Meeting Abstract
Female condition significantly influences timing of reproduction, age at first breeding and offspring survival, and is proposed as a driver of offspring sex ratio. The Triver’s-Willard hypothesis (TWH) predicts high-quality mothers should invest more into sons because males have higher variance in individual fitness for species in which reproductive success is more variable in one sex. Thus, females should adjust offspring sex ratio in response to factors that could modify both their own lifetime reproductive success and that of their progeny. Though well studied in vertebrates, it is poorly understood if or how the TWH applies to polytocous species, those that produce several offspring per litter, because the trade-offs between size and number of offspring must also be taken into consideration. Williams’ hypothesis (WH) accounts for these possible trade-offs on sex ratio variation. The extrinsic modification hypothesis (EMH) predicts modulation of offspring sex ratio in response to environmental conditions. Using wild pigs as a model, we tested whether (1) maternal attributes modulated litter sex ratio (TWH), (2) maternal mass influenced production cost, based on litter size and sex-ratio (WM), and (3) environmental conditions influenced litter size (EMH), in a polytocous species. Older females, generally larger and with more parental experience, had male-biased litters, providing support for the TWH. Increased maternal size and condition (an index of resources) both positively influenced litter production cost, supporting the WH and EMH. Increased maternal size and condition also positively influenced litter size but not sex ratio. Our results suggest that for species with large litters, the benefits from adjusting litter size outweigh those from modulating offspring sex ratio.
