Maternal condition and offspring sex ratio in polygynous ungulates: a case study of bighorn sheep

The Trivers and Willard model (TWM) predicts that for sexually dimorphic polygynous mammals, mothers able to provide a high level of care should bias offspring sex ratio in favor of sons. Contradictory results of empirical studies, however, suggest that selective pressures for adaptive offspring sex ratio vary with species and environmental conditions. We report the results of a 29-year study of marked bighorn sheep (Ovis canadensis) in a population that underwent wide changes in density and where most females were weighed each year. Lamb sex ratio was independent of absolute ewe mass and yearly deviations from individual or population average mass, but there was a nonsignificant trend towards fewer males being born at high population density. Bighorn sheep satisfy all the assumptions of the TWM but not its prediction: lamb sex ratio is independent of maternal ability to provide care. Recent hypotheses to explain the lack of relationship between maternal condition and offspring sex in ungulates are unlikely to apply to bighorn sheep. We suggest that the TWM may only apply when social rank strongly affects the ability to provide maternal care. Those circumstances are likely to occur for only a few species and within a narrow range of environmental conditions.