Prolactin and helping in birds: has natural selection strengthened helping behavior?

The evolution of helping behavior in birds has been hotly debated.Hamilton's inclusive fitness theory has received much supportfrom ecological cost-benefit studies; however, the hypothesisthat helping has not been selected per se but is simply a phenotypicallyplastic response to altered social conditions has been proposed.In this view helping by nonbreeding birds occurs when at independencethey fail to leave their parents and are exposed to the criticalstimulus, the begging of young birds to be fed. We report thatlevels of prolactin, a hormone associated with parental behavior,are conspicuously higher in an avian species with helpers, theMexican jay (Aphelocoma ultramarina), than in a congeneric andsympatric species without helpers, the western scrub jay (A.californica). Specifically, prolactin in the nonbreeding membersof the helping species is higher than the level found in thebreeders of the congeneric nonhelping species. In addition,prolactin levels in nonbreeders rise well before the appearanceof begging young. These findings reject the phenotypic plasticityhypothesis based purely on a response to begging young and suggestthat prolactin is involved in the physiology of helping behaviorin birds as part of a complex adaptation.