SEX-BIASED PARENT-OFFSPRING CONFLICT

In species showing sexual dimorphism, parents may obtain different fitness returns per unit of parental expenditure from sons and daughters. Under these circumstances, parents are expected to invest extra resources in offspring of the most profitable sex. However, it is unclear whether sex-biased expenditure is the result of selection acting on parents, their offspring, or both. Current parent-offspring conflict theory is used to investigate whether sex biases in parental expenditure should be accompanied by sex biases in parent-offspring conflict. It is suggested that, in general, greater conflict should be expected between parents and offspring of the favoured sex. Specifically, greater conflict is predicted among mother-son dyads than among mother-daughter dyads in most polygynous birds and mammals. Data on domestic sheep, as well as empirical evidence available for other species (mainly ungulates), lends support to the prediction. The prediction is further extended to cercopithecine primates, a group which lacks clear sex-biases in parental investment. In this case, differences in fitness returns per unit of parental expenditure between the daughters of dominant and subordinate mothers are positively related to differences in the extent of mother-daughter conflict. The results from this study highlight the important role that selective pressures acting on the offspring phenotype may have played in the evolution of sex-biased patterns of parental investment.