Low birth rates and reproductive skew limit the viability of Europe's captive eastern black rhinoceros, Diceros bicornis michaeli

Ex situ populations play a critical role for the conservation of endangered species, especially where in situ populations face imminent threats. For such populations to act as vital reserves, they must be viable and sustainable. Eastern black rhinoceros (Diceros bicornis michaeli) epitomise the delicate nature of conservation, as a steady increase in the in situ population over the last two decades is threatened to reverse due to intense poaching pressures on rhinoceros across sub-Saharan Africa. This study utilized population viability analysis to evaluate the demographic and genetic viability of the European captive population of eastern black rhinoceros, and compared demographic parameters to in situ reference populations. Although self-sustaining, the ex situ population performs poorly relative to in situ counterparts, growing at a rate of only 1-2 % per annum compared to 6-8 % for managed wild populations. Captive females start reproducing later, have longer inter-calving intervals, and a lower proportion breed each year. Furthermore, over 40 % of reproductive-age animals have yet to reproduce, with additional implications for the maintenance of genetic diversity. Pedigree analysis highlights the unequal contribution of wild-caught founders to the current population; 69 % of which have no living descendants, and more than a third of the current population are related to five founders. This results in a current genome equivalent of just 13.39 equally reproducing founders. Although reproductive skew is not unusual in wild populations, it severely undermines efforts to maintain genetic and phenotypic diversity in captive breeding programmes. We suggest that understanding and alleviating the causes of reproductive skew must be an important consideration for small population management to maintain the genetic and demographic viability of ex situ populations.