Intra-sex and inter-sex sibling interactions as sex ratio determinants

Abstract

Fisher¹ produced the first general argument that a random-mating sexually-reproducing population should devote equal reproductive resources to the production of male and female offspring, while Hamilton² considered several unusual situations in which the expenditure of reproductive resources should be biased towards one sex or the other. His ‘host’ model has been widely cited^3–6 and at least three factors proposed as the cause of the sex ratio bias: local mate competition, sib-mating and inbreeding. The problem of which of these factors is the real cause of the biased sex ratio seems important in view of the large number of apparently different but often closely related sex ratio models which are now being proposed. It would be useful to have ways of classifying them according to the presence of different factors which affect the equilibrium sex ratio. Here, I present a general equation (equation (2)) for differential change in fitness which exhibits the equilibrium sex ratio as achieving a balance between the factor contained in Fisher's standard grandchild argument¹ and a second factor resulting from intra-sex sibling interactions over reproductive resources. In particular, I analyse Hamilton's host model² for the presence of these different factors.