Kin selection can explain the evolution of cooperative breeding and the distribution of relatives within a population may influence the benefits of cooperative behaviour. We provide genetic data on relatedness in the cooperatively breeding cichlid Neolamprologus pulcher. Helper to breeder relatedness decreased steeply with increasing helper age, particularly to the breeding males. Helper to helper relatedness was age-assortative and also declined with age. These patterns of relatedness could be attributed to territory takeovers by outsiders when breeders had disappeared (more in breeding males), betweengroup dispersal of helpers and reproductive parasitism. In six of 31 groups females inherited the breeding position of their mother or sister. These matrilines were more likely to occur in large groups. We conclude that the relative fitness benefits of helping gained through kin selection vs. those gained through direct selection depend on helper age and sex.

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