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 phylogenetic ally 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.Key words: predation, carnivores, ungulates, sex ratios, sexual selection, sexual dimorphism, large mammals Despite sex ratios at birth that approach parity, abundance of males and females in wild mammals at adulthood displays striking variation. In the dasyurid Antechinus swainsonii, the skew in sex ratio is at times so great that few or no adult males remain alive and the entire popUlation is made up of females, a pattern best explained by abrupt post-mating mortality of males ::51 year of age and before the synchronized birthing season, and female survival for ::53 years (Cockburn et al., 1985). Among por- (Mills, 1990;Schaller, 1972). Perhaps the most profound gender-specific skew due to predation is in African buffalo where 7.3 males were preyed on for every female or in reedbuck (Redunca redunca) where only females were killed (Schaller, 1972). Why sexes differ in abundance at sites not affected by humans is a continuing source of evolutionary, ecological, and economic interest. Darwin (1871:232) commented that "the practice of polygamy leads to the same results as would follow from an actual inequality in the num...

Poor horse traders: large mammals trade survival for reproduction during the process of feralization

“…Male-biased sex ratios have also been found in feral populations of other feral ungulates, cattle (Berteaux & Micol 1992) and sheep (Réale 1996), supporting the suggestion that this general principle may be true for all these species of ungulates. The between-sex differences reported in populations of wild large herbivores are quite different: males suffer relatively more than females from adverse conditions such as high density or harsh climate ( Toïgo & Gaillard 2003), leading to female-biased sex ratios among adults (Berger & Gompper 1999). …”

Sex-biased survival predicts adult sex ratio variation in wild birds

“…Sex ratios may already be biased at conception ( primary SR), or at birth or hatching (secondary SR), or male and female juveniles may die at different rates [17,[21][22][23]. Second, differential adult survival may bias ASR, because one sex may be predated more often than the other [24,25], or be more susceptible to parasites and pathogens [26]. Consistent with the latter argument, adult survival is sex-biased in a number of fishes, birds and mammals [27,28].…”

Group size, sex and age composition of chital (Axis axis) and sambar (Rusa unicolor) in a deciduous habitat of Western Ghats

“…De and Spillet (1966) suggested that more or less 1:1 sex ratio may usually be found in an area free from selective shooting or predation. The skewed female ratio is probably due to the solitary male dispersing from mixed group experience greater mortality and is more vulnerable to predation by large carnivores (Karanth and Sunquist, 1995;Ramesh et al, 2009;Ramesh 2010), and in most ungulate populations the intensity of intra-male competition results in greater male mortality (Berger and Gompper, 1999). Male ungulates are selected for rapid growth rates and driven to risky behaviour by high testosterone levels (e.g., wider movements into less well known areas, reduction of alertness during pursuit of females, fighting between rivals, etc.)…”