Sex allocation

Sex allocation is the apportioning of parental resources to male versus female offspring in a sexually reproducing species. Sex allocation theory assumes that natural selection favours parents that modify their investment into male and female offspring in such a way that it maximizes the parent’s fitness (Fisher 1930; Charnov 1982). Fisher’s equal allocation theory (1930) states that selection should favour an unbiased sex ratio at the population level. Because each offspring has a mother and a father, males and females will on average make equal genetic contributions to the next generation. Offspring of the minority sex will yield a greater per capita return on investment, putting a premium on the production of a larger number of offspring of this sex. Thus, frequency-dependent selection acts to favour an equal number of male and female offspring provided that males and females are equally costly to produce. In the case where the costs of producing males and females differ, the prediction must be phrased in terms of the investment ratio into each sex rather than the sex ratio. For example, parents may be selected to bias the offspring sex ratio towards the cheaper sex, such that the overall resource investment in the sexes is equal. Hamilton (1967) was the first to identify conditions that violate the assumptions underlying Fisher’s (1930) equal allocation theory. Hamilton pointed out that equal allocation is not expected in species where within-group interactions have a differential effect on the fitness of males and females. For example, if brothers compete over a limited number of mates (i.e. local mate competition), parental fitness will increase if more daughters are produced, as shown in studies on parasitic wasps (Hamilton 1967). Thus, selection should favour a sex ratio that is biased towards the sex experiencing the least amount of competition with close kin. In some cases, this is likely to be the dispersing sex (Clark 1978; Silk 1983). Conversely, selection may also favour a sex ratio that is biased towards the sex that improves within-group conditions for kin (i.e. local resource enhancement). This might for example be the helping sex in cooperatively breeding birds (Emlen et al. 1986; Lessells and Avery 1987). Finally, Trivers and Willard (1973) showed that individuals could be selected to adjust the sex of their offspring in response to environmental conditions. When environmental conditions have a differential effect on the fitness of male and female offspring, parents should bias the sex ratio of their offspring towards the sex that contributes more to parental fitness (Trivers and Willard 1973). Such facultative biasing by individual parents can occur despite strong selection for equal investment in daughters and sons in the population. Theory predicts a number of situations in which individuals should adjust their relative allocation to male and female reproduction. From 1980 onwards there was a profusion of empirical studies testing the various forms of the Trivers–Willard hypothesis (Charnov 1982; Frank 1990; Hardy 2002; West 2009). Many of these studies were conducted on haplodiploid Hymenopterans (ants, bees, wasps), where females can control the sex of their offspring precisely because females develop from fertilized eggs and males develop from unfertilized egg (Trivers and Willard 1973; Charnov 1982; Frank 1990; Hardy 2002). For example, the clearest patterns of individual sex ratio adjustment come from studies on parasitic wasps where females lay a higher proportion of female eggs in large than in small hosts. The reason for this is that there is a strong positive relationship between the size of the host and the number of eggs