Inbreeding and outbreeding in African rhinoceros species

Effective breeding strategies in ex situ conservation require an optimal balance between inbreeding and outbreeding, as both can lead to a decrease in population fitness. Thus the optimization of breeding strategies to maintain genetic diversity entails a profound knowledge of the actual situation (including conservation units). This study examines the consequences of inbreeding and outbreeding in captive populations of two threatened species, the white and the black rhinoceros (Ceratotherium simum and Diceros bicornis), based on data from recent studbooks (2004). We also assessed the conservation units of the black rhinoceros, whose classification into subspecies remains a matter of discussion. Theory predicts that juvenile mortality increases with increasing degree of inbreeding. We calculated inbreeding coefficients and examined possible correlations with juvenile mortality rates. To assess the conservation units of the black rhinoceros, we compared juvenile mortality rates of outbred animals with those of non-outbred animals and additionally performed a geographical distance analysis. With both approaches we aimed to draw breeding borderlines between putative conservation units to preserve the genetic diversity. Our assessment of the current inbreeding situation based on records in international studbooks was severely limited by small sample sizes in both the white and the black rhinoceros. For the same reason we could not evaluate the conservation units in the black rhinoceros. Nonetheless, we conclude that inbreeding and outbreeding must be followed closely in both species, as their consequences can be severe.