Dominance Rank, Secondary Sex Ratio, and Reproduction of Offspring in Polygynous Primates

Abstract

One question in evolutionary biology which is central to understanding reproductive strategies and parental efforts at rearing offspring is why organisms produce certain offspring sex ratios (Trivers and Willard 1973; Charnov 1982). Fisher (1930) argued that since each individual has a mother and a father, both sexes contribute genes equally to the next generation. All else being equal, if members of one sex were overproduced, their ability to reproduce would be limited by the availability of members of the less abundant sex and natural selection would favor any predisposition to produce offspring of the less abundant sex. In this fashion, Fisher reasoned, selection would adjust the offspring sex ratio in an outbreeding population until total parental investment (PI, Trivers 1972) in the two sexes was equal. Fisher (1930) assumed, but did not clearly state, that all parents were alike in their ability to rear reproductively successful sons and daughters (Patterson and Emlen 1980). A model proposed by Trivers and Willard (1973), however, specified how deviations from equal PI may be favored by natural selection according to the influence of maternal condition on the reproduction of offspring. The TriversWillard model proposed that in polygynous species if some condition of mothers (e.g., dominance rank) influences the condition and, as a result, the reproductive success (RS) of sons more than daughters, then selection should result in the evolution and maintenance of mechanisms by which females in poor condition invest less in sons than daughters and/or females in good condition invest more in sons than daughters. This model is particularly relevant for primates, since the majority of species are polygynous (Clutton-Brock and Harvey 1977). Since the dominance rank of a monkey's social group and family has a strong