Kin selection and reproductive value in social mammals

Empirical studies have tested and confirmed a positive relationship between relatedness and the degree of altruistic behavior, as predicted by kin selection theory. In contrast, researchers have studied the relationship between reproductive valuethe extent to which an individual of a given state contributes to future generationsand the degree of altruistic behavior far less frequently, although reproductive value should be considered when testing kin selection. Here, we reviewed empirical studies examining such relationships in social mammals. Overall, >50% of these studies demonstrated that the degree of altruistic behavior (1) decreases with the reproductive value of the actor, and (2) increases with the reproductive value of the recipient, supporting the importance of reproductive value in altruistic behavior. However, these studies cannot rule out all other explanations than kin selection. Few empirical studies have adopted a stronger test that uses the product of relatedness and reproductive value (i.e., their two-way interaction in a linear model). Such tests should be particularly useful when studying long-lived vertebrates, for which the direct estimation of fitness benefits and costs is impractical. To achieve a better understanding of the evolution of kin-directed behaviors, future studies should use the product of relatedness and reproductive value.