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 phenotypicalfy plastic response to altered social conditions has been proposed. In this view helping by nonbreeding birds occur* 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 uttramarina), than in a congeneric and sympatric species without helpers, the western scrub jay (A. caUfornica). 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 die physiology of helping behavior in birds as part of a complex adaptation. Key words: alloparental behavior, Aphelocoma, altruism, helping, jays, prolactin. [Behav Ecol 9:541-545 (1998)] A contentious issue in the evolution of the behavior of hu/~\ mans and other animal* is the origin of altruism. Hamilton's concept of inclusive fitness (Hamilton, 1964) was a major advance in evolutionary theory that helps to explain potentially altruistic behaviors (with a net loss in direct fitness), such as alloparental or helping behavior. Helping behavior is the feeding of young together with the genetic parents by individuals that are not the genetic parents. It is common in humans and has been much studied in social insects (Crozier and Pamilo, 1996), birds (Brown, 1987a; Stacey and Koenig, 1990) and mammal* (Solomon and French, 1996). Cost-benefit studies on birds in the last 25 years have revealed that some helpers may behave altruistically in Hamilton's (1963) original sense (e.g., primary helpers in the pied kingfisher, Cerylt rudis, Reyer, 1980, 1984; helpers in the white-fronted bee-eater, Mewps buUockaides, Emlen and Wrege, 1988,1989), in agreement with inclusive fitness theory applied to birds (Brown, 1974, 1987b; Emlen and Wrege, 1994). Nevertheless, the hypothesis that helping by nonbreeders has not been influenced directly by natural selection but is simply a response to altered social conditions, or "flexible strategy," has been advanced both for social insects (West-Eberhard, 1987) and birds (Jamieson, 1989, 1991; Jamieson and Craig, 1987). In this view, helping by nonbreeding birds occurs when at independence they fail to leave their parents because of environmental constraints and are thereby exposed to the critical stimulus, the begging of young birds to be fed, without involving natural selection. An understanding of the physiological basis of a behavior, when used with die comparative method, can provide a powerful tool to test hypotheses in evolutionary ecology (Real, 1994). We used this approach to show that helping and its associated physiology are not mere responses to begging young. To distinguish between hypotheses that invoke selection on helping and one that does not, we compared levels