Prolactin and the shared regulation of parental care and cooperative helping behavior in white-browed sparrow weaver societies

In many cooperatively breeding species non-breeding individuals help to rear the offspring of breeders. The physiological mechanisms that regulate such cooperative helping behavior are poorly understood, but may have been co-opted, during the evolution of cooperative breeding, from pre-existing mechanisms that regulated parental care. Key among these may be a role for prolactin. Here we investigate whether natural variation in circulating prolactin levels predicts both parental and helper contributions to nestling provisioning in wild cooperatively breeding white-browed sparrow weavers, Plocepasser mahali. In sparrow weaver groups, a dominant pair monopolize reproduction and non-breeding subordinates help with nestling feeding. We show that: (i) among parents, females feed their nestlings at higher rates, make longer provisioning visits, and have higher prolactin levels than males; and (ii) among subordinates, engaged in cooperative helping behavior, those within their natal groups feed nestlings at higher rates and have higher prolactin levels than immigrants. Accordingly, continuous variation in prolactin levels positively predicts nestling provisioning rates and provisioning visit durations when all bird classes are combined. These relationships are principally driven by differences among bird classes; the more limited within-class variation in prolactin and provisioning traits were not evidently correlated, for which a range of potential explanations exist. While these correlative findings are broadly consistent with the hypothesis that parental care and cooperative helping behavior are regulated by a shared mechanism, experiments are now needed to establish whether prolactin plays a causal role, and if so, whether it regulates continuous variation in provisioning rate or simply activates provisioning behavior.