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 support from ecological cost-benefit studies; however, the hypothesis that helping has not been selected per se but is simply a phenotypically plastic response to altered social conditions has been proposed. In this view helping by nonbreeding birds occurs when at independence they fail to leave their parents and are exposed to the critical stimulus, the begging of young birds to be fed. We report that levels of prolactin, a hormone associated with parental behavior, are conspicuously higher in an avian species with helpers, the Mexican jay (Aphelocoma ultramarina), than in a congeneric and sympatric species without helpers, the western scrub jay (A. californica). Specifically, prolactin in the nonbreeding members of the helping species is higher than the level found in the breeders of the congeneric nonhelping species. In addition, prolactin levels in nonbreeders rise well before the appearance of begging young. These findings reject the phenotypic plasticity hypothesis based purely on a response to begging young and suggest that prolactin is involved in the physiology of helping behavior in birds as part of a complex adaptation.