Birth sex in an expanding black rhinoceros (Diceros bicornis minor) population

Despite a now-extensive literature on sex allocation, facultative sex allocation in mammals remains controversial. We studied the complete birth-sex record of a black rhinoceros (Diceros bicornis minor) population from its reintroduction in 1986 through 2008. Neither explicit models of birth sex as Bernoulli trials nor mixed-model logistic regression yielded evidence for an influence on birth sex of birth sequence, maternal identity, year of conception, rainfall, population size, maternal age, or adult sex ratio despite a small female-biased population birth sex ratio that nevertheless appears to be unusual for black rhinoceros. One interpretation of our results is that our study population did not facultatively control birth sex, either because black rhinoceros cannot or because environmental and demographic circumstances did not elicit conditions that favor pronounced adaptive control of birth sex in this population. Alternatively, variation in sex allocation could have occurred in a manner not captured by our modeling or at a scale we could not probe. If the observed female bias is not purely stochastic, it may result from the perturbed age structure of the introductions, which was biased toward subadults.