Reproductive Performance of Black and White Rhinoceroses in Relation to Rainfall, Greenness and Density

Costs of reproduction and reproductive success depend on the timing of reproduction with favourable environmental conditions. For species like the black and white rhinoceros, which live in seasonal environments, changing climate might desynchronize reproduction and periods of favourable conditions. Additionally, fluctuations in environmental conditions can affect calf sex ratios. We thus investigated the reproductive life histories of black and white rhinoceroses at Lapalala Wilderness Nature Reserve, and their response to variations in rainfall, vegetation greenness and population density. We used historical rhino-monitoring data to assess age at primiparity, inter-calving intervals and offspring sex ratios. White rhinos had their first calf earlier than black rhinos (9.4 versus 11.7 years, respectively) but the inter-calving intervals of white rhino were longer than those of black rhino, averaging 38 and 33 months, respectively. Conceptions for both species peaked during the wet season and were associated with periods of high forage availability. Effects of greenness on age at primiparity differed between species, being positive for white rhinos and negative for black rhinos. We suggest that increases in greenness during conception periods might indicate increased grass production which the black rhinos do not have access to and might not consume. The birth sex ratio was male-biased for both species; however, there was evidence for an environmental effect on calf sex allocation. In particular, black rhinos favoured male calves during periods of decreased vegetation greenness. Although reproduction for both species appears to be related to resources, observed differences are possibly influenced at least in part by differences in the dynamics of plant growth forms that each consumes and the responses of each rhino to those dynamics.