Abstract
Inclusive fitness theory predicts that sex investment ratios in eusocial Hymenoptera are a function of the relatedness asymmetry (relative relatedness to females and males) of the individuals controlling sex allocation. In monogynous ants (with one queen per colony), assuming worker control, the theory therefore predicts female-biased sex investment ratios, as found in natural populations. Recently, E.O. Wilson and M.A. Nowak criticized this explanation and presented an alternative hypothesis. The Wilson-Nowak sex ratio hypothesis proposes that, in monogynous ants, there is selection for a 1:1 numerical sex ratio to avoid males remaining unmated, which, given queens exceed males in size, results in a female-biased sex investment ratio. The hypothesis also asserts that, contrary to inclusive fitness theory, queens not workers control sex allocation and queen-worker conflict over sex allocation is absent. Here, I argue that the Wilson-Nowak sex ratio hypothesis is flawed because it contradicts Fisher's sex ratio theory, which shows that selection on sex ratio does not maximize the number of mated offspring and that the sex ratio proposed by the hypothesis is not an equilibrium for the queen. In addition, the hypothesis is not supported by empirical evidence, as it fails to explain 'split' (bimodal) sex ratios or data showing queen and worker control and ongoing queen-worker conflict. By contrast, these phenomena match predictions of inclusive fitness theory. Hence, the Wilson-Nowak sex ratio hypothesis fails both as an alternative hypothesis for sex investment ratios in eusocial Hymenoptera and as a critique of inclusive fitness theory.
Highlights
Inclusive fitness theory aims to provide a framework for understanding the evolution of sociality and social behaviour across all scales of biological organization (Queller, 2000; Boomsma, 2009; Bourke, 2011a)
Linking inclusive fitness theory (Hamilton, 1964) and Fisher’s (1930) sex ratio theory, Trivers & Hare (1976) showed that, within populations of eusocial Hymenoptera, the stable sex investment ratio for any party within the colony equals that party’s relatedness asymmetry, that is its relative relatedness to females and males
The Wilson–Nowak sex ratio hypothesis contradicts Fisher’s (1930) sex ratio theory for both nonsocial and social organisms and is a post hoc explanation for patterns of sex allocation observed in ants
Summary
Inclusive fitness theory aims to provide a framework for understanding the evolution of sociality and social behaviour across all scales of biological organization (Queller, 2000; Boomsma, 2009; Bourke, 2011a). Linking inclusive fitness theory (Hamilton, 1964) and Fisher’s (1930) sex ratio theory, Trivers & Hare (1976) showed that, within populations of eusocial Hymenoptera, the stable sex investment ratio for any party within the colony equals that party’s relatedness asymmetry, that is its relative relatedness to females and males.
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