Kin selection theory predicts that animals will direct altruism toward closer genetic relatives and aggression toward more distantly related individuals. Our 18-y study of wild banded mongooses reveals that, unusually, dominant individuals target females who are more closely related to them for violent eviction from the group. This puzzling result can be explained by selection for unrelated individuals to resist eviction and for related individuals to submit more easily. In support of this idea, we show that kin are targeted for aggression only when individuals are capable of resisting. Our results suggest that, where potential victims can oppose aggression, the usual predictions of kin selection theory can be reversed.

Abstract

Kin selection theory predicts that, where kin discrimination is possible, animals should typically act more favorably toward closer genetic relatives and direct aggression toward less closely related individuals. Contrary to this prediction, we present data from an 18-y study of wild banded mongooses, Mungos mungo, showing that females that are more closely related to dominant individuals are specifically targeted for forcible eviction from the group, often suffering severe injury, and sometimes death, as a result. This pattern cannot be explained by inbreeding avoidance or as a response to more intense local competition among kin. Instead, we use game theory to show that such negative kin discrimination can be explained by selection for unrelated targets to invest more effort in resisting eviction. Consistent with our model, negative kin discrimination is restricted to eviction attempts of older females capable of resistance; dominants exhibit no kin discrimination when attempting to evict younger females, nor do they discriminate between more closely or less closely related young when carrying out infanticidal attacks on vulnerable infants who cannot defend themselves. We suggest that in contexts where recipients of selfish acts are capable of resistance, the usual prediction of positive kin discrimination can be reversed. Kin selection theory, as an explanation for social behavior, can benefit from much greater exploration of sequential social interactions.