Adoption as an offspring strategy to reduce ectoparasite exposure.

Adoption occurs frequently in colonial species where both the cost of parasitism and the opportunity for dependent young to find a foster family are typically high. Because ectoparasites show highly aggregated distributions among colony members, we tested two central predictions of the novel hypothesis that adoption is driven by selection on young to reduce ectoparasite load: first, that nest-based ectoparasites cause offspring to seek adoption, and second, that an individual's parasite load will be reduced after it has been adopted. In agreement with these predictions, experimentally infested Alpine swift Apus melba offspring sought adoption significantly more often and at an earlier stage than young kept free of ectoparasitic louse-flies. Second, the parasite load of experimentally infested young was reduced after adoption via a redistribution of ectoparasites among the foster family members. Our findings emphasize what we believe to be a novel role for parasites in the evolution of adoption and, by extension, in the emergence of social interactions.