Kangaroo Fathers Modulate Maternal Control of Offspring Sex but Not Postpartum Maternal Expenditure

When sons and daughters have different fitness costs and benefits, selection may favor deviations from an even offspring sex ratio. Most theories on sex ratio manipulation focus on maternal strategies and sex-biased maternal expenditure. Recent studies report paternal influences on both offspring sex ratio and postpartum sexbiased maternal expenditure. We used long-term data on marked kangaroos to investigate whether and how paternal mass and skeletal size, both determinants of male reproductive success, influenced (a) offspring sex in interaction with maternal mass and (b) postpartum sex-biased maternal expenditure. When mothers were light, the probability of having a son increased with paternal mass. Heavy mothers showed the opposite trend. A similar result emerged when considering paternal size instead of mass. Postpartum maternal sexspecific expenditure was independent of paternal mass or size. Studies of offspring sex manipulation or maternal expenditure would benefit from an explicit consideration of paternal traits, as paternal and maternal effects can modulate each other.