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Hybridization following population collapse in a critically endangered antelope - PubMed

  • ️Fri Jan 01 2016

Hybridization following population collapse in a critically endangered antelope

Pedro Vaz Pinto et al. Sci Rep. 2016.

Abstract

Population declines may promote interspecific hybridization due to the shortage of conspecific mates (Hubb's 'desperation' hypothesis), thus greatly increasing the risk of species extinction. Yet, confirming this process in the wild has proved elusive. Here we combine camera-trapping and molecular surveys over seven years to document demographic processes associated with introgressive hybridization between the critically endangered giant sable antelope (Hippotragus niger variani), and the naturally sympatric roan antelope (H. equinus). Hybrids with intermediate phenotypes, including backcrosses with roan, were confirmed in one of the two remnant giant sable populations. Hybridization followed population depletion of both species due to severe wartime poaching. In the absence of mature sable males, a mixed herd of sable females and hybrids formed and grew progressively over time. To prevent further hybridization and recover this small population, all sable females were confined to a large enclosure, to which sables from the other remnant population were translocated. Given the large scale declines in many animal populations, hybridization and introgression associated with the scarcity of conspecific mates may be an increasing cause of biodiversity conservation concern. In these circumstances, the early detection of hybrids should be a priority in the conservation management of small populations.

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Figures

Figure 1
Figure 1. Distribution of sable (Hippotragus niger) and roan (H. equinus) antelopes in Africa and location of the study area.

The giant sable antelope (H. niger variani) has a very small range, which is far apart from the species core distribution in eastern Africa. Giant sables only occur in Angola (inset), where they are restricted to the Cangandala National Park and the Luando Integral Nature Reserve. Geographic ranges are adapted from IUCN (2008a. Hippotragus niger. The IUCN Red List of Threatened Species. Version 2014.3) and IUCN (2008b. Hippotragus equinus. The IUCN Red List of Threatened Species. Version 2014.3). The Figure was produced by Luís Verissimo.

Figure 2
Figure 2. Temporal variation in estimated population size and births of giant sable antelopes and sable x roan hybrids in Cangandala National Park, Angola.

The number of hybrids recorded grew progressively over time (red line), while the number of sable observed increased up to 2006 and declined thereafter (blue line). We recorded births of male (dark blue bars) and female sables (light blue bars) between 2002 and 2005, and then again in 2010–2011, when all sables in Cangandala were taken to a large outdoor enclosure, to where sables captured in Luando National Park were translocated. Male (dark orange bars) and female (light orange bars) putative hybrids were estimated to be born each year until 2010, when access of roan to sable females was prevented. Arrow indicates the first introduction of a giant sable bull translocated from the Luando Integral Nature Reserve.

Figure 3
Figure 3. Schematic representation of diagnostic field characteristics of giant sable antelopes, roan antelopes, and their F1 hybrids and backcrosses observed in Cangandala National park, Angola.

Drawings are based on photographs taken during camera trapping and animal capture sessions carried out in 2009–2011. The hybrids and backcrosses represented were confirmed through genetic analysis. The sable x roan hybrids (robles) are phenotypically intermediate between the parental species, while the hybrids x roan backcrosses have features more closely resembling roan. The female backcross shows the abnormal horns observed in a single individual.

Figure 4
Figure 4. Genetic evidence for sable x roan hybridization in Cangandala National Park, Angola.

(a) First and second components of a principal components analysis (PCA) of 51 microsatellite genotypes from 90 Hippotragus spp. samples; ovals are 95% inertia ellipses; the inset shows the distribution of eigenvalues for all principal components. (b) Individual assignment to genetic clusters (K = 2) inferred by Bayesian analyses (STRUCTURE); arrows indicate first generation backcrosses to roan confirmed by NewHybrids analysis. Phenotypically intermediate individuals are located between roan and sable in the PCA biplot, and they are partially assigned to both species in the STRUCTURE analysis.

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