Offspring sex in mountain goat varies with adult sex ratio but only for mothers in good condition

Abstract

Studies of mammals have often produced results inconsistent with theories predicting adaptive sex-ratio manipulation. Some apparently strong trends weaken or disappear over time, suggesting that multiple variables affect sex ratio and that their relative importance may change over time. Mountain goats are sexually dimorphic ungulates that satisfy all the assumptions of the Trivers-Willard hypothesis. Therefore, females able to provide high levels of maternal care are expected to produce an excess of sons. An earlier study found that older females, that are larger and dominant, produced more sons than did younger females, which are smaller and subordinate. Here, we show that, as sample size tripled, that trend disappeared. A large-scale climate index, reproductive status at conception, and population density had no consistent effects on offspring sex ratio. A composite measure of female condition at conception revealed that offspring sex ratio varied with adult sex ratio, as predicted by the homeostatic hypothesis, but only for mothers in good condition at conception. These females were dominant, heavier, and older. Their probability of producing a son decreased from about 80 to 20 % as the adult sex ratio became more male-biased. For mothers with a low condition index, however, adult sex ratio had no effect on offspring sex ratio. Our research suggests that offspring sex ratio is affected by complex interactions between individual condition and other variables, whose importance may vary over time and can only be elucidated by long-term studies.