Successful sons or advantaged daughters? The Trivers-Willard model and sex-biased maternal investment in ungulates.

The Trivers-Willard model predicts that in polygynous species, given certain assumptions, mothers with extra resources should bias investment towards sons by adjusting sex ratio at birth and/or by differential provisioning of sons and daughters. Evidence for facultative adjustment of the birth sex ratio from recent empirical studies on ungulates is equivocal at best. However, studies on maternal input and reproductive costs suggest that ungulate mothers do discriminate against daughters during maternal investment in the most polygynous, dimorphic species. Better data on the relationship between maternal input and fitness returns for producing sons or daughters are needed to interpret patterns of sex-biased maternal investment.