Abstract
How can altruism evolve or be maintained in a selfish world? Hamilton's rule shows that the former process will occur when rb > c-the benefits to the recipients of an altruistic act b, weighted by the relatedness between the social partners r, exceed the costs to the altruists c-drives altruistic genotypes spreading against nonaltruistic ones. From this rule, we infer that altruistic genotypes will persist in a population by forming a stable heritable polymorphism with nonaltruistic genotypes if rb = c makes inclusive fitness of the two morphs equal. We test this prediction using the data of 12 years of study on a cooperatively breeding bird, the Tibetan ground tit Pseudopodoces humilis, where helping is performed by males only and kin-directed. Individual variation in ever acting as a helper was heritable (h(2) = 0.47), and the resultant altruism polymorphism remained stable as indicated by low-level annual fluctuation of the percentage of helpers among all adult males (24-28%). Helpers' indirect fitness gains from increased lifetime reproductive success of related breeders statistically fully compensated for their lifetime direct fitness losses, suggesting that rb = c holds. While our work provides a fundamental support for Hamilton's idea, it highlights the equivalent inclusive fitness returns to altruists and nonaltruists mediated by rb = c as a theoretically and realistically important mechanism to maintain social polymorphism.