Sex Ratios in Extant Ungulates: Products of Contemporary Predation or Past Life Histories?

Abstract

Variation in mammalian adult sex ratios (ASR) is striking both within and among species. Darwin (1871) originally suggested that causes of variation included competition among males for females and predation. He also recognized that intensity of competition might be greater in species adorned with secondary sexual traits. Assuming that sexual dimorphism is a reasonable indicator of intrasexual competition, we predicted that ASR would become increasingly skewed among dimorphic species and this pattern would be exacerbated by the intensity of predation. With effects of common ancestry removed by computing phylogenetically independent contrasts, we failed to detect strong relationships between ASR and either sexual dimorphism or body mass of males or females. Presence of predators also had no consistent effect on these broader patterns, and even within Cervidae and Bovidae, there was not a solid relationship between dimorphism and ASR. The only generalized pattern that emerged was that males were killed disproportional to their abundance (74% of 31 species). However, differences between live and killed sex ratios in a population were not correlated with female or male body mass or sexual dimorphism. Our results suggest that relationships between sexual dimorphism and mortality are not as straightforward as presumed. Nevertheless, at a proximate level, predation directly affects patterns of sex ratio variation among adult ungulates, but differences in survival of sexes may arise as a direct consequence of greater age-specific mortality among males, for which the ultimate cause is likely to be selection operating differently on males and females. A challenge for the future lies not so much in the separation of proximate from ultimate factors but in evaluating what, if any, life-history traits predispose sexes to differential mortality and the extent to which predation may shape these characteristics.