Density-dependent dispersal reduces conflict over the sex ratio

Haplodiploids—in particular, wasps—are the workhorses of sex-allocation research. This owes to their unusual system of sex determination which provides a ready means of sex-ratio adjustment. Notably, their sexually asymmetrical mode of genetic inheritance leads mothers and fathers to come into conflict over the sex ratio of their offspring. In the simplest outbreeding scenario, a mother is favoured to employ an even sex ratio while a father prefers that all his mate’s offspring are female. An important modulator of evolutionary conflict between mating partners is genetic relatedness, raising the possibility that this sex-ratio conflict is reduced in low-dispersal settings with mating occurring between relatives. However, the impact of population viscosity on sex-ratio conflict in haplodiploids remains unknown. Here, we develop and analyse a kin selection model to investigate how the rate of dispersal modulates sex-ratio conflict in a haplodiploid, viscous-population setting. We find that population viscosity is associated with a reduction in the extent of sex-ratio conflict—the effect being very weak under density-independent dispersal and much stronger under density-dependent dispersal.