Sex ratio adjustment and maternal allocation in pronghorn (Antilocapra americana): A comprehensive test of the Trivers-Willard Hypothesis in nature

The Trivers and Willard Hypothesis (TWH) (1973) is one of the most famous explanations for why selection should support uneven birth sex ratios and differential resource allocation towards sons. This publication has also helped to popularize "condition" as an indication of individual fitness. In the 39 years since publication, the TWH has remained contentious.;The TWH proposes that mothers are able to transfer their condition to offspring, and that offspring condition in early development persists into adulthood. The TWH predicts that, in a polygynous mating system, where male reproductive success is more variable than female reproductive success, females in good condition will maximize fitness by investing in sons, and females in poor condition will maximize fitness by investing in daughters. To complete a comprehensive test of the TWH, I used an isolated population of pronghorn on the National Bison Range, MT. Many empirical studies have tested different aspects of the hypothesis, but no study has tested all assumptions and predictions in a single species, while accounting for random effects that influence offspring growth and survival, and simultaneously testing the overall adaptive value of male performance in adulthood. I tested the TWH assumptions and predictions using measurements of maternal condition and social rank. I also examined the effect of maternal condition on current and future maternal reproductive success, and I investigated a proximate hormonal mechanism for sex ratio adjustment. I found that females in superior condition did not produce more male offspring than females in poor condition, nor was there an effect of maternal condition on fawn survival to weaning. There was a dominance-related bias in birth sex ratios, but I found no effect of maternal social rank on fawn survival or male reproductive success. I found that current reproductive success lowered female condition, but did not cause reduced female reproductive success in subsequent years. I also found that female social rank correlated with reproductive and adrenal hormones during the summer and breeding season. I conclude that pronghorn do not support TWH, and the increased production of sons in dominant females is likely a non-adaptive physiological phenomenon.