The neural regulation of fatherhood, insights from the anemonfish amphiprion ocellaris

Parental care is a fundamental social behavior exhibited ubiquitously in diverse lineages across the animal kingdom, and when present is critical for offspring survival. Parental care represents a conflict: should parents invest in offspring fitness at the consequence of forgone reproductive opportunities? Or should they relinquish their parental duties in pursuit of additional mating events and offspring production? When parents do care for their young, specific mechanisms in the brain facilitate parental behavior. Thus far, the vast majority of research directed at understating the neural control of parental care has focused on females. Female care, or maternal care, is far more common among the common animal research models, and hence our understanding of its proximate regulatory mechanisms is well beyond our comprehension of those underlying paternal care. However, when paternal care is present, it is tantamount to maternal care in its contributions to offspring survival. While some of the mechanisms may be conserved, others may differ as males and females present dramatic sex differences in physiology. Hence, research efforts directed at paternal care are needed in order to understand the neural mechanisms underlying why fathers father. Two promising systems in understanding fathering behavior include nonapeptide signaling processes, and circulating steroid hormones. Nonapeptides and steroid hormones are key modulators of a variety of social behaviors in diverse taxa, and have also been identified as critical in the regulation of parental care. This dissertation aims to uncover how nonapeptide signaling, in concert with steroid hormones, regulate male parental care in the anemonefish Amphiprion ocellaris. In the introductory chapter 1, the conundrum of paternal care is presented, and the anemonefish A. ocellaris is introduced as a promising model in furthering our understanding of its neural regulation. Chapter 2 establishes A. ocellaris as a useful compliment to some of the more classically studied models of paternal care within vertebrates. Sex differences in levels of parental effort and in circulating steroid hormones during high levels of parental care were quantified. Results established that males are the predominant caregivers, spending more time in the nest and displaying a more total parental behaviors compared to females. Additionally, males showed higher levels of the circulating steroid hormone 11-ketotestosterone (11KT), which remains stable throughout the breeding cycle, indicating high androgens are not a hindrance to paternal effort. Females displayed higher circulating estradiol, which peaks 6 days after the eggs are laid, presumably in response to egg production and subsequent spawning events. Chapter 3 explores the role of arginine vasotocin (AVT) and isotocin (IT) signaling during paternal care. Here, using antagonists specific to each peptide receptor, I demonstrated that blockade of IT signaling dramatically reduced total parental effort in males. Moreover, results suggest this was specific to paternal care, as the antagonist had no effect in aggressive dyadic interactions. Furthermore, I show that blockade of AVT significantly increased paternal care, a surprising finding given how high baseline levels of parental effort are in this species. A previous report found that the AVT antagonist reduced aggression. Consequently, I hypothesize this promotion of paternal care in response to the AVT antagonist may be a result of reduced vigilance and aggression, thereby allowing more effort allotted towards parental care. In chapter 4, I demonstrate that aromatase and IT receptor gene expression are dynamically regulated in response to paternal care, independent of AVT V1aR receptor expression as well as the circulating steroid hormone 11KT. Individuals actively parenting had higher levels of aromatase and IT receptor expression than those not currently parenting. IT receptor expression was higher in males compared to females, a pattern reflective of the sex differences in parental effort. Brain aromatase expression was higher in females compared to males, which I hypothesize is an induced response of higher circulating E2 levels in females. No differences in AVT receptor expression or circulating 11KT were found in contrasting parental contexts. Chapter 5 utilizes a unique aspect of anemonefish behavior, step fathering. In this study naïve males were given a batch of eggs to care for, and following 90 minutes of parental behaviors, paternal brains were examined for the co-localization of IT and the ribosomal protein S6 (an neuronal activation marker), in order to determine the extent to which activation of IT neurons is related to paternal effort. Results indicate that paternal effort is negatively related to the activation of IT neurons within the POA. However, the time step-fathering males spent in the nest was positively correlated with the amount of eggs lost, signifying that males spending a lot of time in the nest may have been eating eggs, and hence were not displaying parental behavior. Furthermore, I have anecdotally noted in previous experiments that males improve as fathers with experience. Therefore, using experience biological fathers, or allowing stepfathers more time to interact with several batches of eggs may yield differing results. Ultimately the efforts of this dissertation provide cumulative evidence supporting a conserved role of IT signaling in the regulation of parental care across taxa, and independent of the sex of the primary caregiver. Findings also suggest AVT signaling as ancillary in the regulation of paternal care, as it may be important in nest defense and vigilance behavior. Moreover, I also show aromatase gene expression is an important aspect of parental behavior, as it is higher in a parental context