Regulation of pigeon cropmilk secretion and parental behaviors by prolactin.

Prolactin stimulates the growth and development of specialized epithelial cells lining the cropsac of pigeons and doves (family Columbidae), leading to formation of "cropmilk," which is fed to the newly hatched squab. This system of milk feeding is unique among birds. To support the feeding of cropmilk, a complex array of behavioral adaptations are also supported by high levels of prolactin secretion in columbids during parenting. These specializations include elevated food intake (hyperphagia), nest attendance, and regurgitation feeding of the squab. Although prolactin is clearly important for these behavioral adaptations, the precise physiological and mechanistic bases for these behavioral effects remain controversial. The molecular mechanisms of prolactin action in the cropsac epithelium have been studied by cloning prolactin-induced genes, by cloning and expressing the pigeon prolactin receptor, and by analyzing the transcription factors that are activated after prolactin treatment. The avian (pigeon) prolactin receptor is a member of the cytokine receptor superfamily and uniquely contains a complete duplication of the extracellular ligand-binding domain. One of the early signal-transducing actions of prolactin in cropsac epithelium is the activation of signal transducer and activator of transcription (STAT) proteins via tyrosine phosphorylation. This fundamental signaling pathway is shared with mammalian prolactin target tissues. The convergent evolution of milk feeding and the behaviors that support parenting in columbids and mammals has depended on adaptation of both conserved mechanisms and divergent physiological processes.