Sex Allocation Theory for Birds and Mammals

Parents divide their reproductive effort into the production of sons and daughters. Darwin (43) was intrigued by the fact that parents usually split their effort so that approximately equal numbers of sons and daughters are raised. He believed that this male: female ratio had been adjusted by natural selection because he understood that the number of females set a limit on reproductive capacity. He could not, however, clearly specify how natural selection shaped the sex ratio. Fisher (5 1 , 52) provided the explanation by noting that frequency-dependent selection stabilizes the sex ratio near equal­ ity. Since Fisher presented his explanation, many examples of biased sex ratios have been observed in nature. For example, Hamilton (7 1 ) observed very female-biased sex ratios in parasitic wasps that mate in small groups. Hamil­ ton explained this bias by showing that in these wasps the mating competition among brothers violates a latent assumption in Fisher's argument. In other examples of observed biases, the explanations put forth provided new dimensions to Fisher's central theory rather than direct exceptions. The most important of these new dimensions for birds and mammals was observed by Trivers & Willard (109). They noted that, in some mammals, healthy mothers tended to produce a relatively higher proportion of sons than did unhealthy mothers. They explained this pattern of variation among families