REGULATION OF PIGEON CROPMILK SECRETION AND PARENTAL BEHAVIORS BY PROLACTIN

Prolactin stimulates the growth and development of specialized epithel ial cells lining the cropsac of pigeons and doves (family Columbidae), leading to formation of ''cropmilk,'' which is fed to the newly hatch ed squab. This system of milk feeding is unique among birds. To suppor t the feeding of cropmilk, a complex array of behavioral adaptations a re also supported by high levels of prolactin secretion in columbids d uring parenting. These specializations include elevated food intake (h yperphagia), nest attendance, and regurgitation feeding of the squab. Although prolactin is clearly important for these behavioral adaptatio ns, the precise physiological and mechanistic bases for these behavior al effects remain controversial. The molecular mechanisms of prolactin action in the cropsac epithelium have been studied by cloning prolact in-induced genes, by cloning and expressing the pigeon prolactin recep tor, and by analyzing the transcription factors that are activated aft er prolactin treatment. The avian (pigeon) prolactin receptor is a mem ber of the cytokine receptor superfamily and uniquely contains a compl ete duplication of the extracellular ligand-binding domain. One of the early signal-transducing actions of prolactin in cropsac epithelium i s the activation of signal transducer and activator of transcription ( STAT) proteins via tyrosine phosphorylation. This fundamental signalin g pathway is shared with mammalian prolactin target tissues. The conve rgent evolution of milk feeding and the behaviors that support parenti ng in columbids and mammals has depended on adaptation of both conserv ed mechanisms and divergent physiological processes.