Being a mother is energetically costly and can entail trade-offs between reproduction and self-maintenance. In biparental species, fathers, as well as mothers, care for offspring and improve offspring survival. It is unknown if fathers in these species, like mothers, experience costs of parenting. It is also not clear whether the beneficial consequences of paternal care for offspring are mediated exclusively through direct effects on offspring or whether they can be mediated indirectly through beneficial effects on mothers. This dissertation investigates energetic costs as well as behavioral and affective changes associated with fatherhood, and morphological, physiological and affective conditions in single mothers rearing offspring without assistance from their mates in the monogamous, biparental California mouse (Peromyscus californicus). Results demonstrate that cohabitation with a female and/or fatherhood decreases blood glucose levels, alters lipid profiles, as well as protects body mass and fat mass from being affected by energetic challenge. Fathers also buffer their mates from potentially adverse consequences of motherhood. Mothers rearing pups without assistance from a mate can maintain pup survival and development as normal, but experience morphological and endocrine changes that might be harmful in the long term. These experiments provide important implications for the understanding of trade-offs between reproduction and self-maintenance in a biparental breeding system.

The neurobiological basis of and factors facilitating male parental behavior are not well understood in mammals. Multiple rodent species show increases in parental behavior during the transition into parenthood or after prolonged exposure to pups. For the first time, we evaluated effects of prior exposure to pups on paternal responsiveness in the biparental California mouse (Peromyscus californicus). Adult virgin male California mice exhibit aggressive or indifferent responses to pups more frequently than fathers, which exhibit robust paternal care. We analyzed behavioral, neural, and corticosterone responses to pups in adult virgin males that were interacting with a pup for the first time, adult virgin males that had been exposed to pups 3 times for 20 min each in the previous week, and first-time fathers. Control groups of virgins were similarly tested with a novel object (i.e., a marble). Previous exposure to pups increased paternal care: responses to pups did not differ between virgins with repeated exposure to pups and new fathers. Neither basal corticosterone levels nor corticosterone levels following acute pup or marble exposure differed among groups. Finally, Fos expression in the medial preoptic area (MPOA) and bed nucleus of the stria terminalis was higher following exposure to a pup than to a marble. Fos expression was not, however, affected by previous exposure to these stimuli. These results suggest that previous experience with pups can increase paternal responsiveness via unknown mechanisms. To investigate these mechanisms, we evaluated synaptic, intrinsic, and morphological properties of MPOA neurons in male virgins or first-time fathers using standard whole-cell recordings in a novel in vitro slice preparation. We measured synaptic currents in response to local electrical stimulation, and we quantified intrinsic excitability by measuring voltage changes in response to square-pulse injections of both depolarizing and hyperpolarizing current. We also analyzed the morphology of MPOA neurons. Most parameters did not differ significantly between virgins and fathers. However, we documented a decrease in synaptic inhibition in fathers. These findings suggest that the onset of paternal behavior in California mouse fathers may be associated with limited electrophysiological plasticity within the MPOA.

The mechanistic basis of paternal behavior in mammals is poorly understood. Assuming there are parallels between the factors mediating maternal and paternal behavior, it can be expected that the onset of paternal behavior is facilitated by reductions in stress responsiveness, as occurs in females of several mammalian species. This dissertation describes studies investigating the role of stress responsiveness in the expression of paternal behavior in biparental, monogamous California mice (Peromyscus californicus). I compared endocrine, neural, and behavioral responses to stress, as well as anxiety and neophobia, in males from different housing conditions, and between males that did or did not respond paternally to an unfamiliar pup. Results from the first study suggested that fatherhood attenuates behavioral responses, but does not influence corticosterone responses, to an acute predator-urine stressor. Cohabitation with a female mate, however, tended to reduce corticosterone responses to repeated predator-urine exposure. I next characterized neophobia, anxiety, and neuroendocrine responses to acute predator-urine exposure. Results indicated that being a father did not modulate neophobia, but did appear to be associated with reduced anxiety. Paternally responsive males exhibited more anxiety-related behavior than nonpaternally responsive males, but did not differ in levels of neophobia. Social isolation, but not fatherhood, modulated the neuroendocrine response to stress exposure, while no effect of paternal responsiveness was found. Finally, to determine whether fathers and nonfathers, or paternally responsive and nonpaternally responsive males, differentially perceive pups as stressful, I compared neural responses to 1-h exposure to a pup. No effect of fatherhood was found; however, paternally responsive virgin males showed lower Fos expression in the paraventricular nucleus of the hypothalamus and bed nucleus of the stria terminalis compared to nonpaternally responsive virgins. My findings suggest that fathers in this biparental species do not exhibit reductions in stress responsiveness similar to those in lactating females of several species. Nonetheless, paternally responsive males show reduced activation of stress-related brain areas in response to pup exposure, as well as altered patterns of neophobia and anxiety-related behavior, as compared to nonpaternally responsive males. These findings will further the understanding of the mechanistic basis of paternal behavior in mammals.

In biparental species, in which both parents care for their offspring, the neural mechanisms underlying the initiation of paternal care are not well understood. The neural mediators of paternal behavior overlap substantially with those underlying maternal behavior; however, few studies have looked at the functional roles of classical neurotransmitter systems, such as norepinephrine (NE), in the initiation of paternal care. Moreover, the cellular and molecular mechanisms that govern neural plasticity during the transition to parenthood remain largely unexplored. To address this gap, studies in this dissertation examined the role of NE in the onset of paternal behavior, as well as the mechanisms underlying neural plasticity resulting from experience with pups in adult California mice (Peromyscus californicus), a biparental species. These experiments mainly focused on adult virgins, as I was interested in examining the neural mechanisms underlying the initiation of pup-affiliative behavior in this species. The first study investigated the effects of nepicastat, an inhibitor of NE synthesis, on pup-directed behavior in adult virgin California mice. I found that nepicastat significantly reduced the number of virgin males and females that approached an experimentally presented pup and that displayed parental behavior. In the second study, I examined the effects of nepicastat on neural activation, as indicated by c-Fos expression, in brain regions associated with parental care and anxiety following exposure to pups. Virgin males injected with nepicastat exhibited significantly reduced neural activation in the medial amygdala following pup exposure compared to vehicle-injected controls. Finally, I examined the impact of exposure to pups on perineuronal net expression (PNN) in the brains of male and female California mice. PNNs are extracellular matrix structures that are known to influence synaptic plasticity. I used a pup sensitization paradigm that promotes the onset of paternal behavior in virgin California mice. I found that dynamic changes in PNN expression in the brain occur with repeated exposure to pups in male but not female California mice. These findings provide insights into the role of the noradrenergic system in the onset of parental behavior as well as the mechanisms regulating neuroplasticity of the paternal brain during the transition to parenthood.

In biparental mammals, becoming a father can significantly alter males’ neural and endocrine systems, but effects of fatherhood on physiology and morphology are less clear. This dissertation tested the hypothesis that fatherhood has significant physiological, morphological and exercise-performance costs, and are modulated by parity and environmental stress. Three studies were performed using the monogamous, biparental California mouse (Peromyscus californicus). In each study, physiology, exercise performance, and morphology were compared among breeding males, non-breeding males (with a tubally ligated female [Chapter One] or an ovariectomized, estrogen/progesterone-treated female [Chapters Two and Three]) and virgin males (with another male [Chapter One] or an ovariectomized, untreated female [Chapters Two and Three]).

Chapter One examined acute effects of fatherhood. New fathers had significantly larger hindlimb muscles than non-breeding males, but virgin males had heavier subcutaneous fat pads than non-breeding or virgin males. No differences were found in physiology (basal metabolic rate [BMR], hematocrit), or locomotor performance (maximum sprint speed, treadmill endurance, maximal oxygen consumption [VO2max]).

Chapter Two characterized long-term effects of fatherhood across multiple reproductive bouts. Breeding males differed from non-breeding and/or virgin males in several physiological (resting metabolic rate [RMR]), exercise performance (sprint speed, predatory aggression), and morphological (lean, fat, and organ masses) measures at specific time points; however, differences among groups were inconsistent, and few differences were found overall. Moreover, the number of differences between breeding males and controls did not increase with parity.

Chapter Three examined effects of cold acclimation and determined how these effects were influenced by males’ reproductive condition. Cold-acclimating virgins had lower body, fat, and lean masses, and higher RMR and VO2max, than virgins at room temperature. Among long-term cold-acclimated animals, breeding males had higher BMR, lower VO2max, longer feet, and larger bacula than non-breeding and virgin males. These results suggest that cold stress may reveal energetic costs of fatherhood.

The results from this dissertation suggest that fatherhood, under the conditions tested, has few pronounced or consistent effects on physiology, exercise performance, and morphology in male California mice. This dissertation has important implications for the understanding of evolution of biparental care and the life-history of this species.

Stress and glucocorticoids are hypothesized to mediate a trade-off between current and future reproduction, as organisms are predicted to invest less in current reproduction under stressful conditions. In this dissertation I explored whether stress and activation of the hypothalamic-pituitary-adrenal (HPA) axis (resulting in elevated glucocorticoid concentrations) suppress current reproductive behavior in the monogamous, biparental California mouse ( Peromyscus californicus). Initially, I characterized HPA function under baseline and post-stress conditions (Aim 1). I next tested the hypotheses that older animals are more stress-resistant than younger animals, as older animals have lower residual reproductive value (Aim 2), and that fathers are more stress-resistant than non-fathers (Aim 3). Lastly, I determined whether chronic stress (Aim 4) and acute glucocorticoid elevation, separate from external stress (Aim 5), would decrease paternal care and alter survival and development of offspring. Results demonstrate that HPA function in California mice differs markedly from that of previously studied rodents (Aim 1), and that changes in residual reproductive value (age; Aim 2) and reproductive status (Aim 3) do not modulate HPA axis activity. Chronic stress reduced paternal behavior (Aim 4), but effects were subtle and not likely mediated solely by changes in circulating corticosterone levels, as corticosterone injection did not alter paternal behavior (Aim 5). Moreover, decreased paternal behavior during chronic stress did not appreciably alter survival, development, growth or HPA function of pups, suggesting that stress-induced decreases in paternal care had no fitness consequences under controlled laboratory conditions. In conclusion, glucocorticoids do not appear to directly mediate the trade-off between current and future reproduction in males of this species; however, stress can decrease paternal behavior. Therefore, while corticosterone concentrations may serve as part of the signal by which parents determine reproductive investment, increases in this hormone alone may not be sufficient to derail current reproductive attempts. Instead, stress-induced changes in other hormones and neuropeptides might be important in regulating stress-related changes in behavior. These experiments provide important insight into interactions between reproductive status and HPA function in a biparental mammal, and could have implications for conservation of biparental species and for understanding effects of stress on human parenting.

In many species, caring for young is essential for reproductive success because it increases offspring survival, development, and quality. However, parental care can be costly by increasing resource expenditure and mortality risk and decreasing likelihood of future reproduction. Therefore, animals are predicted to adjust their levels of caregiving based on both intrinsic (e.g., sex, physical ability) and extrinsic factors (e.g., season, social interactions). Within-family dynamics are particularly interesting because an animal’s level of care determined by intrinsic factors may be influenced by responses to the behaviors of mates, offspring, or siblings. In biparental species, an individual’s social environment includes not only offspring, which can manipulate parents to garner greater levels of care, but also its mate. Little is known about how interactions within biparental families affect parental behavior. Thus, I examined intrinsic and social influences on caregiving behavior and the consequences for offspring in a biparental mammal, the California mouse (Peromyscus californicus). The first study investigated sex as a factor by determining whether parent-offspring discrimination differs between mothers and fathers. Throughout the pup-rearing period, I presented individual parents simultaneously with two mesh balls containing their own pup in one and an unrelated pup in the other. Although parents behaved similarly on individual test days, fathers showed significant changes in behaviors directed towards the non-kin ball over time, suggesting that fathers may show increased interest in unrelated young as their offspring become independent. In the second study, I examined parental behavior of both parents and offspring quality are affected by paternal age. Mothers mated to young fathers nursed pups more than those mated to old fathers; however, few other measures differed between families of old and young fathers. In the last study, I explored the effects of overlapping litters, in which subsequent litters are raised concurrently, on parent-offspring and sibling-sibling interactions and on offspring development.

In many biparental mammalian species, such as California mice (Peromyscus cali- fornicus), new fathers exhibit affiliative behavior toward unfamiliar infants, whereas reproductively naïve males show highly variable behavioral responses to infants. The sources of this variation are unknown. We investigated the effects of acute stress on pup-directed behavior in reproductively naïve male California mice. Each mouse underwent three 10-minute tests with an unfamiliar pup at 48-hour intervals. Males in the stressed group (N=22) were stressed using subcutane- ous oil injections, a common experimental stressor used in rodents, immediate- ly before each of the first two tests. The controls (N=22) were left undisturbed to avoid any experimentally induced stress. Compared to controls, stressed mice spent significantly less time performing paternal behavior in tests 1 and 2, while only marginal differences were seen in test 3. In tests 2 and 3, signifi- cantly fewer stressed mice interacted with the pup than controls. These findings suggest that acute stress experienced by reproductively naïve males might con- tribute to both short-term and long-term differences in pup-directed behavior.