Costly young and reproductive skew in animal societies

Many recent models of reproductive skew explain subordinate reproduction as a staying incentive offered by dominants, who can produce more young with a helper present than without. Here, we present a new alternative explanati on for subordinate reproduction, which applies whenever the fitness cost to a parent of producing young is an accelerating function of the number prod uced (as commonly assumed in optimal clutch size theory). Under these circu mstances, a dominant individual may be selected to offer a share of reprodu ction to a related subordinate, not as an incentive to stay, but because ad ditional offspring that would be expensive for the dominant to produce are cheap for the subordinate. "Beneficial sharing" of this kind is more likely the more closely related the subordinate is to the dominant, so that the m odel predicts a negative relationship between skew and relatedness. This re sult runs directly counter to the positive relationship predicted by previo us incentive-based models. We explore the interaction of these contrasting effects by developing an integrated model that allows for both beneficial s haring and staying incentives. When offspring are cheap to produce, this in tegrated model predicts that the incentive effect sill dominate, and skew w ill increase with relatedness. When young are costly, in contrast, benefici al sharing will be of greater importance, and skew will decrease with relat edness.