Whole brood mortality increases the opportunity for female-biased sex ratios under local mate competition

Fisher predicted that sex ratios should be even when the cost of producing each sex is equal, assuming large panmictic populations. Hamilton showed that when individuals breed exclusively in small groups (local mate competition), the sex ratio does not comply with Fisherian theory, but instead favours a female bias. Local mate competition has been suggested to explain female biases most often in haplo-diploids and parasitoids, where a small number of foundresses make up a breeding group. Here I show how pre-reproductive mortality can affect the predicted ESS sex ratio under local mate competition. Specifically, whole brood mortality reduces the number of families in the breeding pool each generation and, therefore, increases the probability that brothers will compete for mating opportunities. This effect is predicted to be strongest when females lay a small number of clutches each generation. These findings suggest that Hamilton’s classic model for group-structured sex ratios may be applicable to a much wider range of systems than was previously believed, since the requirements of the model are not always confined to small deme sizes.