THE INFLUENCE OF DEVELOPMENTAL MORTALITY ON OPTIMAL SEX ALLOCATION UNDER LOCAL MATE COMPETITION

In panmictic populations, optimal sex allocation is, under the assumptions of Fisher's model, not influenced by the probability of offspring developmental mortality, or by differences in mortality between the sexes. In contrast, when mating opportunities are confined to siblings, developmental mortality can influence optimal sex allocation. Many animal species have both local mating and developmental mortality. We show that when developmental mortality is random for individual offspring, optimal sex allocation is influenced by mortality among males but not among females. Male mortality increases the allocation to males, but this should never be male biased, even under extreme male mortality. This result applies both when mothers are able to control the sex of individual offspring precisely, and when sex is allocated with binomial probability. The influence of mortality becomes progressively larger when the variance of the distribution of mortality over clutches diminishes. The reduction in fitness is greater than the proportion of mortality, especially at small clutch sizes, and mortality reduces the advantage of producing precise sex ratios, and of local mate competition in general.