Costly young and reproductive skew in animal societies

Many recent models of reproductive skew explain subordinatereproduction asa staying incentive offered by dominants, whocan produce more young with ahelper present than without. Here,we present a new, alternative explanationfor subordinate reproduction,which applies whenever the fitness cost to aparent of producingyoung is an accelerating function of the number produced(ascommonly assumed in optimal clutch size theory). Under thesecircumstances,a dominant individual may be selected to offer a share ofreproductionto a related subordinate, not as an incentive to stay, butbecauseadditional offspring that would be expensive for the dominanttoproduce are cheap for the subordinate. "Beneficial sharing"ofthis kind is more likely the more closely related the subordinateis to thedominant, so that the model predicts a negative relationshipbetween skew andrelatedness. This result runs directly counterto the positive relationshippredicted by previous incentive-basedmodels. We explore the interaction ofthese contrasting effectsby developing an integrated model that allows forboth beneficialsharing and staying incentives. When offspring are cheap toproduce,this integrated model predicts that the incentive effect willdominate,and skew will increase with relatedness. When young are costly,incontrast, beneficial sharing will be of greater importance,and skew willdecrease with relatedness.