Abstract

The Trivers and Willard hypothesis (1973) states that in a species where reproductive success is more variable in one sex, natural selection could lead to females having the ability to bias the sex allocation of her offspring according to her body condition. The extrinsic modification hypothesis (EMH) recently suggested that offspring sex can also be influenced by environmental conditions experienced by mothers. I investigated the influence of rainfall, population size, vegetation type and burning history, in the year prior to conception and during pregnancy, on sex allocation in the black rhinoceros ( Diceros bicornis minor ) population of Mkhuze Game Reserve, South Africa. I found that increases in rainfall and population size during preconception and pregnancy decrease the probability of having a female calf. The results also show that the frequency of burn and the elapsed time since the last burn of the area used prior to conception have a positive influence on the probability of having a female calf. Vegetation type in the area used by mother rhinos during preconception and pregnancy did not influence sex allocation. In conclusion, I found that environmental conditions may influence sex allocation in the black rhinoceros, thereby supporting the EMH. This knowledge can be applied to improve population structure assessments and management, especially in enclosed reserves, which is essential to maintain the productivity of endangered species.