Group composition impacts reproductive output and population viability in captive white rhinoceros

Captive breeding programmes can play an important role in the conservation of species threatened with extinction in the wild. White rhinoceros, Ceratotherium simum, have suffered drastic declines due to illegal poaching across their range, and captive populations have the potential to safeguard this species from extinction. However, white rhinoceros birth rates in captivity have been extremely low, and there is substantial variation in breeding success between different institutions. A better understanding of the factors limiting their reproduction in captivity could improve their breeding management and conservation potential. This study used studbook data (n = 467) and comparisons with wild populations in Kenya (n = 236) to assess the breeding performance and viability of the European captive southern white rhinoceros, Ceratotherium simum simum, population. Our results show that the European population is declining 2% annually under current demographic parameters. On average, just 10% of females calved annually, in comparison to almost 40% across wild populations. To become self-sustaining, this must increase to a minimum of 17%, though reaching this target may lead to additional management challenges. Further analyses using studbook (n = 134 individuals) and multi-institutional questionnaire data (n = 20 institutions) suggest that modifying the social structure of captive groups could improve female breeding success. Institutions housing larger groups had proportionally greater breeding success, and females were more likely to copulate if housed with another breeding female, and more receptive to bulls if housed in a group with a lower mean age. These findings highlight the importance of considering social conditions in captive breeding programmes, particularly for species with complex social systems and low reproductive rates.