S15-3 Fitness consequences of cooperative breeding in the Seychelles warbler

Inclusive fitness benefits have been suggested as the selective force behind the evolution of cooperative breeding. Assessing the benefits accrued to individual males and females is crucial to understanding the sex-specific helping behavior observed in many cooperatively breeding species. We investigated the fitness consequences of male and female helping behavior in the Seychelles warbler (Acrocephalus sechellensis). Until 1988, the entire world population of Seychelles warblers was confined to Cousin Island (29 ha), where the carrying capacity has been maintained since 1973. Due to intense competition for breeding vacancies, many young become subordinates within a territory and often help by provisioning nondescendent offspring. On high-quality territories, the benefits accrued by subordinates are higher for females than males. Female subordinates remain on their natal territory and obtain higher inclusive reproductive success by helping closely related relatives, by co-breeding within the group, and through experience in parenting. Males often become subordinates on non-natal territories and so do not gain indirect reproductive success by helping. They only rarely gain direct benefits through co-breeding, and do not gain through the eventual inheritance of the territory. The disparity in the benefits gained by each sex may explain why the majority of subordinates are female. It should be kept in mind that the benefits of cooperation may be later offset by competition between same-sex offspring, and that the balance between these forces determines the reproductive value of sons and daughters.