THE NEUROENDOCRINE ROLE OF PROLACTIN IN LIFE-HISTORY TRADEOFFS AND TRANSITIONS IN A BIPARENTAL BIRD

During parental care, animals must allocate resources towards offspring care that may come at a cost to personal survival, growth, and future reproductive efforts. Hormones physiologically mediate many of these tradeoffs. Prolactin, a pituitary peptide best known for its role in mammalian lactation, also promotes parental behaviors across vertebrates. However, less is known how prolactin affects reproductive behavior and physiology, or the endocrine stress response, during parental care. Further, how prolactin enacts these effects at the neuroendocrine level remains less studied. To address these gaps, I examined the effects of prolactin on behavior, hormones and gene expression in a biparental bird, the rock dove (Columba livia). Rock doves exhibit nearly-egalitarian biparental care and prolactin-driven pseudolactation, making them an ideal system to study prolactin’s effects on physiology in both sexes. First, I characterized prolactin and prolactin receptor gene expression across the endocrine axis controlling reproduction: the hypothalamic-pituitary-gonadal (HPG) axis in both sexes, as well as in the crop sac, the site of pseudolactation. Next, I used hormone manipulations to determine the causal role of prolactin in transitions from mating to parental behavior. I found that treatment with prolactin promoted parental behaviors, even after nest removal, but did not significantly alter expression of courtship or copulation behaviors as compared with vehicle-treated controls. I found that prolactin-treated birds showed increased sensitivity to hypothalamic hormones in the pituitary and sex-specific changes in the gonads, where males showed increased sensitivity to gonadotropins, but females did not. These results suggest the HPG axis may compensate for elevated prolactin (within the physiological range) in order to maintain reproductive behavior and function.Lastly, I examined how prolactin system regulation compares across birds with a range of parental experience and ages. I found that neural expression of prolactin receptors nor vasoactive intestinal peptide (a peptide involved in prolactin release) changed with increasing parental experience. This result suggesting that increased fitness with parental experience may be mediated through other mechanisms or elsewhere in the brain. However, when I examined the effects of parental experience on the hormonal stress response, I found that experienced parents released lower levels of glucocorticoids and maintained higher plasma prolactin levels after an acute stressor than inexperienced individuals. This hormone pattern appears to be mediated by increased glucocorticoid receptor density in the hippocampus. Taken together, this dissertation integrates across neurobiology, endocrinology and animal behavior to provide novel insights into reproduction and stress in vertebrates