Cooperative breeding and immunity: a comparative study of PHA response in African birds

Cooperatively breeding birds might be expected to suffer from higher costs of parasitism than pair-breeding species because of two aspects of their ecology which should facilitate horizontal transmission and possibly select for higher parasite virulence: first, they interact regularly with more individuals than pair-breeding species, and second, these individuals are commonly close relatives that could share similar resistance alleles. This hypothesis predicts that cooperative breeders should invest relatively more in immune defence than closely related species which breed in pairs. I tested this prediction comparatively in African birds by examining the response of the immune system to the mitogenic lectin, phytoahemagglutinin (PHA response) in relation to cooperative breeding. Among 66 species, of which 18 breed cooperatively, PHA response was significantly higher in cooperatively breeding species. This association appeared not to be confounded by body size, clutch size, nest position, coloniality or similarity owing to common phylogenetic descent. These results suggest that cooperatively breeding birds may have been selected to invest more than pair-breeders in defences against parasites. If so, then additional costs of philopatry and helping behaviour might be imposed on breeders, helpers and offspring.