Various species across the animal kingdom utilize a biparental strategy to rear offspring to adulthood and to maximize fitness. Various studies have been conducted to date that demonstrate the effects of single parenthood on offspring. However, much less is known about the physiological, neural, and longer-term effects of single parenthood on the parents. For this dissertation, I set out to increase our understanding on this subject using the biparental avian species Columba livia (pigeon, rock dove). Both females and males of this species are capable of rearing offspring to adulthood after the loss of a mate, and both sexes are capable of pseudo- lactation to feed the offspring during the early stages post-hatching. For these reasons, pigeons present us with a powerful opportunity to examine sex-specific behavioral, neural, and physiological changes of single parenting in a biparental species. This present dissertation sets out to elucidate: 1) The general behavioral and physiological short term effects of single parenthood, 2) The differences in crop milk quality and provisioning between single and paired parents, as well as the similarities and differences between single mother and single father crop milk quality and provisioning, and 3) The neural and physiological differences between single and paired parents and the similarities and differences between single mothers and fathers over the course of rearing offspring to independence.
In chapter 1, I report my findings from a short-term study I conducted on the behavioral, neural, and physiological effects of single parenting on single parents at day 5 post their chicks hatching. My team and I found that single parents maintained similar provisioning levels to paired parents, but spent less time brooding their offspring. The chicks of single parents were smaller than paired-parented chicks at 3 days post-hatching. Mothers exhibited higher glucocorticoid receptor (GR) gene expression than fathers in general. Single parents exhibited lower prolactin (PRL) gene expression in the pituitary gland compared to paired parents. These findings collectively provided a robust foundation for understanding the dynamics at play within avian parenting after a major parental disturbance, offering a stepping stone for further exploration into the molecular and physiological underpinnings that shape the parenting landscape in avian species.
In chapter 2, we assessed variations in crop milk quality and offspring development among single-mothered, single-fathered, and paired-parented nests. Single fathered chicks exhibited reduced size compared to those from paired-parented and single-mothered nests, with second-hatched chicks of single parents being particularly affected. Single-fathered chicks also received less crop milk with a lower dry weight percentage compared to paired-parented and single-mothered chicks. The crop tissue of fathers was heavier than mothers and fathers retain crop milk with a higher dry weight percentage than mothers. Fathers also expressed more crop tissue mesotocin receptors (OxtR) than mothers. Paired fathers demonstrated higher prolactin receptor (PRLR) gene expression when compared to paired mothers, single mothers, and single fathers, and single parents once again expressed less pituitary PRL than paired parents. Single mothers expressed higher paraventricular nucleus (PVN) GR expression compared to paired mothers. Two-chick brood single parents exhibited higher baseline corticosterone (CORT) concentrations than their paired counterparts. Our comprehensive data demonstrate that single parents, regardless of sex, undergo physiological and neurobiological changes to sustain crop milk production and offspring care. However, these changes do not fully compensate for the absence of a partner. Our findings will open avenues for further investigation of potential trade- offs and sex-specific disparities in avian pseudo-lactation.
In chapter 3, we manipulated presence of parental partners and measured offspring growth and gene expression associated with glucocorticoids, prolactin, mesotocin, and gonadotropins at different stages of parenting from the early (day 5 post-hatching), middle (day 15 post-hatching), and late stages of parenting (day 3 post-fledging) in the pituitary and PVN. Additionally, we measured baseline circulating plasma concentrations of CORT. Single-fathered chicks were the smallest chicks overall at day 5 post-hatching, while single-mothered chicks were the smallest overall at day 15 post-hatching. Single mothers had more PVN GR gene expression than paired mothers, and single parents had higher baseline circulating CORT compared to paired parents in general at day 5 post-hatching. Single parents at day 15 post- hatching had lower baseline circulating CORT than paired parents. Mothers expressed more PVN mineralocorticoid receptors (MR) than fathers at day 15 post-hatching, and paired mothers expressed more PVN MR than paired fathers, single mothers, and single fathers at day 3 post- fledging. Paired fathers expressed more pituitary PRLR than paired mothers, single mothers, and single fathers. Single parents experienced lower PRL gene expression in the pituitary as compared to paired parents at day 5 post-hatching. Fathers at day 15 post-hatching and day 3 post-fledging had higher PVN PRLR than mothers. Mothers expressed less PVN OxtR than fathers at day 5 post-hatching, Single parents at day 15 post-hatching expressed less pituitary GnRHR than paired parents. Single mothers at day 15 post-hatching also expressed less PVN GnIH than paired fathers. This investigation, conducted from the initial stages of parental care through to the point of offspring achieving independence, reveals the myriad of neurological and physiological changes that male and female parents undergo in the face of a major disturbance in their parenting strategy.
As a whole, the comprehensive data I present for my dissertation work demonstrates that single parents, regardless of sex, undergo profound physiological and neurobiological changes in their journey to rear offspring. However, these changes, while remarkable, do not completely compensate for the absence of a partner. My findings open up exciting new avenues for future research of potential trade-offs and sex-specific disparities in parenting behaviors following a major disturbance in parent-offspring dynamics. This work holds the promise of furthering our understanding of the complex world of avian parenting and promises to shed light on broader aspects of reproductive biology.
