Influence of environmental conditions on sex allocation in the black rhinoceros population of Mkhuze Game Reserve,South Africa

According to the Trivers–Willard and local resource competition (LRC) hypotheses, for species where reproductive success is more variable in one sex, natural selection may lead to a bias in sex allocation of a female’s offspring according to her body condition. The extrinsic modification hypothesis (EMH) suggests that offspring sex can also be influenced by environmental conditions experienced by mothers. We investigated the influence of rainfall, El Niño‐Southern Oscillation (ENSO), population size and burning, in the year before conception and during pregnancy, on sex allocation in the black rhinoceros population of Mkhuze Game Reserve, South Africa, during 1970–2007. Females were more likely to have a male calf as rainfall during pregnancy increased, supporting the Trivers–Willard hypothesis. Also, the probability of having a male calf increased with population size, supporting the LRC hypothesis. Calf sex allocation was not influenced by ENSO. In conclusion, local environmental conditions may influence sex allocation in black rhinoceros, thereby supporting the EMH. Burning and population size may influence sex allocation in black rhinoceros, and yet can be manipulated by managers. Thus, this knowledge can be applied to improve population structure assessments and management regimes, especially in enclosed reserves, which is essential to maintain endangered species’ productivity.