Mammalian sex ratios and variation in costs of rearing sons and daughters.

In red deer, the sex ratio of calves at birth (calculated as the proportion of calves born that are male) increases with the dominance rank of the mother, whereas opposite trends exist in several populations of macaques and baboons. Here we show that the subsequent survival and reproductive success of subordinate female red deer is depressed more by rearing sons than by rearing daughters, whereas the subsequent fitness of dominant females is unaffected by the sex of their present offspring. By contrast, among subordinate female macaques, the rearing of daughters has greater costs to the mother's subsequent fitness than does the rearing of sons, although again, no difference in the costs of rearing sons and daughters is found among dominant mothers. These findings indicate that both differences in the relative fitness of sons and daughters and differences in the relative costs of rearing male and female offspring can favour variation in the sex ratio.