SEX-BIASED PARENT-OFFSPRING CONFLICT

In species showing sexual dimorphism, parents may obtain different fit ness returns per unit of parental expenditure from sons and daughters. Under these circumstances, parents are expected to invest extra resou rces in offspring of the most profitable sex. However, it is unclear w hether sex-biased expenditure is the result of selection acting on par ents, their offspring, or both. Current parent-offspring conflict theo ry is used to investigate whether sex biases in parental expenditure s hould be accompanied by sex biases in parent-offspring conflict. It is suggested that, in general, greater conflict should be expected betwe en parents and offspring of the favoured sex. Specifically, greater co nflict 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 ungula tes), lends support to the prediction. The prediction is further exten ded to cercopithecine primates, a group which lacks clear sex-biases i n parental investment. In this case, differences in fitness returns pe r unit of parental expenditure between the daughters of dominant and s ubordinate mothers are positively related to differences in the extent of mother-daughter conflict. The results from this study highlight th e important role that selective pressures acting on the offspring phen otype may have played in the evolution of sex-biased patterns of paren tal investment.