Offspring sex ratio in relation to maternal age and social rank in mountain goats (Oreamnos americanus)

In polygynous mammals, high-quality females may increase their fitness by producing a high proportion of sons. During a 9-year study of marked mountain goats (Oreamnos americanus), we assessed the relative effects of maternal age, social rank and reproductive status on offspring sex ratio. The sex ratio of kids in the population did not differ from unity (75 males, 85 females). The proportion of female kids decreased markedly with maternal age. Young females (less than or equal to6 years old) produced approximately 70% daughters, while old females (greater than or equal to 10 years old) produced about 25% daughters. The proportion of females born did not vary with maternal social rank when accounting for mother's age. Weaning success one year did not affect the probability of producing a son or a daughter the following year. Kid sex was independent of the sex of the kid produced by the same female the previous year. Because the proportion of daughters produced decreased dramatically with age, and because males appear more costly to rear than females in sexually dimorphic ungulates, our data suggest that the ability to provide maternal care may increase with age in mountain goats. Accepting that older mothers are better mothers because of higher social rank and experience, our data support the predictions of the Trivers and Willard hypothesis.