Investigating population performance and factors that influence reproductive success in the eastern black rhinoceros (Diceros bicornis michaeli)

摘要

With fewer than 5000 black rhinoceros remaining, ex situ populations play a vital role in the conservation of this species. To reinforce in-situ conservation efforts, captive populations must be self-sustaining, both demographically and genetically, to maximise future viability. The aim of this thesis was to determine the sustainability of the European captive population of eastern black rhinoceros, and investigate factors that may influence population performance in this species. Population viability analysis of demographic data from in situ managed black rhinoceros populations were used to establish the variability in population growth rates across reserves. Secondly, these data were used to calculate fecundity, mortality and population structure performance indicators, as a reference for how this species can perform under natural conditions. The same analyses were then applied to the European ex situ population, to determine whether the population is demographically and genetically self-sustaining, and identify areas for potential improvement. The European captive population of eastern black rhinoceros, although currently self-sustaining, is performing sub-optimally both with respect to their in situ counterparts, and to a proposed target of 5% growth per annum. Population performance is primarily limited by sub-optimal reproduction, both in terms of individuals producing fewer calves per annum, and due to a high degree of reproductive skew across the population, leading to a large proportion of individuals failing to produce offspring. A multi-institutional study was conducted on 90% of the European population, to investigate intrinsic differences in faecal reproductive hormone metabolites between breeding and non-breeding individuals. In females, irregular oestrous cyclicity was observed, with longer than average cycles observed more frequently in females that had never bred, and periods of acyclicity more common in females that had not bred for at least seven years. Non-proven females also had higher body condition scores, and were less likely to exhibit regular signs of oestrus. In males, non-breeding males had reduced faecal testosterone compared to males that had previously sired offspring. Extrinsic factors were also investigated, to determine whether differences in reproductive success could be attributed to aspects of the social or physical environment. However, no consistent relationships were observed between breeding and non-breeding males or females. Furthermore, adrenal activity and testosterone concentration were not correlated with environmental factors, indicating that extrinsic factors alone may not explain differences in reproductive success. However, within females, differences in hormone concentration were associated with irregular oestrous cyclicity. Long cycles were associated with increased glucocorticoid metabolite concentration, and oestradiol metabolites were lower during periods of acyclicity. The duration of the preceding luteal phase varied between cycle types, indicating that the occurrence of regular and irregular oestrous cyclicity may be influenced by hormone exposure during the preceding oestrous cycle. In summary, a number of intrinsic differences in reproductive hormones in both males and females have been identified, which may be related to differential reproductive success. A better understanding of the causes of these differences would be beneficial to maximise growth rates and overall population performance of this ex situ population.