Birth sex ratio in captive mammals: Patterns, biases, and the implicationsfor management and conservation

Sex allocation theory predicts that a female should produce the offspring o f the sex that most increases her own fitness. For polygynous species, this means that females in superior condition should bias offspring production toward the sex with greater variation in lifetime reproductive success, whi ch is typically males. Captive mammal populations are generally kept in goo d nutritional condition with low levels of stress, and thus populations of polygynous species might be expected to have birth sex ratios biased toward males. Sex allocation theory also predicts that when competition reduces r eproductive success of the mother, she should bias offspring toward whichev er sex disperses. These predicted biases would have a large impact on capti ve breeding programs because unbalanced sex ratios may compromise use of li mited space in toes. We examined 66 species of mammals from three taxonomic orders (primates, ungulates, and carnivores) maintained in North American toes for evidence of birth sex ratio bias. Contrary to our expectations, we found no evidence of bias toward male births in polygynous populations. We did find evidence that birth sex ratios of primates are male biased and th at, within primates, offspring sex was biased toward the naturally dispersi ng sex. We also found that most species experienced long contiguous periods of at least 7 years with either male- or female-biased sex ratios, owing i n part to patterns of dispersal (for primates) and/or to stochastic causes. Population managers must be ready to compensate for significant biases in birth sex ratio based on dispersal and stochasticity. (C) 2000 Wiley-Liss, Inc.