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 explanation for subordinate reproduction, which applies whenever the fitness cost to a parent of producing young is an accelerating function of the number produced (as commonly assumed in optimal clutch size theory). Under these circumstances, a dominant individual may be selected to offer a share of reproduction to a related subordinate, not as an incentive to stay, but because additional 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 model predicts a negative relationship between skew and relatedness. This result runs directly counter to the positive relationship predicted by previous incentive-based models. We explore the interaction of these contrasting effects by developing an integrated model that allows for both beneficial sharing and staying incentives. When offspring are cheap to produce, this integrated model predicts that the incentive effect sill dominate, and skew will increase with relatedness. When young are costly, in contrast, beneficial sharing will be of greater importance, and skew will decrease with relatedness.
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