Complementary interactions between indirect and direct fitness in a cooperatively breeding bird.

Altruism is difficult to explain evolutionarily and to understand it, there is a need to quantify the benefits and costs to altruists. Hamilton's theory of kin selection argues that altruism can persist if the costs to altruists are offset by indirect fitness payoffs from helping related recipients. Nevertheless, helping nonkin is also common and in such situations, the costs must be compensated for by direct benefits. While previous researchers tended to evaluate the indirect and direct fitness in isolation, we expect that they have a complementary interaction where altruists are associated with recipients of different relatedness within a population. The prediction is tested with 12 years of data on lifetime reproductive success for a cooperatively breeding bird, Tibetan ground tits . Helpers who helped distantly related recipients gained significantly lower indirect benefits than those who helped closely related recipients, but the opposite was true for direct fitness, thereby making these helpers have an equal inclusive fitness. Helping efforts were independent of helpers' relatedness to recipients, but those helping distantly related recipients were more likely to inherit the resident territory, which could be responsible for their high direct reproductive success. Our findings provide an explanatory model for the widespread coexistence of altruists and recipients with varying relatedness within a single population.