use gliding for limited distances (as a shortcut/climbing alternative, for escape, or similar) where this isn’t a problem, soaring animals (and many human built gliders) can use rising air currents to gain height, powered flyers (animal or human) push against the air to speed themselves up or climb.

All flying animals use flapping as their source of propulsion. Wings in flapping move up and down, on the downstroke, the wing is held close to horizontal and extended, this catches as much air as possible and shoves it downward and usually backward, sometimes in other directions on some animals. On the upstroke, the wing is rotated to cut through the air, this generates less drag. The large amount of air pushed on the downstroke then upstroke means overall a flapping wing is pushing air down and in the opposite direction to whichever way the animals is moving. Wings or other gliding surfaces, sometimes other flat body parts like bird tails, also act as control surfaces. By angling/twisting or similar the surface, air is directed various directions to push the animal where it wants to go. Since staying up in the air requires energy, which increases proportionally to weight, flying animals need to produce a lot of power for their size.

In the opposite direction, burrowing/tunneling animals have to move through tougher materials than any other kind of movement. Soil, sand, or other sedim…uh…powders don’t flow unless some minimum force is applied, and are quite dense. To move it out of the way, animals need a lot of strength, since the burrow needs to be larger to fit a bigger animal, high strength per size is needed (so large animals won’t burrow much partly due to Square-Cube Law.) A long thin shape is useful if an animal uses its body to dig, this reduces the size of the burrow needed for the same size of animal. Animals with appendages can use them to dig, scoop shaped ones will do a better job similar to shovels, but many shapes can work if the animal will use the burrow for a long time. An option used by some animals is to eat what they are burrowing through, which might be the animals main food source if it eats detritus or smaller creatures living underground.

Animals fight for a number of reasons: to catch prey, fend off attackers or possible threats, or fighting for resources such as territory, mating opportunities, or food. Whether an animal fights and how hard depends roughly on the resources at stake and how good a chance the animal has to win the fight: fighting to keep small number of offspring alive will generally get much more intense then a fight to kill one particular prey animal when another will likely show up soon enough.

The Sun Tzus quote about winning without fighting applies to animals, fights take energy, even a winning fight can result in injuries that may cause more damage then the fight is worth, and losing a fight is obviously less preferred then not having the fight in the first place. Because of this, intimidation and avoiding fights are quite common, if an animal can scare off a rival or attacker, or avoid being noticed by a threat, the animal avoids the costs o fighting. Intimidation uses various ways to make an animal seem threatening, such as loud noises or posture to seem larger, or looking like a different, more dangerous kind of animal, or various body displays to show a willingness to fight. Interactions over time have led to poisonous animals having particular bright color schemes which warn attackers off, which some nonpoisonous animals have evolved to have as well to gain the intimidation benefit. Camouflage, fast movement, and finding or creating places to hide are used to avoid threats. Camouflage could be similar color effects like countershading (underside of animal is light, upper side is dark, to blend in water with a light sky seen from below or dark deep water seen from above) to color changes (Cephalopods are particularly known for this) and exact copying of some environmental thing (Some insects look almost exactly like leaves, sticks, etc.). Sound muffling and smell muffling can be used as well.

Predators catching prey will need to kill an enemy, this means weapons to kill whatever they are catching, and an ability to get close enough to use them. The second part usually means ambushes or speed or a combination, ambushes rely on ability to stay hidden plus a sudden burst of power for an attack, chasing relies on short sprints to longer runs of speed to outrun prey. Potential prey animals, if they don’t have have similar weapons themselves to catch animals, are typically adapted more to avoid predators then fight back, and will tend to have some or all of good senses to detect threats, speed, ability to hide or camouflage, intimidation, or armor or thick skin to make predator attacks ineffective.

Territory or other resource fights can be lower stakes, depending on the exact resources being competed over. If within a social group, group members provide some benefits to each other (otherwise they probably wouldn’t be living together) so competition within social groups might also be less intense. Though “might” is the important word, some of these competitions do still get intense and deadly, especially if the resources are valuable. If a type of animal doesn’t use weapons for other purposes, it may have special display structures for these types of fights (such as many antlers), or may just use its existing body in a more aggressive way.Fights within social groups, or at least within a particular species, can often seem ritualized, with a specific fighting method and sequence of steps used.

If an animal does fight, the variety of weapons and defenses available might be familiar to human built weapons and armor, they will become adapted to defeat each other as animal evolve. Hard body parts are useful for both attacking and defending, to handle the physical stresses of fighting. Cutting usually requires something like an edge or point in a hard body part (triangular looking teeth on predators are an example, as are thin, sharp claws on the same). Though not as sharp as human built tools or glass fragments or similar can get. These can be used to cause bleeding if the opponent has that type of circulation or simply cut chunks of flesh out of soft body parts. Hard shells or equivalent can break such weapons, to defeat these, teeth or claws or equivalent may simply be made stronger and/or replaceable, or use blunter body parts with raw force to crush the shell or bone or hard outer part to get inside. Simple physical strength can be used to cut off circulation or breathing, slowly crush shells, or to push or force the opponent around if the animal in question has an advantage in this area. Raw strength, however, tends to be slower, which may give opponents a chance to fight back or allow the attacker to be attacked themselves.

Poisons can be used to attack other animals or for defense. “If it bites you and poisons you, it is venom, if you bite it and are poisoned, it's a poison” describes this difference and how the word venom’s scientific use. Animal poisons work in a variety of ways, some by attacking various body systems (neurotoxins interfere with nerve transmission, some venoms stop muscles working, etc.), some interfere with common chemical reactions like energy production, some directly cause tissue damage to various degrees. To attack with poisons, animals need to get them into their target’s body, various injection systems are used to do this. Defensive poisons can be located anywhere in an animal (Which might not protect the individual, but can protect the species by poisoning animals that try to eat it.), though many animals with such poisons will demonstrate that face with certain colors. While poisons can be potent weapons, if adapted well, the poisons themselves take a lot of energy to produce, many animals don’t carry enough for lots of attacks. Targets can also develop resistance to poisons.

Physical defenses include hard body parts of thick skin to block attacks. Defenses are of course heavy and require extra nutrition, and can restrict movement or other body functions, but many are usable for multiple functions (such as many skeletons both protecting an animal and supporting their body.) The shape of physical defense can be important, any shape can resist scratching, but more rounded/less flat shapes and bracing/reinforcement are useful to deal with stronger bites, crushing, strong blows, or other powerful forces.

Senses are anything used to gather information from the outside world. You may have heard of human senses like vision, hearing, proprioception, and others, other animals have these as well, plus some weird ones. Here’s roughly how different senses work:

• Sight: Information from visible light, infrared, near ultraviolet. Visible light and nearby electromagnetic radiation on the spectrum have a number of useful qualities for animal senses. It can see small details, radiation like microwaves or radio lacks fine enough resolution to do this. The sun emits more visible light than other parts of the spectrum, and Earth’s atmosphere is transparent (mostly) to it, so there is a lot available to see with. It interacts differently with many materials, so reflected or emitted provides lots of information about where it came from. It also doesn’t destroy or damage most biological molecules as higher energy electromagnetic waves (like ultraviolet or gamma rays) do, so it can be safely interact with a sensitive sense organ.

  Light sensors range in complexity from a simple light/dark sensor which can tell shadows and night/day cycles, to full on eyes that can see several colors and fine detail. All eyes have to have a receptor, usually this is a molecule that absorbs some range of frequencies. When light is absorbed, the molecule enters something called an excited state, this excited state then reacts with something in a neuron to send a neural signal, or chemical signals in simpler life forms. Eyes that see a picture need some way of resolving images. Some of these eyes (including vertebrate and cephalopod eyes, the “lens in front of a ball” eyes) use a single lens and sheet of light receptors, the retina, in back to do this. The lens focuses the image, the array of receptor cells processes light from different directions. Some other animals (including snails) use a pinhole eye, where a very small opening produces the image. Compound eyes use lots of tubes next to each other to see different parts of surroundings, building an image from these.

  Light comes in a range of frequencies, and different materials reflect or emit a different combination of these. Color vision is how animals detect these differences. If the animal has one type of receptor that sees brightness, it is equivalent to black and white vision. In animals with multiple color receptors, the types of receptors respond differently to the various frequencies of light, the animal then combines this information to make color. Humans have three receptors, one reacts most strongly to blue and gets weaker away from this color, one the same with green, and one the same with yellow. Red light activates this last receptor but not the first two, so emitting different combinations of red, blue, and green light is a good way to simulate all colors for humans, including RGB displays and subtractive color printing using Cyan, Magenta, and Yellow (Which absorb some colors and leave the right combination of red, gren, and blue) . Other animals may have other numbers of receptors, “dichromatic” “trichromatic”, “tetrachromatic” etc. describe the number of types of receptors each animal has.

  Heat vision is actually a form of sight, all bodies radiate electromagnetic waves, the exact amount depends on temperature. At earth surface temperatures, the most radiated are infrared, so infrared based vision roughly sees the temperature of the object emitting it. Some animals see near ultraviolet, the least dangerous frequencies closest to visible. Both of these use the same sorts of receptors, with the same considerations, as visible light, though heat vision needs some protection from the animal’s own emissions from body heat.

  Light is dispersed and/or absorbed a little bit by air, more by water, and dirt obviously blocks light almost completely, this allows long distance sensing in some environments. It is obviously affected by the day/night cycle, it is likely the first light sense in life was useful for telling day or night. Animals in well lit areas obviously get the most use out of sight. Animals living in poorly lit areas such as deeper in the ocean, or ones active at night, dusk, or dawn, might either lose sight, or develop vision that can pick up what light is there, either by catching more light or having eyes optimized more for sensitivity to any light then for color vision or other detail. The deepest ocean and underground far from tunnel or cave entrances lose all light and animals can be blind at little cost, though many animals in these areas travel to other environments where vision is still useful.
  * Sound and pressure changes or vibrations: Sound is made of pressure waves, which quickly compress and expand materials while travelling through them. Sound is produced in quite a lot of different ways, and has a huge range of frequencies, so sensing the strength and combination of sound frequencies brings a lot of information about the environment. Sound can also travel relatively far, including around objects, even more so in water than air. Not strictly sound, but closely related, and non continuous pressure waves or other vibrations. These, if an animal experienced them, can be sensed in similar ways and also tell a lot of information about surroundings.

  Sensing sound relies on nerves that respond to motion or vibrations, these nerves send signals when their orientation changes, which occurs when movement caused by pressure, flow, or other changed position happens for one section and not another. Some of these nerves might respond to any kind of motion, which can detect pressure or the existence of sound. Individual frequencies can be detected by neurons attached to structures that only vibrate when a particular frequency of sound hits. A general physical effect called resonance makes this possible, every structure has certain frequencies of change that make it vibrate much more than others, with the right design, the range that creates this response can be kept narrow, allowing accurate detection of some frequency. The principle has been used in radios (for picking up only desired radio frequencies), avoided in large structures like buildings (where resonances can destroy the structure, methods are used to reduce the response, in animals, ears have evolved with a lot of structures such as hairs that each can narrowly hear particular frequencies. Have lots of these, and an animal can have a good hearing range and accurately detect particular types of sounds.

  Sound travels a good distance through almost all environments, though is more easily dispersed then light. It travels best through water, moving quickly and losing little energy with distance. Air is less effective at transmission, as well as dirt, but these still transmit sounds relatively well. Sound also transfers more easily between materials with similar properties, so if sensors are inside a body, land animals need a way to efficiently transfer the energy from air to body, while water to body and dirt to body happen much more easily.
  * Chemical Senses: Includes smell and taste. Receptors have a molecule shaped in a way that interacts with very specific types of other molecules. When such an interaction occurs, the sensor molecule adjusts, and sends a signal. Scientists used to think shape was the determiner, a correctly shaped molecule fits into the receptor and attaches strongly enough to send a signal. Now, other mechanisms are also involved with some detections. Animals have a wide range of sensors, allowing a wide variety of molecules to be sensed.

  Chemicals don’t travel as fast as sound or light, and spread out more where they travel, so they won’t produce as precise a picture of surroundings as light or sound. However, they last a long time, and if detection is good enough, very small amounts can be found. This makes chemical senses good for detecting that something has happened in an area.
  * Body Position, Movement, Gravity: Proprioception is the fancy word for sensing where all the parts of a body are in relation to each other. Fast moving or larger animals in particular need these senses or something similar, they are very important for proper balance and direction control.

  Several senses together detect these things. Sensors inside a body detect pressure and muscle contraction, together these can determine how body parts are positioned in relation to each other. Gravity, acceleration, and rotation can be detected with fluid filled capsules, or heavy material. Both of these will accelerate slower then the rest of the body, and will be pulled different then the body by gravity, this relative motion and pressure can be detected by movement detecting neurons.
  * Pressure, touch, temperature, and other contact senses: A variety of senses found on skin or equivalent, some of these might be found internally for various body sensations. It is not known how touch senses work, distortion of the neurons causing a signal to be sent may be involved. Temperature sense relies on a collection of different molecules that change form as temperature changes. The equilibrium between different forms of any particular molecule will change smoothly as temperature changes rather then at a single point, added to having lots of molecules, and a wide range of temperatures can be sensed.

  You might hear that “The temperature you feel senses heat flow, not temperature’’: This isn’t strictly true, but there is a reason for the saying: Since skin is a boundary between the body and the environment, it takes a temperature between them. The exact temperature depends on heat transfer rate, a faster loss of heat from your body requires a lower skin temperature (heat conduction is faster between your body’s interior and the surface of the skin if the temperature difference is greater), so skin temperature makes a good proxy for heat loss.
  * Pain: Pain detects tissue damage. Animal pain is sensed when certain nerve cells are damaged, sending a strong signal.
  * Internal body senses: Hunger, thirst, heat, and other senses use equivalents to external senses to get information with the body. A cavity or organ forced to expand is detectable by pressure sensors, such as fullness after a meal or needing to pee. Internal chemical senses can detect if there are too much or too little of some chemicals, stimulating control processes in an animal’s body or need to eat, breathe, pee, or similar.
  * Electrical, Magnetic. A few animals can detect electric or magnetic fields. Electrical reception only happens in water, since other materials do not conduct electricity well enough for an electric field to be detectable at any distance. Magnetic sensing is used for longer distance travel and is usable in any material.

  Electric fields are sensed by certain molecules, these fields cause a structure change that makes the molecule function differently, this change causes events that set off a nerve signal. The way(s) animals detect magnetic fields isn’t know: Possible methods include a magnetic magetial similar to a compas magnet, that rotates and/or presses on surrounding tissue to be detected by pressure sensors, or by electrical sensors that detect currents generated if the animal and magnetic field are moving differently, or by the effects on spin of subatomic particles (see descriptions of chemical bonding or quantum mechanics fir a more complete explanation.) Electric fields are useful for detecting nearby living things: nerve impulses, muscle contractions, and some other body functions create electric field changes, and animal bodies made of water with salt dissolved can conduct electricity, which interacts with existing electric fields. Magnetic detection isn’t as useful in the immediate environment, but can be used to navigate like a compass, some rocks are also more magnetic and this might be useful for smaller scale navigation.
  * Direction, sensitivity location of sensors: Most animals you think of have more then one sense organ: 2 or more eyes, 2 ears, lots of skin sensations around the body, maybe lots of electrical sensors on some fish. And of course, within each sense organ there are lots of whatever kind of cell does the sensing, plus things like compound eyes. Any organ takes energy and nutrients to grow, and more than one of a sense organ might just provide duplicate information not worth much.

  Aside from safety margins, extra sense organs serve a few roles. Different sense organs, positioned at different places on the body, will pick up different information. Bodies block light and sound somewhat, so eyes and ears pointing different directions give a wider field of view. Multiple small eyes, and multiple light sensors in on eye, pointing in slightly different directions allows an image to form. Contact senses need to be right next to whatever they are sensing, so need to be all throughout any parts of a body where that sense will be useful. Multiple receptor types detecting different things, as described for each sense, are needed for color vision, smelling or tasting different chemicals, different sound frequencies, etc. Overlapping or close together sensors sensing the same thing allow direction and depth perception. Your own depth perception partly relies on images from two eyes being slightly different, the differences allowing distance to what is being seen to be determined (Parallax and perspective are worth searching for more information on this.) Different strengths of sound, smells, or electric fields n nearby sensors work similarly, allowing a direction to be determined, more sensors provide finer information this way.

  Sense organs are obviously more useful if they can collect more information, particularly more information of interest: Eyes might be higher up to see farther and over objects, chemical sensors in a mouth provide important information about food, similar logic applies to others. An animal’s body interferes with some senses, so they may need to be, or at least be more effective, on the surface and not be covered by protection. Locations that don’t get attacked as much may be helpful. Being close to a brain or equivalent means less nerve cells are needed to transfer information, and possibly a somewhat faster reaction time (depending how long nerve connections are needed to organs that carry out the reacting.) Making sense organs movable (either the organs or body part they are attached to) can help with all of these, organs can be moved to an area of interest to collect information, moved somewhere else if protection is needed, moved to a different interesting area, and so on. It is like the typical animal head with a brain, mouth, and lots of sense organs exists for some of the previous reasons: Some animals (the bilaterals) tend to move a particular direction, catching food means moving towards it so a mouth naturally pointed this direction, the animal would find it useful to sense what was in front of it and which way to go, so sense organs were most useful towards this front end, and a cluster of sense organs meant a brain nearby made the most sense.

  Brains of course are where decisions get made. Which leads to:

At one end, we have the Clever Octopus, elephants that never forget, playful dolphin, and Clever Crows. On the other end, sponges, bryozoans, and other attached ocean filter feeders don’t think much or at all. In the middle, everyone reading this page. Intelligence is difficult to define exactly: memory, problem solving skills, tool use, learning, adaptability to new situations all play a role, and different animals considered intelligent are good at different things. Scientists can test intelligence by seeing how animals perform various tasks, such as the mouse in a maze, or escape or tool use tests.

Any multicellular life needs a way for different parts to coordinate. Even in the simplest types, cells that move together or form coordinated shapes will do better than ones that act independently even while clustered. If the life form has senses, this information has to be processed in some wary to be useful. To accomplish this, various communication methods are used. Chemical communication is the most basic, using various signaling molecules for this role, which diffuse or are pumped to appropriate receptors, cells with correct receptors receive a signal and act accordingly. Hormones are an example, in humans and many other animals.

More precise and faster, but less long lasting, are electrical signals. These take the form of nerves most of the time. Neurons, the cell that transmits signals, have a long main tube with several branches on both ends. Signals are transmitted by ion exchange along the tube, when the end is reached the neuron releases chemicals to another connected neuron or to another type of cell that does something, in neurons these chemicals set off another electrical signal. Neurons can deliver a signal to exactly where it is needed, instead of diffusing like chemical signals, and electrical signals travel a higher speed then diffusion or most fluid flow, but the signal disappears quickly. The chemicals that communicate between neurons, called neurotransmitters, can act like chemical signals/hormones, lasting longer and diffusing over a wider area. Learning and memory are thought to come from neural connections being strengthened or weakened to form particular arrangements, but the nervous system is complex and other processes may play a role. Whatever these processes, they influence how future nerve signals travel, which leads to adjusted information processing and decision making.

Chemical signals in theory could produce what we call intelligence, but in practice neurons take this role. Neural webs in animals like jellyfish which are spread out through the body make decent decisions, but as animals get more complex, lumps of neurons forming a particular organ are used to fill this role. In animals with heads, the brain is the biggest lump, sometimes a few additional ones exist in the body. A measure called “encephalization quotient" compares an animal’s brain size to an “expected” size for the animal’s weight, which is usually calculated by looking at lots of similar animals, checking ratios, and calculating a trend. This measure roughly corresponds to measurements of intelligence, but not in an exact way. Larger bodies need larger brains for basic body functions, muscle control, and more incoming sensory information, otherwise more neurons allows more types of complex networks and connections and should allow more of all kinds of intelligence, but some animals may use what they do have more efficiently, by neurons that individually can process more information, or a more efficient arrangement of neurons.

Neural tissue eats up a lot of energy, in your own body about a quarter of the energy you use goes to your brain, so intelligence has to have a good payoff, an animal's environment and lifestyle benefiting in some way from adaptability, memory, and the like. Oceanic nonmoving filter feeders don’t move or make much of any decisions at all, so intelligence tends to be low. Social Animals are often more intelligent than solitary ones, as keeping track of members of their group and navigating relationships can get complicated. Animals in complex environments use intelligence to navigate them, remembering routes and figuring out patterns for where things are. Predators are thought to generally be more intelligent than herbivores, since their food sources can move and have to be tracked or searched for, plus figuring out safe and powerful ways to attack. Animals with more generalized diets are likely more intelligent, this allows them to adapt to new food sources and adjust among a greater variety.

Almost all animals interact with members of their own species, even if only to mate or they happen live near each other and interact that way. Some take this further, living in groups of varying complexity, ranging from temporary groups coming together to do something like hunt, mate, or scare off predators, to long lasting groups like herds, to hive insects with inborn specialized roles and huge numbers of members. Wherever animals contact each other, some sort of social interaction and communication occurs.

Grouping up, like anything else animals do, has tradeoffs. Disadvantages include that more animals in a single space means more competition for resources. Animals must also manage interactions between each other. Advantages of grouping up are that members can work together in ways that benefit everyone in the group, such as working together to get resources they could not individually, like building things or hunting large prey if carnivores, Animals can specialize, some taking different roles like different hunting positions, or some animals watching for danger while others do something else.

The combination of competition for resources and cooperation can make social interaction rather complicated, so more social animals tend to be more intelligent than equivalents. A number of structures can exist, such as highly specialized eusocial animals where every individual is specialized to do something, to family groups, among others. Common is hierarchy, where higher ranking members get more resources than lower ranking ones, animals will compete for positions in the hierarchy at points but otherwise accept a position. This can reduce fighting in some situations compared to all animals competing all the time for resources.

For animals to interact, they must communicate. Communication can use any external sense an animal has, with one sending a signal to trigger that sense and the other detecting the signal and acting on it. Calls, songs, and other sounds are most familiar, sound is great for communicating as it travels around obstacles, goes a long distance, and has a lot of potential variety allowing lots of potential messages to be quickly sent. Vision may be used with body position and movement, or in a few animals light or color displays, Chemicals used for smell communication are usually called pheromones, these can last a long time in an area but are not as fast as sound or light. Touch can be used as well, as in rubbing someone to comfort them.

Cross species social interaction is somewhat common. Most well known to readers are human and domesticated animals, but different nonhuman animals will sometimes join together. Examples are some multispecies herds, or occasioanlly different types of animals teaming up to hunt. Animals must be able to follow each other's communication, which will make this process less common, but combining different capabilities, or different requirements, is beneficial. Multispecies herds, for example, might have some species as better lookouts and some as better fighters, and different food requirements reduce competition.

Reproduction is one of the defining features of life, animals are no exception. Like other multicellular life, animals reproduce by creating a smaller, separate body that grows into a full grown organism. A few can reproduce asexually, one animal producing offspring on its own, almost all reproduce sexually, combining genes from two parents in each offspring. Thus an entire collection of Sex Tropes. Sexual reproduction allows more mixing of genetic material, creating more variety in offspring, though the two sets of genetic material need some way to meet, which involves risk and energy.

Sexually reproducing animals start from a single cell, created by merging a cell from each parent. The merged cell divides to form a mass of cells, this cluster grows large enough to form a small body, portions of which specialize into different organs. Chemical signals control how this specialization goes, based on the location of cells within the structure, ultimately the whole process is controlled genetically in some nonobvious ways, there are often not specific genes for, say, a tentacle or lung, but combinations that produce a particular pattern. Mutations in these genes are a common way to evolve unusual new body types. At some point, the body is large enough and has the correct organs to live as an independent organism.

In the simplest form of reproduction, reproductive cells are released by parents, randomly meet, carry enough starting nutrients to form a small, independent offspring, and that offspring than goes on to live with no further assistance from the parents. This type of reproduction has the advantage of the parents needing little energy per reproductive cell, they can produce a lot, some of which will survive. Over time, reproductive cells have specialized, some are larger and carry nutrients to start growth, these are eggs. Others are smaller and cal usually move faster, these are sperm. In animals that specialize in producing one or the other, egg producers are females, sperm producers are males, you are probably aware of other differences that have come from this division in your favorite animals of choice. Producing both is an option evolved in some animals, it requires these animals to maintain organs to produce both but adds flexibility.

Some animals invest a bit more energy into each offspring, these can’t throw out as many reproductive cells, but the ones they do release have a better chance of surviving. Eggs can carry more nutrients, often the egg cell is in a larger structures such as the chicken eggs we eat, that carries lots of starter nutrients for a good sized offspring. Parents can guard eggs, maybe burying them, maybe watching them or carrying the eggs, or dropping the egg structure and growing offspring inside their own body. When the offspring hatches or is born, parents may further protect and take care of it for some time, thus many Parenting Tropes.

Simply spitting out reproductive cells on the chance some will meet works for some animals, but most have to meet up close in some way to bring sperm and eggs together, a.k.a. mating (...Sex Tropes). This also allows animals to choose who to reproduce with, though in some sparsely populated areas animals will go with whoever they meet first as otherwise they might not reproduce at all, At minimum, the animals can stay near each other which makes sure sperm and eggs are close as well and can meet, animals can do internal fertilization where one type of cell (usually the sperm) goes into the other’s body, using a range of different methods in different animals. In between options exist, where sperm is given to the receiver, who puts it on their own eggs or into their own body. These methods are important for land animals, without water reproductive cells can’t move freely to meet.

The choice of who to mate with has goes a number of different ways. Animals often go for qualities with survival advantages, such as faster or stronger partners. Some animals fight for the chance to reproduce, or use competitive displays in a similar role. A lot of animals go for qualities without obvious survival benefits, such as birds liking nice songs or colorful display structures, a combination of animals with the characteristic and animals attracted to it can cause runaway evolution where the feature takes over a population or forms a new one. Meeting potential mates is easier if everyone is close together and reproducing at the same time, this the Mating Season Madness trope in many animals if their way of living allows it. Animals may have a few to many partners, and spend varying amounts of time with them, from attached filter feeders that release reproductive cells and never meet reproductive partners, to one off encounters, to herds where one dominant animals reproduces with many others from the opposite sex, to longer and shorter term monogamous pairings.

Two animals have reproduced, a new creature is hatched, born, or otherwise come into existence. Now it has to grow to adult size. Growth rates vary from animal to animal, as a general trend, animals with faster metabolism will likely grow faster. Food availability is also important, the more food, the more an animal can eat, the more it can grow. Still, there is wide variation. Animals with parental care will be fed or otherwise taken care of for part of this time, animals in social groups are often cared for by members of the group.

Some animals are born like miniature adults, some go through changes. The most extreme of these changes are a process called metamorphosis, where the animal goes from a larval form to a completely different body shape. Caterpillar to butterfly is the most well known example, other insects, some bony fish, starfish, amd a number of more obscure animals do something similar, including a number of attached, sessile animals that have swimming larvae before changing to settle down. In between are creatures like frogs with tadpoles or young wingless dragonflies, which noticeably change but still kee[ big elements of their bodies from young to adult forms. Being hatched or born adult like obviously requires less change over time, saving some energy and requiring less genetics or signaling for development. However, changing over time allows different forms to specialize into different roles, or do things when young not necessary for adults. Insect larvae often specialize to eat and don’t use energy on reproductive organs, sessile nimal’s larvae can swim to find new locations, before attaching somewhere as an adult. Among many animals, metamorphosing or not, adults and young eat different foods and possibly live in different places, reducing competition with themselves.

At the other end of the lifecycle, animals face a lot of deadly threats, plus aging in many. Lots of deaths are of course caused by diseases, attacks, accidents, extreme weather or lack of food or other bad luck, and other such things, animals in human controlled environments can almost always outlive animals in the wild. However, some internal controls do seem to limit how long animals live even if nothing external kills them, though mechanisms of aging aren’t well understood. Some animals die after reproducing, cephalopods and some insects are the most famous, the tradeoff here is likely that putting energy into reproduction instead of surviving gives more offspring, which compensates for the original parents dying.

Disease and injuries are constant threats, they can kill directly, or weaken an animal to where it dies some other way. Animals have a number of ways to survive and heal from them. Protective systems like skin or acidic or alkaline stomach stop many microbes and parasites from entering in the first place, usually either with barriers that physically stop an infector from getting in, or chemicals that poison or destroy it. Large enough parasites can be physically removed, as with horse or cow tails that can swat flies, or birds and whales rubbing themselves in dirt to clean off skin parasites. Some animals specialize in cleaning parasites from other species, usually behavior evolves so the animal being cleaned allows this to happen.

If microbes get into a body, animals have further defenses. Some chemicals inside a body can harm microbes. Specialized cells can eat them, other cells can attack infected existing cells, which are part of a virus reproductive cycle. Vertebrate have additional components to this system, that can adapt to specific invaders and more effectively target them, cleaning up the disease agent much faster if exposed again. This is the basis for vaccines, which expose the body to something harmless that activates the immune system as a disease would, so you catch the actual disease your body fends it off far faster. These disease fighting systems also attack cancer cells, some smaller parasites, and other random things that may enter the body. To tell an outside body from a part of the animal, animal cells have signaling molecules on their surfaces that invading disease microbes usually lack, immune cells can also respond to common molecules found on disease causing microbes to detect them.

To heal from injuries, an animal needs to remove damaged tissue if it can’t be repaired, and regrow whatever was missing. Doing this takes nutrients and energy, and some body parts on some animals can’t be fully healed if damaged. To remove unrecoverable tissue, some will be rubbed off, sometimes regrowing tissue will push it away, immune cells might digest some if the tissue comes in small enough pieces. Regrowing tissues means lots of cell division, with chemical signals to tell what type of cells should grow where, and chemicals such as bone or shell are secreted as needed. The process is about the same as normal growth, since regrowing what is damaged is what the animal is doing.

If an animal isn’t killed by an attack, accident, or infection, or other bad events, a few patterns have been found for how long they can live. Larger species tend to live longer, though smaller animals within a species tend to live longer also. Animals that face fewer threats/have some protection, like flying or shelled ones, tend to live longer, likely the benefits of reduced aging matter less when something else has a good chance of killing it anyway. Slower metabolism animals can live longer, though this isn’t as consistent a trend, it seems the lowered speed of chemical reactions means those associated with aging also happen slower. Some cnidarians are unique in switching between two forms, each of which effectively rebirths the animals, these theoretically can live forever this way. Some others don’t seem to age at all, though they still suffer other causes of death. The longest lived animals as far as we know are some several thousand year old sponges and corals, out of larger animals some deep sea sharks may be the longest lived.

Several tabs up described how an environment affects the animal, but how does an animal affect its environment? What happens if you place one in an environment or take one away? (Careful about doing this in Real Life, ecology can get quite complicated and weird things happen.)

Obviously, everything an animal does affects its surroundings in some way, large or small, but there are a few big categories of ways animals influence things around them. Most of these also apply to other lifeforms you might add, but this is useful notes animals, so animals will be the ones talked about.

One way is to directly physically change its surroundings. Burrowing, building, knocking things down, moving objects around, that sort of thing. In extreme examples, animals can create whole new environments, such as coral (and occasionally other animals) building reefs or beavers creating lakes by damming rivers. Another less extreme but important examples are burrowers mixing and aerating soil/seabed sediment, which can change nutrients available to plants or algae, and allows different living things (including microbes, fungi, and such) to use the soil or seabed.

The second big effect on surroundings is through eating. Animals have to eat things, and can be eaten by other things (not just predators, also parasites, somewhat scavengers as well.), making what they eat less common and what feeds on them more common. Animals that can learn will also change their behavior in response (hunting differently. Changing activities to avoid predators or remove a new parasite, that sort of thing.) Sounds simply enough, but the knock on effects when food and feeders themselves eat things and are eaten by things, and those organisms are eaten by and eat other things…..eating behavior is a powerful way that animals can influence their surroundings. If an organism like trees or coral that is the basis for the environment is part of its food connections, an animal might be vitally important=ant for maintaining an environment (often happens with predators of things that eat important plants or similar), or might cause a collapse if too common or rare.

Larger scale and longer term, sound counterintuitive effects occur as eaters and eaten adapt to each other. The existence of creatures that eat a certain type of lifeform tend to increase that type’s diversity, both by reducing competition for resources which allows room for other living things to use them, and by forcing adaptation, as an eater can’t optimize as well for a variety of types of food as for a single type. Longer term, life forms eating other life forms are a big driver of evolution, as features useful for defense or getting food now have value where they would be wastes of energy and nutrients otherwise. It is thought, for example, that the evolution of good senses, hard body parts, faster movement, among other features were driven by the appearance of predators of animals back in prehistory. These features have proven useful for other things once evolved, and make up a big component of animal diversity today.

A third big animal influence in this useful notes is by moving things around. Animal’s ability move makes them much more important in this role then most other organisms (or moving microbes which can’t move that quickly). Within an environment, animals might help spread things out from a single source, such as moving pollen and seeds around which helps some types of plants reproduce. Over a larger scale, migrating animals might move diseases from place to place, or move nutrients if they tend ot eat on one place while pooping, peeing, and/or dying in another (leaving a decomposing body in the new place). A spectacular example of the last is Whale Fall, where corpses of whales which tend to live near the surface might fall to the ocean floor, feeding large numbers of many kinds of animals from scavenging the carcass, on an otherwise nutrient poor ocean floor.

A final effect is on interactions with living things that aren’t eating. This could include harmful interactions (parasites technically means this, this useful ntes has mostly used to narrower sense of animals that eat from another without killing it, like bloodsuckers.) or helpful interactions. Obviously, removing or adding an animal on one side of an interaction effects those on other other. These could include cross animal social cooperation, competition between different types of animals for things like territory, and a range of behaviors that are hard to summarize here.

How important an animal is depends on how much overlap its activities have with other animals (same with other lifeforms), and how large an effect its activities have. No two animals in an area will do exactly the same thing for long, in these situations one will outcompete another, but there can be varying amounts of overlap (such as two animals that eat similar food, but are active at different times, or have different preferences of both types are available, vs. animals that might occasionally eat the same thing but mostly have different food sources). Extremely important animals can be called keystone species, removing them destroys or greatly degrades the environment they were in, some examples include major predators, including ones who’s prey eat the major plants or other organisms making up an environment, or environment builders described above, or on the invasive species side, an animal that can eat something that previously had no threats. By contrast, an animal that eats food also covered by lots of other animals, that is also not all that common, and does no building or burrowing or similar won’t have much influence.

Horses, Cows, Dogs, and many more: Human society uses a lot of animals, mostly for food, work, and companionship, also entertainment like cockfights or horse races, and various other random small roles. Animals might be wild, as with hunted ones or pollinators, that happen to serve a role for people. They may be tamed or kept in captivity, still the same as a wild animal but controlled or learned to live around humans, as with aquarium fish, honeybees, or War Elephants. The most significant involvement is domestication, where an animal has changed in big ways from a wild form into something adapted to live with humans. Wolves to dogs, most livestock and horses, are all changed in this way. Some animals cannot be domesticated for reasons that aren’t entirely clear. Even if not domesticated, animals for taming should be useable if they are comfortable around people or can be controlled appropriately.

Food animals might be eaten directly as meat, or kept for products such as milk, eggs, or honey. Good food animals historically ate foods that people wouldn’t want, such as waste fed to pigs or grass eating livestock: if the animal diet did overlap, plant food was needed. This way the animals weren’t competing with people for food. Though in modern times with extremely productive farming, food animals mostly get a much richer diet of food crops. Animal products supply nutrition that is hard to get elsewhere, meat milk and eggs give people fat, protein, and a number of vitamins that are possible but trickier to get from plants. Fish historically have been caught wild, but in the past few decades controlled fish farming has gotten more important, especially as fisheries in many places have become less productive thanks to overfishing and pollution. Fast growth is useful, for meat animals this often means less food needed per meat produced. It also often means less food for the animal is needed to produce whatever it is being raised for, an important quality for anything being grown.

Work animals like working dogs or draft horses obviously need to be able to do the work expected: draft animals need to be strong and have good endurance, hunting dogs need their good smell and hearing, plus endurance for a longer hunt, you’ll know what qualities such an animal needs for the work they do. Just like with food animals, it is good for work animals to eat something different than human food, so as to not compete directly, however the roles work animals are playing may be valuable enough that this isn’t a concern. Among work animals are war animals: Horses, War Elephants are famous, camels have been used to great effect in some battles. In battles before industrialization, these animals give their riders height and speed, and large animals coming to kill you, often making powerful noise, are very scary; getting enemies to run away is a major part of these battles.

As for pets, entertainment, etc., go for whatever makes you happy and isn’t too expensive or causes too much trouble. Using animals for human society does bring a lot of problems. Animals moved around the world can become invasive species, outcompeting native animals and causing problems in nearby environments. Rats, dogs, and cats are known to do this numerous places in the world, pythons in Florida are a more exotic example. Animals can be dangerous, large livestock kill a good number of people each year. Animals pollute, in premodern times dung was the main problem to clean up, today the land use that often replaces forests or other denser plant cover with animals farming is the big problem, contributing to global warming. Beef cows are notorious for this, also giving off methane that adds to the problem, plus the pollution associated with feed often used for them. Human used animals are often densely kept, spreading diseases. On a smaller scale, catching exotic pets often uses highly destructive practices for the amount captured.

Still, despite these issues, animals will be used for a long time.

The “primitive” animals, either with no symmetry or radial/circular symmetry. They have lots of unusual features and abilities you don’t find among bilaterians.

Non-moving filter feeders, these are probably the closest to the earliest animals. Sponges have a similar form, a water channel or series of channels on the inside, lined by a layer of tissue, that coats a layer of structural material of some sort, followed by an outer lining. The inner layer is lined by cells with cilia, the cilia pull water through the inner channel, where particles and microbes are absorbed as food. More complex sponges have lots of small channels, giving more surface area for the cilia to pump water and absorb nutrients. Sponges use a number of structural materials for support, some use a material called spongin similar to collagen, some use minerals. Many sponge larvae are swimming, using cilia, others drift, they lack specialized organs just like the adult forms. Most sponges capture sperm inside themselves and grow larva there, rather then releasing eggs to be fertilized in the open as many other similar animals do.

If you are wondering whether sponges the animal were used for sponges the cleaning tool, yes, they were. Spongin is elastic, and the numerous water channels and holes in some sponges are excellent for holding water, combine them and you get the soft, elastic, water soaking thing found in kitchens and cleaning equipment everywhere. Sponges were used so much for this that they were overfished in some areas, synthetic materials got rid of this problem and are the main component of cleaning sponges today.

You’ve probably seen these if you’ve watched deep sea documentaries: hollow, transparent swimming animals with curved rows of waving hairs on the side: they often fill the role of ‘generic swimming animal” shown for a quick shot but not commented on. The main body itself is quite small, with a simple mouth and digestive system, the large cluster of cilia on the sides moves the animal and helps funnel food in, mostly small plankton. The tissue of these animals is jellyfish like, held together by a similar material.

The transparent cilia carriers on comb jellies can have a number of shapes: lobes, ovals, twisted, or squash shapes are some examples.

Coral, Sea Anemones, Jellyfish, and some others. Cnidarians have an unusual reproduction system, cycling between a fixed in place form and a free floating or swimming form; a few species can in theory live forever by changing between these forms. Cnidarians are radially symmetric, the same going around in a circle, and have a digestive system where food goes in and waste goes out through the same opening, many have some sort of tentacles or arms utilized in capturing prey. Cnidarians are also characterized by the possession of cnidocytes, cells that are shaped like tiny harpoons that serve to inject venom into their prey.

——
Jellyfish

Tropes:
Electric Jellyfish

Used often as background animals swimming in an ocean, or documentaries as a generic ‘simple animal”. Jellyfish are not any particular branch of cnidarian, instead are found in several groups. The tentacles sting things, and draw food towards the jellyfish mouth where it is eaten. Their pulsing swimming motion takes advantage of water turbulence, helping the cup structure relax. The floating jellyfish we are familiar with is just one part of a reproductive cycle, also part is a seabed living fixed form that spawns many swimming forms. The floating form (Medusa) reproduces with sperm and eggs, which fertilize and grow into the fixed in place form (polyp), this can either turn into a swimming form, or release several smaller sheets that become swimming bodies.

Jellyfish stings: The sting actually injects a structure into the victim, this structure releases poison. To get rid of the poison, the best way is washing with seawater..yes, a sea creature’s sting is solved with seawater. This washes off the jellyfish structures without popping them as freshwater might.

These days, with lots of ocean creatures dying off for various reasons (pollution, temperature, acidification, overfishing) jellyfish have been filling in the gaps. Not too economically useful, and can cause further problems if jellyfish eat too much larvae of other creatures. Though many sea turtles eat the things, they too are negatively impacted by the ocean's changing climate and pollution, so may not be enough to curb the jellyfish.

——
Siphonophores

Tropes:
The Worm That Walks

Also not a specific branch of cnidarian. Siphonophores form big colonies of specialized animals, in a vaguely similar way as cells form multicellular life, and live much like jellyfish, eating and often stinging what they can catch.

Among their number are some of the few really huge non-vertebrate animals. One of them is longer than a blue whale, though not as heavy and technically made of lots of animals.

——
Coral

Makers of reefs, with lots of beautiful, colored shapes where tons and tons of other life makes its home. These shapes are actually rocky structures built by coral, the actual animals are small bodies not too different from jellyfish or sea anemones, with tentacles surrounding a mouth. Many coral have algae within them that supply much of their energy, otherwise they catch whatever floats by.

The reefs are much more known than the animals themselves, creating fantastically productive ocean systems that produce a lot with often limited nutrients, and act as breeding grounds for many animals elsewhere in the ocean. Not surprisingly, these are threatened by pollution, temperature changes, and acidification, a common effect is “bleaching” where coral lose their algae, go white, and die, causing problems for everything else living nearby.

The closest thing to a Blob Monster, though much smaller then usually seen in movies. They were discovered only around 1880, and are very small (a few mm across), as a result not much is known about them. Placozoans are pancake shaped, with a few types of cells organized into layers. They use cilia to move, and eat by engulfing food on one side of the pancake shape, then excreting enzymes to digest their food, which appears to be microbes and other debris. The structure is consistent enough that only genetic testing tells species apart. Poisons have been found for protection from predators. They mostly reproduce by budding, but can perform sexual reproduction occasionally if conditions are stressed.

As mentioned, the exact relationship of placozoans to other animals groups is not well figured out. Their extreme simplicity suggests an early diverging, primitive type, their cell layers resemble a simplified version of comb jellies or cnidarians. Genetic testing sometimes suggests a close relationship to cnidarians or bilaterals, the exact relationships are still not nailed down even in these analyses. Obviously they have lost complexity if this is the case.

This is a small group of flatworm like animals. They were originally classified as flatworms, which were assumed to be early bilaterals. However, genetic evidence points to flatworms and this group actually having separate ancestry, It is thought that this group diverged from other bilterals early on, but some evidence points to them being related to echinoderms instead. Al of them live in salty or brackish water.

They lack a lot of internal organs and are generally flattened top to bottom. They are missing some internal organs found in other bilaterals, such as a body wall or brains, some lack a standard digestive system. Eating is split between eaters or small things and parasites, mostly living near the bottom of whatever water they are in. Reproduction is sexual, all members are hermaphrodites.

Roundworms and lots of other little tiny things, including lots of parasites. You don’t notice them day to day unless you or a pet gets infected, but they are one of the most common animals in the biosphere, found in all environments, including on you. Some estimates suggest they have about a million species, but scientists have only discovered a few tens of thousands. Probably the most unusual environment is in the deepest mine in the world, where some nematodes eat bacteria living on the rock. It is thought they descend from worms introduced when people first dug the mine. Nematode diets are pretty much anything small they can get their hands on: bacteria, small particles like skin flakes, decomposing material, bits of flesh and body nutrients as parasites, etc.

As the name roundworm suggests, these are pretty simple animals, a worm that is round, with a standard gut and head, with sometimes an organ for sucking liquids. Nematode mouths are somewhere between a Flower Mouth and a regular jaw, with several components that open and close together.

Technically, nematodes are part of a bigger group, the nematoida, which includes several similar worms.

This group of animals is extremely recently discovered, specimens were only fully captured and analyzed in the 1970s and 1980s. They are extremely small, and live by attaching themselves to the sea bottom, hiding in crevices, and feeding on debris. The animals are named for an outer shell called a lorica, which has one big opening for the head. These organism have most of the standard internal organs, including brains, but do not have blood circulation. They have typical male and female reproduction, with larvae that mostly are kind of like adults, but swimming to find a location to settle.

More worms, living in the ocean, eating random bits of stuff. Mainly included because of their name, meaning “Penis worm”. (Same base as “Priapism”.) This both references their general shape, and a particular organ.

The Kinorhyncha, close relatives, live in mud instead, and eat random bits of stuff there. These are more obviously segmented, but still wormlike.

Both these animals have an outer hardened cuticle, which has to be molted.

Despite the name, these look something like caterpillars, and unusually are fully land based, though must have water based ancestors as their relationship to arthropods seems to go back to when these were water living. Instead of legs, they are supported by cones of their body extending to the ground on which they wriggle. These lower portions are hardened, while the rest of the skin is softer.

They are mostly carnivores, ambushing prey, and some show herdlike behavior, defending regions and developing a dominance hierarchy.

Also called water bears. They look something like an armadillo bug if such a thing existed, with several feet attached to shelled segments bigger near the middle and smaller at the head and end. And they are very, very microscopic. Mostly they eat microbes and plants, a few eat fellow tardigrades.

Their main claim to fame is surviving all sorts of nasty stuff: high and low temperatures, drying, outer space, quantum entanglement, among others. A simple genome helps with this, the animals also are able to enter a dormant state that can tolerate these conditions and restart under better conditions. They also have a number of proteins that preserve cellular structure.

Tropes:
Big Creepy-Crawlies

In many ways, the stars of the animal show, living in almost all environments, by far the most species both discovered and estimated, filling almost all possible ways of eating and living. Prehistorically, members of this group were the first animals onto land and into the air.

Arthropods are covered in a hard exoskeleton, and form in segments, you can see these clearly in a lobster shell or on centipedes or millipedes. In most creatures the segments have evolved to merge together into a few large body sections, as in three part insects or two part spiders. This exoskeleton commonly has appendages, forming legs, arms, claws, antennas, or wings based on the creature. This skeleton provides protection and support, but for whatever reason never evolved to grow with the animal, the danger during moulting and slowed growth limit arthropod's modern size. Compound eyes are common, with lots of different lenses adding up to a single picture, but a number of eye structures exist in the group.

With such a large, well known group, it's worth writing up arthropod subgroups as their own sections.

——
Chelicerates

Tropes:
Arachnid Index
Scorpions

Arachnids and horseshoe crabs, named after their mouthparts. Arachnids include spiders, scorpions, and ticks, among others. Prehistorically, these may have been the first animals onto land, legs and organs easily adapted to book lungs made the transition relatively easy. Today, horseshoe crabs still make this transition, going on on a beach to mate and lay eggs, while arachnids mostly live on land permanently.

Spiders and scorpions have merged body segments into a head and rear section, the head uses organs called pedipalps to eat, the rear section contains most internal organs. Spiderwebs are famous, but spiders can also ambush prey, or create other types of traps, sometimes using poison. Ticks are parasites, of course. Horseshoe crabs look like a dome with a spiny tail that walks along the seafloor, the tail’s main use isn’t stinging as it may appear, but flipping the crab over if it gets stuck.

Many of these animals use book lungs, a breathing system where flat, body fluid containing structures are stacked together and surrounded by air.

——
Myriapods

Tropes:
Creepy Centipedes

Centipedes and millipedes. Long critters with lots and lots of legs. Centipedes are carnivores that lack eyes, millipedes are usually herbivores or random debris eaters that can see, among other differences. They are a candidate for early land animals, breathing through leg structures, today all of them live on land. Centipede bites may be painful, but rarely dangerous.
——
Crustaceans

Tropes:
Giant Enemy Crab
Red Live Lobster

Technically not a phylogenetic group, since insects descend from some of these, but still recognizable as a type of animal. Often 10 legs, shelled, includes culinary favorites shrimp, crabs, and lobsters. And, surprisingly, barnacles, which resemble free floating crustaceans until they attach to something and filter feed, to the bane of large sea creatures and ships everywhere. Mostly they live in water, some go on land for short periods, and a few have gone fully land based.

Crustaceans come in many sizes, from the largest arthropod today to krill, and fill lots of ecological roles in the ocean, from detritus eaters to somewhat large predators, one the seafloor or floating or swimming, and act as a source of prey for lots of larger animals. The commonly known lobsters and crabs, and many shrimp, are in a group called decapods, meaning 10 feet, but other groups have more segments and look like worms with lumps or millipedes. Lots of animals are called shrimp, unlike the curled cooked version, most of these are flat when alive. They are not red as the cooked lobster trope suggests, see uncooked shrimp as a better representation of colors, though some crabs will look red.

Caddisflies, or Trichoptera

These live most of their lives as larvae in freshwater, adults live for a much shorter time. The larvae are important animals where they live, eating a variety of foods and being food themselves for lots of fish and other animals.

——
Flies, or Diptera

Tropes:
Flies Are Evil
Messy Maggots
Super Fly Reflexes

Another “true” group (see true bugs), officially known as the true flies. These have a bad reputation among people, thanks to mosquitos biting and spreading diseases, annoying houseflies and fruit flies and such, maggots, and several species whose larvae live in people and animal’s skin, causing problems in the larger animal, this is called myiasis or being fly-blown.

Flies have just one pair of wings unlike other insect’s double set, and with this pair plus good senses are very agile in the air, thus swatting a fly as a show of reflexes. Fly’s dodging comes heavily from detecting air currents, which solid objects such as a hand generate more of than flexible or perforated objects, thus flyswatter designs with lots of holes. Pieces of cloth can also do the job if flicked or whipped in the right way.

Fly larvae eat lots of types of foods, but detritus and dead things are most common, thus many types of maggots. A few of these maggots can be used medically, sterilized (very important!) members of some species preferentially go for dead tissue while leaving living tissue mostly alone, and also have bacteria killing chemicals: these can remove dead tissue and reduce infections in some ulcers or other difficult to heal wounds. A lot of adult flies have sucking mouthparts, which can feed off of liquid as in fruit flies, or, as with mosquitos, suck blood. Thanks to fast reproduction and being well studied, some fruit flies are used as lab animals.

——
Scorpionflies, or Mecoptera

Named for the end of theri abdomen which looks like, surprisingly enough, a scorpion tail. These look like the other group of flies, but instead live in moist areas and eat dead animals, or other random bits of what is available.

——
Fleas, or siphonaptera

Tropes:
Flea Episode

Ntice the a-ptera, so no wings on these. Fleas are flightless, but are known for jumping seemingly absurdly high compared to their body size, another example of Strong Ant style Square-Cube Law in action. All of them live on mammals or birds, sucking blood. Larvae also live on skin, eating dead skin cells and other debris that accumulates. Like many blood suckers, they are known to transmit diseases (bubonic plague/black death being the most well known.)

Animals whose early embryos divide in a spiral way, in other words, as the fertilized egg cell divides, the resulting cells make a spiral pattern. The other group of protostomes.

Microscopic, mainly water living animals, with unusual hard mouthparts. Some are attached, some float, and some swim, eating up bits of debris and small animals. The mouthparts open into a brisly looking front looking something like a vase, the rest of the body can have a variety of shapes, though always symmetrical, such as conelike with a tail, or cup like.

Worms that are flat. This is not just being silly, flatworms have lost a lot of the internal organs that most bilateral animals have. Among these are a respiratory system, flatworm’s flatness lets them abrosb oxygen and lose carbon dioxide and some wastes through diffusion through the skin. They were thought to be primitive relatives for some time, but DNA studies have placed them as spiralians that almost certainly lost internal organs at some point. Some can be cut in pieces and regrow, among other unusual properties.

The most well known of these are probably tapeworms, among many parasites, lots of flatworms are free living as well, living in water or moist environments. The free living types have a variety of lifestyles, some parasites do the bloodsucking thing, some like tapeworms and flukes live inside hosts, feeding off surrounding fluids.

Tropes:
Instant Leech: Just Fall in Water!
Pitiful Worms
Worm in an Apple
(Neither technically have to be annelids, but you probably imagine something earthworm like))

Earthworms and leeches, among others. Includes a number of deep sea worms, attached to the seafloor or nutrient sources such as sunken whale corpses or hydrothermal vents (including the red and white tubeworm pictures), polychaetes are the main group of these. Annelids grow in segments, each segment repeats some organs with a similar structure, with blood vessels, nerves, a digestive system running through, and similar connecting organs running through; they were once united with arthropods for this reason, but molecular analyses don't support this. Most have little hairlike structures called setae off the sides to help them move. The larger than normal ring you see on earthworms is a reproductive section, used to store eggs.

Both ocean and earthworms often burrow into the ground, allowing circulation into seabeds and soils and improving their quality, in addition to the usual feeding roles. Decomposers/debris eaters are the most common, some are predators or herbivores. Leeches are known for bloodsucking, though many are predators also.

While some of this group can regrow from cut up pieces, earthworms aren’t among them. Most whose reproduction is known take care of eggs, providing nutrients and/or protecting them in some way.

Resembling clams, but with a different ancestry and different shell anatomy, with the two halves acting more like a mouth than side structures as in clams. The creatures attach to the sea floor using a long structure, and the shell either hangs in water, or extends out of a burrow for feeding. Larvae tend to resemble small adults, but swimming in the water.

Brachiopods are rarer today than in the past, they were the main clam type animal at points in prehistory, but clams have generally replaced them for various reasons as time went on.

——

A small group of worms, they live on seafloor, stand upright, and use a web looking structure on top to catch food.

——

A couple groups are usually included, with some different internal anatomy, but scientists aren’t sure if they are completely related. They are written as one group for simplicity.

Bryozoans are another group of attached, filter feeding ocean animals, with a variety of forms from stalklike to kelplike to others, the name means “moss animals” and some do look like this. Reproduction releases sperm that are either captured by bryozoans acting as females (in most species individuals can switch between the two) or fertilize free released egg. These hatch into mobile larvae that filter feed and/or rely on yolk until they find a place to settle down.

A group of usually very thin worms, mostly living in sediment, most act as predators, using a proboscis to capture prey. They are among worms that can be broken into pieces which regrow into complete animals. They don’t have a heart or blood vessels, instead using a tube network to move body fluids around, acting to both circulate nutrients and control movement of the animal.

Starfish, sea lilies, sea cucumbers, sea urchins among others. Minerals just underneath the skin surface provide some protection, yu are likely familiar with the hard signs of starfish and/or sea urchins. A water tube system both acts like the equivalent of blood and moves animals that can move by pressurizing and/or emptying different sections. Some echinoderms, including the original ones, are attached filter feeders: sea cucumbers, sea urchins, and starfish can move slowly and eat things on the seafloor. Sea cucumbers generally scavenge/eat debris in sediment, sea urchins mainly eat algae but can grab animals sometimes also. Movement is done by tube feet, which use water water pressure to move the small feet to push the animal or other objects around. Some sea cucumbers crawl instead like worms.

Echinoderm larvae are wormlike, but adults of most types except sea cucumbers are not, instead showing 5 side symmetry as is obvious in starfish (sea cucumbers have both, looking wormlike but also having 5 side symmetry circling around the cylinder).

Sea cumbers and starfish are known for being able to throw out organs, sea cucumbers in defense, starfish can extrude their stomachs to digest things outside the body, useful for getting into clamshells.

Acorn worms and pterobranches. Acorn worms are wormlike, but have an unusual anatomy where some organs are in a large lobe in front, as a result they look something like earthworms with a collar, smaller front, and larger back, but the mouth is actually on the collar. These draw water through their mouths and filter feed it, releasing it from gills. The mostly live burrowed in the seabed.

Pterobranches have a similar body shape, but have lots of tentavles/feathery things sticking out the top to help catch debris. They are closely related to extinct grapholites, described in Prehistoric Life.

A small group of wormlike animals, mostly seabed burrowing filter feeders. They resemble tunicate larvae and early vertebrates, with a nerve cord down the back, plus a cartilage rod. Though they have small fins that help swim, they mostly burrow into seabed with their mouths sticking out, filter feeding.

If you go back before other apes, before elephants, before birds, frogs, sharks, and other fish, your closest relatives are…some coral looking tubular filter feeding things. Yes, really. The resemblance is in lancelet like larvae, with a nerve cord and cartilage rod down the back, but as they become adults they lose these features.

Most tunicates (sea squirts) are attached filter feeders, forming a roughly tubelike shape where water can flow through and be filtered. Some (salps) are free floating, often transparent, and act as predators. A few called larvaceans don’t change much from the larval form, instead adapting the swimming tail to help catch debris.

Vertebrates are a relatively successful group as animals go, members live in just about every environment, including being one of two groups with flyers, and the group has more species than most other types of animals. However, vertebrates get disproportionate attention in media, probably for a couple reasons: 1. Most media is made by and for one of their number and 2. Vertebrates get much larger than other animals. For prehistoric life, add that bones and teeth fossilize well. Combine these, and most animals you interact directly with/notice will be vertebrates, with media reflecting this.

The big defining feature is a backbone, in most species extended into a full internal skeleton of cartilage or bone. Other features include adaptive immunity, where an immune system can change a response and “learn” to respond to different invaders, allowing vaccines in medicine, hemoglobin in blood, and eyes with a single lense and ball, with a retina on the back, a structure only shared with cephalopods with some minor differences. The skeleton is almost certainly the main reason vertebrates can get so big, it provides support that doesn’t exist in soft bodied animals, and can grow continuously unlike arthropod exoskeletons which must be molted. The skeleton also attaches to muscles and includes limbs, allowing walking, flying, and manipulating things. Common to chordates in general (lancelets and tunicates as well) but not as obvious in those others, are tails extending past the anus, other animals with back sections like scorpions have extended abdomens instead.

Vertebrate evolution is a great example of Early-Installment Weirdness, the first creatures were very small lancelet like animals, if they’d stayed like this an alternate intelligent species might call them “fin worms” or "chord worms" or similar. Instead, over tens of millions of years, they got bigger, full skeletons, fins including the classic fishtail, jaws, better senses and bone evolved, plus possibly some soft features that don’t fossilize; at around 400 million years ago some of the hugely diverse fish became the biggest things in the ocean and some others were able to walk onto land. Most of PrehistoricLife describes vertebrates, you can check out more details there. (Or The Other Wiki, or books, etc.)

——
Hagfish and Lampreys, or Cyclostomes

Tropes:
Lamprey Mouth

Most vertebrates have jaws, hagfish and lampreys are the exception. Cyclostome roughly means “circle mouth”, a good description of the lamprey mouth. Both are relatively low metabolism animals, hagfish tend to scavenge, though diets are somewhat flexible and they occasionally grab other meat, lampreys have some bloodsucking parasite species, some species that filter feed as larvae and die shortly after reproducing as adults. Both are eel like, and hagfish even lack backbone that defines vertebrates, though they do have skulls which show membership in the group.

Hagfish are known for making lots and lots of slime, partly as a defense against predators, and their skin makes good leather. Lampreys are sometimes eaten, though have few other uses.

——
Sharks and Rays, or Cartilaginous Fish

Tropes:
Threatening Shark

Also includes things called ratfish and chimeras. These have a cartilage skeleton, not as hard as bone but lighter, and some muscles also attach to skin. On that skin are toothlike scales, helping reduce friction and providing some protection. Most cartilaginous fish fertilize internally instead of releasing eggs and sperm into water, some hold eggs in their body to hatch and a few give live birth. This is a problem with pollution, overfishing, accidental killing, and other such issues, as these populations take time to recover.

Most of these fish are meat eaters or filter feeders. Some standouts besides the famous meat eaters are filter feeding whale sharks, the biggest fish around today. Also quite large are basking sharks, whose weird appearance alive and as a decomposing corpse is probably responsible for lots of sea monster sightings. Sleeper and Greenland sharks are deep sea animals, with slow metabolisms and very long lives.

——
Ray Finned Fish

Tropes:
These Tropes Are Fishy

About half of vertebrate species are in this group, if it wasn’t mentioned in another group of fish in this useful notes, it goes here. Ray finned fish have a few traits: a bony skeleton with relatively thin fin bones, and a swim bladder, a bag of gas the fish can inflate or deflate to control buoyancy. Fish are in every water environment on the planet, eating anything it is possible to eat, ranging in size from several ton sunfish to dwarf minnows that are a few millimeters long.

Food lovers and makers may know about oily vs. non oily fish: the oil in the fish helps the fish float and stores energy, useful for fish that must swim long distances. Less intense swimmers don’t need this oil and become whitefish. (There are some weird ones like fatty catfish, of course.) Caviar is the eggs of certain species, most bony fish reproduce by putting lots of eggs and sperm into the water to fertilize, though in a large group there are always exceptions.

Bony fish have a variety of shapes, most with the classic fish form, but rounded, flat, snakelike, and other shapes exist for some species with unusual lifestyles. Some, such as sturgeons, have partial cartilage skeletons. Most modern fish are in the “Teleosti” group, with thinner scales and more agile swimming.

——
Lobe Finned Fish

There are only a few species of these, lungfish and ceolacanths. Coelacanths are a famous animal thought extinct, but rediscovered in the 20th century, extinct relatives had been found as fossils from only long before. Lungfish live in some places in the southern hemisphere.

The main characteristic of lobe finned fish is thick fins, as the name suggests. They also have lungs, as the name lungfish suggest, though in Coelecanths this has evolved into an oil and air filled organ that helps flotation. Lungs most likely evolved first and later some became swim bladders in ray finned fish. There are a few other random differences with ray finned fish as well. Lungfish tend to live in rivers and can go dormant when they dry out, while coelecanths drift and swim through water and grab fish.\\
——
Amphibians

Tropes:
Reptile and Amphibian Tropes

It isn’t known if modern amphibians have a common ancestor different than land vertebrates different theories of ancestry go either way, but they make a useful group for this useful notes.

Frogs, salamanders/newts, and small snakelike creatures called caecilians are the modern amphibians. They all must lay eggs in water, and have relatively thin skin that easily absorbs or loses water and other materials in it. This makes amphibians particularly sensitive to pollution, pictures of mutated frogs with extra legs or arms were a common news story in the 1990’s about pollution. The thin skin also means amphibians can absorb oxygen through it in addition to through lungs, and prevents them from living in salt water, so no amphibian versions of whales or plesiosaurs or penguins or such. Frog legs have specially evolved for jumping, which they can do for surprisingly long distances.

Frogs and salamanders tend to eat whatever small animals they can catch, sometimes using a tongue to catch food. Caecilians are highly specialized for burrowing, and seem to eat whatever they find underground.

——
The Great Reptile Mix-Up

Land vertebrates that don’t need to lay eggs in water, or at least had an ancestor with this quality, are called amniotes. Some have returned to water, but these are either live birthing in some way, possibly keeping eggs inside the body, or return to land for reproduction. Includes mammals, birds, and rept…actually, we run into some issues here.

No group has been more messed with by phylogenetic classification than “reptiles”. When the original linnean system was created, land vertebrates naturally divided into furry warm blooded animals with milk (mammals), feathered warm blooded animals with wings (birds), thin skinned cold blooded animals laying eggs in water (amphibians), and cold blooded animals with drying resistant eggs often with scales, shells, or osteoderms (reptiles). This system still makes sense for vets and zookeepers and other people working with modern animals. However, it is not phylogenetic, birds and crocodiles have a common ancestry that lizards don’t, turtles may be associated with this group also.

Prehistoric animals mess this up even further. Because of various bone features, and later because cold blooded but laying eggs out of water was a stage modern amniotes clearly went through, but the animals didn’t have mammal or bird features, lots of prehistoric creatures were called “reptiles”, such as dimetrodon, therapsids (“mammal like reptiles”), dinosaurs, pterosaurs, large sea creatures, and many others. The whateversaurus tropes exists for this reason, -saurus mean lizard, and is a common suffix used for these animals. However, the phylogenetic problem still exists, lots of creatures will be more closely related to birds than lizards, probably also turtles, and “mammal like reptiles’ are closer to mammals than anything else, and equally related to birds and modern reptile groups. Classifying them by traits doesn’t work well either based on what we know, cold blooded lizards, live birthing possibly warm blooded ichthyosaurs, high metabolism fur like thing covered pterosaurs, feathered dinosaurs, turtles, and somewhat more active crocodile ancestors would not get classified into one group while leaving out birds and mammals, if someone were doing it from scratch today.

So, though Prehistoric Life uses the term reptile in this older sense, the official classification term is now used for diapsids, and this useful notes will describe group within it separately.

——
Mammals and other synapsids

Tropes:
Mammal Tropes

Amniotes split into synapsids and sauropsids early on, the groups are based on molecular evidence, and fossils where they have different numbers of holes in a skull for lightening and provided room for muscles. Synapsids have an up and down history, the first land vertebrates to get really big, than replace by the other ‘psids for a couple hundred million years over a couple mass extinctions, than with a third mass extinction became the big animals again. And at least one of them is reading this right now.\\
Mammals are probably the most tropperiffic group of animals, even if you don’t cheat and leave out all the society, culture, technology, romance, etc. tropes. They make out our livestock, pets, several pests, and the big wild animals we stay away from, or smaller wild animals we watch do things. Mammals are defined by milk, fur (at least in ancestors), and some skull structures such as a different ear and jaw shape. Most have live births, in marsupials like kangaroos to a small infant that crawls into a pouch and grows further, in placentals into a more or less partly grown baby that can survive with some care outside. Milk gives us dairy products and breastfeeding, it must supply lots of nutrients and energy, and will therefore be fatty,...have lots of nutrients, including protein, and be a good consistency for babies to drink. Milk production and live birth means mammals provide good parental care by animal standard, definitely no ‘release reproductive cells into water and hope for the best” as lots of other groups do, or “die after reproducing”. Fur is useful as insulation, it also protects from sun and provides color for displays.

By species, rodents and bats are the biggest groups, including a number of pests, or just annoyances as with bats in a house. Mammals include eaters of most types of foods, you probably know roughly which groups eat what. Insect eating is more common in smaller animals, larger ones tends towards larger prey or plants or a mix. Grass eaters are common livestock, a very useful quality as grass is one of the few foods people don’t handle well, allowing the use of an extra resource. The largest animals around today are a group of baleen whales, as described in the “size” section of this useful notes.

——
Lizards, Snakes, Tuatara, or Lepidosaurs
Tropes:
Hollywood Chameleons
Lovable Lizard
Malicious Monitor Lizard
Snake and Serpent Tropes

Probably the animal group that most resembles the earliest amniotes, though still evolved over time. Lepido- means scaled, and scales are a key feature of these animals. Lizards are the familiar four legged, long tailed things you know about, snakes are the scary animals with no legs you don’t want biting or constricting you, tuatara are a group found only in New Zealand that look roughly like lizards, though with a third eye on their head and some skeleton differences. The scales give some protection, and help with other skin functions. Snakes have evolved from within lizards, lizards and tuatara have a common ancestor. The scales are overlapping, and have to be shed, sometimes as a big piece shaped like the whole animal.

Lizards and tuatara tend to move in a sprawling posture, with legs splayed out to the side. This.takes less energy when standing up or laying down, but limits their movement as sprawling creates more side to side movement, plus constricts lizard lungs and interferes with breathing Some can run on two legs for a time. Snakes obviously crawl along the ground in a number of ways, either moving some stomach sections forward while pushing with others, or using a wide to side motion that pushes using the sideways sections.

As usual for this sort of large group, lizards fill a variety of ecological roles, eating a variety of diets. Most lay eggs to reproduce, but a few give live birth, and some females can reproduce without mating. Most have slow metabolism, a few have something in between birds and mammals and most lizards. Snakes are almost all carnivores, with rodents as the big source of prey. It is known that some are venomous, some people think all snakes have at least a little venom (though obviously not all dangerous to people), and are part of a large group with some venomous lizards. Constrictor snakes are the name for the ones that squeeze prey, mostly big muscular types for obvious reasons, the exact killing mechanism could be causing heart failure, or cutting off breath.

——
Turtles

Tropes
Index in a Half Shell
Turtle ancestry is tricky, they for a long time seemingly showed up fully formed in the fossil record, and don’t have several features scientists normally use to classify land vertebrates, best guess was they were primitive/early diverging branch on the amniote . Today, molecular studies mostly point to the bird + crocodile group as close relatives, but a few point to lizards being closer.

Whatever their ancestry, turtles are obviously defined by their shell, created of bone or a thick leathery material. This shell obviously provides protection, but also limits movement, limits space for organs, including space to breathe, and requires nutrients to grow. In a more intimate issue, turtle penises must be really long to reach between the mating couple. Most of these problems are less extreme in water, fins can provide propulsion for example, and turtle lineages have evolved from land to water and back many times. Turtles also lack teeth, having beaks instead.

Most turtles eat whatever is available, with land turtles eating more plants and sea turtles eating more meat, as might be expected from how easy walking is vs. swimming. Among the more specialized turtles are leatherbacks, eating jellyfish. Metabolism is generally slow, apart from some sea turtles that maintain a high temperature partly through physical activity. The combination of slow metabolism and protection is probably what allows turtles to live such a long time, close to 200 year lifespans have been measured in a few.

——
Crocodilians

Tropes:
Never Smile at a Crocodile
Sewer Gator
Green Gators

The crocodile group, pseudosuchians, has a lot of interesting prehistoric members which are worth reading about, but the only remaining members are the alligators and crocodiles (and gharials and caimans) we all have heard of today. All of these mainly live in rivers, eating fish, land animals they can ambush, or a mix of the three. Crocodiles and alligators are differentiated by random bits of anatomy, caimans also have some of these and are usually smaller, and gharials have a very thin snout that trades ability to catch land prey for fast closing to catch fish.

While fish are a big part of lots of crocodile’s diets, catching land animals is what they are famous for, and they have a lot of anatomy to do it. Crocodiles are famous for a very strong bite, and for their ridiculously weak jaw opening muscles, this bite is used to hold animals in water to drown them, followed by the crocodile eating, sometimes rolling to separate bits of meat to swallow. They are adapted to swim low in water, eyes and nostrils on top of the head. Greyish slight greenish (exaggerated by pictures) color adds camouflage, resembling logs or natural river color. The plate like things in their skin are called osteoderms, which are bony and include structures for good heat transfer and other functions in addition to protection.

Crocodiles have a lot of anatomy suggesting a more active ancestry: 4 chambered hearts, found in birds and mammals, plus the ability to stand and walk upright and some can even gallop in a way, though they can't do this for long without tiring. See prehistoric life for more details.

——
Birds and other Avemetatarsalia

Tropes:
Bird Tropes
Dinosaur Tropes
Terror-dactyl
Dinosaurs

Another highly troperiffic group, including the Pterosaur, a bunch of large prehistoric land animals you played with as a kid, and modern birds are no slouch either, such as the majestic Eagle, the Circling Vultures waiting for something to die, Chicken which all meat tastes like, Clever Crows or Creepy Crows as fits your preference, and many others.

Birds today are known for being warm blooded, having beaks, and feathers most obviously. Other adaptations include air filled bones, very efficient lungs, and large breast muscles used for pulling the wings. All in some way contribute to flying, beaks and air filled bones lighten the animal, powerful chest muscles work the wings, high metabolism and efficient lungs give the bird energy to fly, a high power to weight for animals, feathers form the airfoil of the wings and keep birds warm to help maintain a high temperature. The lungs have a very different structure than what you are familiar with, instead of lungs that draw air in and out and absorb it, birds have a rigid organ than absorbs oxygen, and a couple systems of air sacs that expand and contract, all attached by tubes. These sac’s movement is timed so that fresh air is always flowing through the oxygen absorber, while stale air bypasses this organ and is exhaled directly. This helps birds handle much lower oxygen levels than almost all animals can handle, they can fly at altitudes where humans need extra air. Fligthless birds maintain these adaptations. The evolution of many of these features is a subject of interest for paleontologists, a lot show up in bird’s flying relatives Pterosaurs as well.

Different birds obviously eat a variety of foods and fill a variety of roles in their environment. Studies of how beak shapes relate to bird’s diets and lifestyles are a common example of evolution at work. Bird reproduction is by eggs, which most birds protect to keep a proper temperature. Parental care is common among birds as well.