Kin selection models for the co-evolution of the sex ratio and sex-specific dispersal

We investigate the co-evolutionary relationship between sex-ratio bias and sex-specific dispersal behaviour using an inclusive fitness approach. We consider two models: (i) DDM, in which dispersal of both sexes occurs before mating; and (ii) DMD, in which male dispersal precedes mating and female dispersal follows mating. Under DDM, at equilibrium, there is no bias in either the sex ratio or the sex-specific dispersal rates unless the sex-specific dispersal costs are different. However, under DMD, and at equilibrium, equal sex-specific dispersal costs imply a female bias in the sex ratio and a female dispersal rate at least as great as that of males. The present work highlights the role of sex differences – in either dispersal costs or the timing of dispersal – to the co-evolution of the sex ratio and dispersal.