Are there physiological constraints on maternal ability to adjust sex ratios in mammals?

Sex ratios at birth or hatching (henceforth termed sex ratios) vary (Clutton-Brock & Iason, 1986" Cameron, 2004) and this variation in the partitioning of resources between the sexes is termed sex allocation. Sex allocation is a key life history variable and evolutionary theory predicts variation in the sex ratio when sex-specific fitness returns vary between individual parents (Fisher, 1930) in relation to current local conditions or ability to invest (Hamilton, 1967" Trivers & Willard, 1973" Clark, 1978" Silk, 1984). Essentially, parents should produce the sex that will leave them the most grand-offspring. For example where one sex has more variable reproductive success and where that reproductive success is influenced by parental investment, parents would be advantaged by investing more in the reproductively variable sex when they have extra resources to invest, but not when they have limited resources (Trivers & Willard, 1973). Therefore, a range of environmental conditions are hypothesized to influence sex ratios. Other hypothesized modifiers of the lifetime reproductive benefits to parents include, local density (Clark, 1978" Silk, 1984), overlapping generations (Schwanz, Bragg & Charnov, 2006), need for helpers (African wild dogs Lycaon pictus: Creel & Monfort, 1998" red-cockaded woodpeckers Picoides borealis: Gowaty & Lennartz, 1985" Seychelles warbler Acroephalus sechellensis: Komdeur et al., 1997), and mate attractiveness (Burley, 1981" Pen & Weissing, 2000) all of which have been linked to sex ratio biases. Despite the logical simplicity of underlying principles of sex allocation, empirical studies of sex ratios typically produce inconsistent results, especially in mammals (West, 2009). In mammals, some of the variation can be explained by methodological inconsistencies between studies (Cameron, 2004" Sheldon & West, 2004), nonetheless, the effect size of sex ratio skews remain unpredictable and are often less than expected, suggesting constraints on the ability of parents to adjust sex ratios (West & Sheldon, 2002" Edwards & Cameron, 2014).