Acute inhibition of dopamine ß-hydroxylase attenuates behavioral responses to pups in adult virgin California mice (Peromyscus californicus).

In biparental species, in which both parents care for their offspring, the neural and endocrine mediators of paternal behavior appear to overlap substantially with those underlying maternal behavior. Little is known, however, about the roles of classical neurotransmitters, such as norepinephrine (NE), in paternal care and whether they resemble those in maternal care. We tested the hypothesis that NE facilitates the initiation of nurturant behavior toward pups in virgin male and female California mice (Peromyscus californicus), a biparental rodent. Virtually all parents in this species are attracted to familiar and unfamiliar pups, while virgins either attack, avoid, or nurture pups, suggesting that the neurochemical control of pup-related behavior changes as mice transition into parenthood. We injected virgin males and females with nepicastat, a selective dopamine ß-hydroxylase inhibitor that blocks NE synthesis (75 mg/kg, i.p.), or vehicle 2 h before exposing them to a novel pup, estrous female (males only), or pup-sized novel object for 60 min. Nepicastat significantly reduced the number of males and females that approached the pup and that displayed parental behavior. In contrast, nepicastat did not alter virgins' interactions with an estrous female or a novel object, suggesting that nepicastat-induced inhibition of interactions with pups was not mediated by changes in generalized neophobia, arousal, or activity. Nepicastat also significantly reduced NE levels in the amygdala and prefrontal cortex and increased the ratio of dopamine to NE in the hypothalamus. Our results suggest that NE may facilitate the initiation of parental behavior in male and female California mice.

Specificity of California mouse pup vocalizations in response to olfactory stimuli.

To investigate flexibility in vocal signaling by rodent pups, we examined whether olfactory stimuli influence characteristics of pup calls and how these calls may be affected by sex and litter size in California mice (Peromyscus californicus). Pups were isolated and recorded during a 3-min baseline period followed by a 5-min exposure to bedding containing scent from their home cage, scent from the home cage of an unfamiliar family, coyote urine, or no scent (control). Latency to call, call rate, and call characteristics (duration, frequency, and amplitude) were compared between the baseline and scent-exposure periods and among olfactory conditions. Compared with the control condition, pups from two-pup litters called more quietly when exposed to odor from a predator, whereas pups from three-pup litters called more loudly. Additionally, pups showed nonsignificant tendencies to reduce call rates in response to odors from their home cage and to increase call rates when exposed to predator urine. Last, males produced higher-frequency calls and more ultrasonic vocalizations than females. These results indicate that pup calling behavior in this species can be influenced by acute olfactory stimuli as well as litter size and sex. The flexibility of pup calling in response to these three variables potentially increases the communication value of pup calls and helps shape the parents' responses.

Plasticity of the paternal brain: Effects of fatherhood on neural structure and function.

Care of infants is a hallmark of mammals. Whereas parental care by mothers is obligatory for offspring survival in virtually all mammals, fathers provide care for their offspring in only an estimated 5%-10% of genera. In these species, the transition into fatherhood is often accompanied by pronounced changes in males' behavioral responses to young, including a reduction in aggression toward infants and an increase in nurturant behavior. The onset of fatherhood can also be associated with sensory, affective, and cognitive changes. The neuroplasticity that mediates these changes is not well understood; however, fatherhood can alter the production and survival of new neurons; function and structure of existing neurons; morphology of brain structures; and neuroendocrine signaling systems. Although these changes are thought to promote infant care by fathers, very little evidence exists to support this hypothesis; in most cases, neither the mechanisms underlying neuroplasticity in fathers nor its functional significance is known. In this paper, we review the available data on the neuroplasticity that occurs during the transition into fatherhood. We highlight gaps in our knowledge and future directions that will provide key insights into how and why fatherhood alters the structure and functioning of the male brain.

Effects of single parenthood on mothers' behavior, morphology, and endocrine function in the biparental California mouse.

Motherhood is energetically costly for mammals and is associated with pronounced changes in mothers' physiology, morphology and behavior. In ~5% of mammals, fathers assist their mates with rearing offspring and can enhance offspring survival and development. Although these beneficial consequences of paternal care can be mediated by direct effects on offspring, they might also be mediated indirectly, through beneficial effects on mothers. We tested the hypothesis that fathers in the monogamous, biparental California mouse (Peromyscus californicus) reduce the burden of parental care on their mates, and therefore, that females rearing offspring with and without assistance from their mates will show differences in endocrinology, morphology and behavior, as well as in the survival and development of their pups. We found that pups' survival and development in the lab did not differ between those raised by a single mother and those reared by both mother and father. Single mothers spent more time in feeding behaviors than paired mothers. Both single and paired mothers had higher lean mass and/or lower fat mass and showed more anxiety-like behavior in open-field tests and tail-suspension tests, compared to non-breeding females. Single mothers had higher body-mass-corrected liver and heart masses, but lower ovarian and uterine masses, than paired mothers and/or non-breeding females. Mass of the gastrointestinal tract did not differ between single and paired mothers, but single mothers had heavier gastrointestinal tract compared to non-breeding females. Single motherhood also induced a flattened diel corticosterone rhythm and a blunted corticosterone response to stress, compared to non-breeding conditions. These findings suggest that the absence of a mate induces morphological and endocrine changes in mothers, which might result from increased energetic demands of pup care and could potentially help maintain normal survival and development of pups.

Maintenance of bone mass despite estrogen depletion in female common marmoset monkeys (Callithrix jacchus).

Estrogen depletion leads to bone loss in almost all mammals with frequent regular ovarian cycles. However, subordinate adult female common marmosets (Callithrix jacchus) undergo socially induced anovulation and hypoestrogenism without clinically apparent adverse skeletal consequences. Thus, we speculated that this non human primate might have evolved a mechanism to avoid estrogen-depletion bone loss. To test this possibility, we performed three experiments in which lumbar-spine (L5-L6) bone mineral content (BMC) and density (BMD) were assessed using dual-energy X-ray absorptiometry: (i) cross-sectionally in 13 long-term ovariectomized animals and 12 age- and weight-matched controls undergoing ovulatory cycles; (ii) longitudinally in 12 animals prior to, 3-4 and 6-7 months following ovariectomy (ovx), and six controls; and (iii) cross-sectionally in nine anovulatory subordinate and nine dominant females. In Experiments 1 and 3, plasma estradiol and estrone concentrations were measured and uterine dimensions were obtained by ultrasound in a subset of animals as a marker of functional estrogen depletion. Estrogen levels, uterine trans-fundus width, and uterine dorso-ventral diameter were lower in ovariectomized and subordinate females than in those undergoing ovulatory cycles. However, no differences were found in L5-L6 BMC or BMD. These results indicate that estrogen depletion, whether surgically or socially induced, is not associated with lower bone mass in female common marmosets. Thus, this species may possess unique adaptations to avoid bone loss associated with estrogen depletion.

Effects of a physical and energetic challenge on male California mice (Peromyscus californicus): modulation by reproductive condition.

Reproduction strongly influences metabolism, morphology and behavior in female mammals. In species in which males provide parental care, reproduction might have similar effects on fathers. We examined effects of an environmental challenge on metabolically important physiological, morphological and behavioral measures, and determined whether these effects differed between reproductive and non-reproductive males in the biparental California mouse (Peromyscus californicus). Males were paired with an ovary-intact female, an ovariectomized female treated with estrogen and progesterone to induce estrus, or an untreated ovariectomized female. Within each group, half of the animals were housed under standard laboratory conditions and half in cages requiring them to climb wire towers to obtain food and water; these latter animals were also fasted for 24 h every third day. We predicted that few differences would be observed between fathers and non-reproductive males under standard conditions, but that fathers would be in poorer condition than non-reproductive males under challenging conditions. Body and fat mass showed a housing condition×reproductive group interaction: the challenge condition increased body and fat mass in both groups of non-reproductive males, but breeding males were unaffected. Males housed under the physical and energetic challenge had higher blood lipid content, lower maximal aerobic capacity and related traits (hematocrit and relative triceps surae mass), increased pain sensitivity and increased number of fecal boli excreted during tail-suspension tests (a measure of anxiety), compared with controls. Thus, our physical and energetic challenge paradigm altered metabolism, morphology and behavior, but these effects were largely unaffected by reproductive condition.

Effects of repeated pup exposure on behavioral, neural, and adrenocortical responses to pups in male California mice (Peromyscus californicus).

In biparental mammals, the factors facilitating the onset of male parental behavior are not well understood. While hormonal changes in fathers may play a role, prior experience with pups has also been implicated. We evaluated effects of prior exposure to pups on paternal responsiveness in the biparental California mouse (Peromyscus californicus). 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 20min each in the previous week, and new fathers. Control groups of virgins were similarly tested with a novel object (marble). Previous exposure to pups decreased virgins' latency to approach pups and initiate paternal care, and increased time spent in paternal care. Responses to pups did not differ between virgins with repeated exposure to pups and new fathers. In contrast, repeated exposure to a marble had no effects. Neither basal corticosterone levels nor corticosterone levels following acute pup or marble exposure differed among groups. Finally, Fos expression in the medial preoptic area, ventral and dorsal 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 facilitate the onset of parental behavior in male California mice, similar to findings in female rodents, and that this effect is not associated with a general reduction in neophobia.

Fathering in rodents: Neurobiological substrates and consequences for offspring.

This article is part of a Special Issue "Parental Care". Paternal care, though rare among mammals, is routinely displayed by several species of rodents. Here we review the neuroanatomical and hormonal bases of paternal behavior, as well as the behavioral and neuroendocrine consequences of paternal behavior for offspring. Fathering behavior is subserved by many of the same neural substrates which are also involved in maternal behavior (for example, the medial preoptic area of the hypothalamus). While gonadal hormones such as testosterone, estrogen, and progesterone, as well as hypothalamic neuropeptides such as oxytocin and vasopressin, and the pituitary hormone prolactin, are implicated in the activation of paternal behavior, there are significant gaps in our knowledge of their actions, as well as pronounced differences between species. Removal of the father in biparental species has long-lasting effects on behavior, as well as on these same neuroendocrine systems, in offspring. Finally, individual differences in paternal behavior can have similarly long-lasting, if more subtle, effects on offspring behavior. Future studies should examine similar outcome measures in multiple species, including both biparental species and closely related uniparental species. Careful phylogenetic analyses of the neuroendocrine systems presumably important to male parenting, as well as their patterns of gene expression, will also be important in establishing the next generation of hypotheses regarding the regulation of male parenting behavior.

Effects of Parental Status on Male Body Mass in the Monogamous, Biparental California Mouse.

Studies of biparental mammals demonstrate that males may undergo systematic changes in body mass as a consequence of changes in reproductive status; however, these studies typically have not teased apart effects of specific social and reproductive factors, such as cohabitation with a female per se, cohabitation with a breeding female specifically, and engagement in paternal care. We aimed to determine whether California mouse (Peromyscus californicus) fathers undergo systematic changes in body mass and if so, which specific social/reproductive factor(s) might contribute to these changes. We compared mean weekly body masses over a 5-week period in 1) males housed with another male vs. males housed with a non-reproductive (tubally ligated) female; 2) males housed with a tubally ligated female vs. males housed with a female that was undergoing her first pregnancy; and 3) experienced fathers housed with vs. without pups during their mate's subsequent pregnancy. Body mass did not differ between males housed with another male and those housed with a non-reproductive female; however, males housed with a non-reproductive female were significantly heavier than those housed with a primiparous female. Among experienced fathers, those housed with pups from their previous litter underwent significant increases in body mass across their mates' pregnancy, whereas fathers housed without pups did not. These results suggest that male body mass is reduced by cohabitation with a breeding (pregnant) female, but not by cohabitation with a non-reproductive female, and that increases in body mass across the mate's pregnancy are associated with concurrent care of offspring rather than cohabitation with a pregnant female. Additional work is needed to determine the mechanisms and functional significance, if any, of these changes in male body mass with reproductive condition.

Differences in placentophagia in relation to reproductive status in the California mouse (Peromyscus californicus).

Parturient females ingest placenta in most mammalian species, whereas fathers may do so in species in which both parents provide care for their offspring. To determine if the propensity to eat placenta varies with reproductive status in the biparental California mouse, we presented placenta to virgin (housed with a same-sex pairmate), expectant (pregnant with their first litter), and multiparous adult males and females. Liver was presented identically, 3-7 days later, as a control. Multiparous females were more likely to eat placenta than expectant and virgin females (p-values <0.016), whereas both multiparous and expectant males had higher incidences of placentophagia than virgins (p-values <0.016). Liver consumption did not differ among groups within either sex. These results suggest that propensity to eat placenta increases with maternal/birthing experience in females, and with paternal experience and/or cohabitation with a pregnant female in males.

Inhibition of alloparental behavior by acute stress in virgin male California mice (Peromyscus californicus).

In many biparental mammals, such as California mice (Peromyscus californicus), fathers display affiliative behavior toward unfamiliar infants whereas reproductively naïve adult males show highly variable responses. Sources of this variability are not well understood, but evidence suggests that stress can either enhance or inhibit alloparental care. We evaluated immediate and delayed effects of acute stress on pup-directed behavior in adult virgin male California mice. Mice underwent three 10-minute tests with unfamiliar pups at 48-hour intervals. Stressed mice (N=22) received a subcutaneous oil injection immediately before tests 1 and 2, whereas controls (N=22) were left undisturbed. In controls, but not stressed mice, latency to approach the pup decreased and duration of alloparental behavior increased across the three tests. At each time point, stressed males were less likely than controls to perform alloparental behavior. Controls spent significantly more time performing alloparental behavior than stressed mice in tests 1 and 2 but not in test 3. Pup-directed aggression did not differ between the groups at any time point. These findings suggest that acute stress can both inhibit alloparental behavior in the short term and prevent the increase in alloparental behavior that typically occurs with repeated exposure to pups in virgin male California mice.

Effects of acute inhibition of dopamine ß-hydroxylase on neural responses to pups in adult virgin male California mice (Peromyscus californicus).

The neural mechanisms underlying paternal care in biparental mammals are not well understood. The California mouse (Peromyscus californicus) is a biparental rodent in which virtually all fathers are attracted to pups, while virgin males vary widely in their behavior toward unrelated infants, ranging from attacking to avoiding to huddling and grooming pups. We previously showed that pharmacologically inhibiting the synthesis of the neurotransmitter norepinephrine (NE) with the dopamine ß-hydroxylase inhibitor nepicastat reduced the propensity of virgin male and female California mice to interact with pups. The current study tested the hypothesis that nepicastat would reduce pup-induced c-Fos immunoreactivity, a cellular marker of neural activity, in the medial preoptic area (MPOA), medial amygdala (MeA), basolateral amygdala (BLA), and bed nucleus of the stria terminalis (BNST), brain regions implicated in the control of parental behavior and/or anxiety. Virgin males were injected with nepicastat (75 mg/kg, i.p.) or vehicle 2 hours prior to exposure to either an unrelated pup or novel object for 60 minutes (n = 4-6 mice per group). Immediately following the 60-minute stimulus exposure, mice were euthanized and their brains were collected for c-Fos immunohistochemistry. Nepicastat reduced c-Fos expression in the MeA and MPOA of pup-exposed virgin males compared to vehicle-injected controls. In contrast, nepicastat did not alter c-Fos expression in any of the above brain regions following exposure to a novel object. Overall, these results suggest that the noradrenergic system might influence MeA and MPOA function to promote behavioral interactions with pups in virgin males.

Balancing Act: an Interdisciplinary Exploration of Trade-offs in Reproducing Females.

Trade-offs resulting from the high demand of offspring production are a central focus of many subdisciplines within the field of biology. Yet, despite the historical and current interest on this topic, large gaps in our understanding of whole-organism trade-offs that occur in reproducing individuals remain, particularly as it relates to the nuances associated with female reproduction. This volume of Integrative and Comparative Biology (ICB) contains a series of papers that focus on reviewing trade-offs from the female-centered perspective of biology (i.e., a perspective that places female reproductive biology at the center of the topic being investigated or discussed). These papers represent some of the work showcased during our symposium held at the 2024 meeting of the Society for Integrative and Comparative Biology (SICB) in Seattle, Washington. In this roundtable discussion, we use a question-and-answer format to capture the diverse perspectives and voices involved in our symposium. We hope that the dialogue featured in this discussion will be used to motivate researchers interested in understanding trade-offs in reproducing females and provide guidance on future research endeavors.

Chronic variable stress in fathers alters paternal and social behavior but not pup development in the biparental California mouse (Peromyscus californicus).

Stress and chronically elevated glucocorticoid levels have been shown to disrupt parental behavior in mothers; however, almost no studies have investigated corresponding effects in fathers. The present experiment tested the hypothesis that chronic variable stress inhibits paternal behavior and consequently alters pup development in the monogamous, biparental California mouse (Peromyscus californicus). First-time fathers were assigned to one of three experimental groups: chronic variable stress (CVS, n=8), separation control (SC, n=7), or unmanipulated control (UC, n=8). The CVS paradigm (3 stressors per day for 7 days) successfully stressed mice, as evidenced by increased baseline plasma corticosterone concentrations, increased adrenal mass, decreased thymus mass, and a decrease in body mass over time. CVS altered paternal and social behavior of fathers, but major differences were observed only on day 6 of the 7-day paradigm. At that time point, CVS fathers spent less time with their pairmate and pups, and more time autogrooming, as compared to UC fathers; SC fathers spent more time behaving paternally and grooming the female mate than CVS and UC fathers. Thus, CVS blocked the separation-induced increase in social behaviors observed in the SC fathers. Nonetheless, chronic stress in fathers did not appear to alter survival or development of their offspring: pups from the three experimental conditions did not differ in body mass gain over time, in the day of eye opening, or in basal or post-stress corticosterone levels. These results demonstrate that chronic stress can transiently disrupt paternal and social behavior in P. californicus fathers, but does not alter pup development or survival under controlled, non-challenging laboratory conditions.

Effects of aging on hypothalamic-pituitary-adrenal (HPA) axis activity and reactivity in virgin male and female California mice (Peromyscus californicus).

Life history theory posits that organisms face a trade-off between current and future reproductive attempts. The physiological mechanisms mediating such trade-offs are still largely unknown, but glucocorticoid hormones are likely candidates as elevated, post-stress glucocorticoid levels have been shown to suppress both reproductive physiology and reproductive behavior. Aged individuals have a decreasing window in which to reproduce, and are thus predicted to invest more heavily in current as opposed to future reproduction. Therefore, if glucocorticoids are important in mediating the trade-off between current and future reproduction, aged animals are expected to show decreased hypothalamic-pituitary-adrenal (HPA) axis responses to stressors and to stimulation by corticotropin-releasing hormone (CRH), and enhanced responses to glucocorticoid negative feedback, as compared to younger animals. We tested this hypothesis in the monogamous, biparental California mouse by comparing baseline and post-stress corticosterone levels, as well as corticosterone responses to dexamethasone (DEX) and CRH injections, between old (∼18-20months) and young (∼4months) virgin adults of both sexes. We also measured gonadal and uterine masses as a proxy for investment in potential current reproductive effort. Adrenal glands were weighed to determine if older animal had decreased adrenal mass. Old male mice had lower plasma corticosterone levels 8h after DEX injection than did young male mice, suggesting that the anterior pituitary of older males is more sensitive to DEX-induced negative feedback. Old female mice had higher body-mass-corrected uterine mass than did young females. No other differences in corticosterone levels or organ masses were found between age groups within either sex. In conclusion, we did not find strong evidence for age-related change in HPA activity or reactivity in virgin adult male or female California mice; however, future studies investigating HPA activity and reproductive outcomes in young and old breeding adults would be illuminating.

The parental umwelt: Effects of parenthood on sensory processing in rodents.

An animal's umwelt, comprising its perception of the sensory environment, which is inherently subjective, can change across the lifespan in accordance with major life events. In mammals, the onset of motherhood, in particular, is associated with a neural and sensory plasticity that alters a mother's detection and use of sensory information such as infant-related sensory stimuli. Although the literature surrounding mammalian mothers is well established, very few studies have addressed the effects of parenthood on sensory plasticity in mammalian fathers. In this review, we summarize the major findings on the effects of parenthood on behavioural and neural responses to sensory stimuli from pups in rodent mothers, with a focus on the olfactory, auditory, and somatosensory systems, as well as multisensory integration. We also review the available literature on sensory plasticity in rodent fathers. Finally, we discuss the importance of sensory plasticity for effective parental care, hormonal modulation of plasticity, and an exploration of temporal, ecological, and life-history considerations of sensory plasticity associated with parenthood. The changes in processing and/or perception of sensory stimuli associated with the onset of parental care may have both transient and long-lasting effects on parental behaviour and cognition in both mothers and fathers; as such, several promising areas of study, such as on the molecular/genetic, neurochemical, and experiential underpinnings of parenthood-related sensory plasticity, as well as determinants of interspecific variation, remain potential avenues for further exploration.

Hypothalamic-pituitary-adrenal (HPA) axis function in the California mouse (Peromyscus californicus): Changes in baseline activity, reactivity, and fecal excretion of glucocorticoids across the diurnal cycle.

The California mouse, Peromyscus californicus, is an increasingly popular animal model in behavioral, neural, and endocrine studies, but little is known about its baseline hypothalamic-pituitary-adrenal (HPA) axis activity or HPA responses to stressors. We characterized plasma corticosterone (CORT) concentrations in P. californicus under baseline conditions across the diurnal cycle, in response to pharmacological manipulation of the HPA axis, and in response to a variety of stressors at different times of day. In addition, we explored the use of fecal samples to monitor adrenocortical activity non-invasively. California mice have very high baseline levels of circulating CORT that change markedly over 24h, but that do not differ between the sexes. This species may be somewhat glucocorticoid-resistant in comparison to other rodents as a relatively high dose of dexamethasone (5mg/kg, s.c.) was required to suppress plasma CORT for 8h post-injection. CORT responses to stressors and ACTH injection differed with time of day, as CORT concentrations were elevated more readily during the morning (inactive period) than in the evening (active period) when compared to time-matched control. Data from (3)H-CORT injection studies show that the time course for excretion of fecal CORT, or glucocorticoid metabolites, differs with time of injection. Mice injected in the evening excreted the majority of fecal radioactivity 2-4h post-injection whereas mice injected during the morning did so at 14-16h post-injection. Unfortunately, the antibody we used does not adequately bind the most prevalent fecal glucocorticoid metabolites and therefore we could not validate its use for fecal assays.

Neural responses to pup calls and pup odors in California mouse fathers and virgin males.

The onset of mammalian maternal care is associated with plasticity in neural processing of infant-related sensory stimuli; however, little is known about sensory plasticity associated with fatherhood. We quantified behavioral and neural responses of virgin males and new fathers to olfactory and auditory stimuli from young, unfamiliar pups in the biparental California mouse (Peromyscus californicus). Each male was exposed for 10 min to one of four combinations of a chemosensory stimulus (pup-scented or unscented cotton [control]) and an auditory stimulus (pup vocalizations or white noise [control]). Behavior did not differ between fathers and virgins during exposure to sensory stimuli or during the following hour; however, males in both groups were more active both during and after exposure to pup-related stimuli compared to control stimuli. Fathers had lower expression of Fos in the main olfactory bulbs (MOB) but higher expression in the medial preoptic area (MPOA) and bed nucleus of the stria terminalis medial division, ventral part (STMV) compared to virgins. Lastly, males had higher Fos expression in MPOA when exposed to pup odor compared to control stimuli, and when exposed to pup odor and pup calls compared to pup calls only or control stimuli. These findings suggest that the onset of fatherhood alters activity of MOB, MPOA and STMV and that pup odors and vocalizations have additive or synergistic effects on males' behavior and MPOA activation.

Acute effects of corticosterone injection on paternal behavior in California mouse (Peromyscus californicus) fathers.

Glucocorticoids are thought to mediate the disruption of parental behavior in response to acute and chronic stress. Previous research supports their role in chronic stress; however, no study has experimentally tested the effects of acute glucocorticoid elevation on paternal behavior. We tested the prediction that acute corticosterone (CORT) increases would decrease paternal behavior in California mouse fathers and would lead to longer-term effects on reproductive success, as even short-term increases in CORT have been shown to produce lasting effects on the hypothalamic-pituitary-adrenal axis. First-time fathers were injected with 30 mg/kg CORT, 60 mg/kg CORT or vehicle, or left unmanipulated. Interactions between the male and its pup(s) were recorded 1.5-2h after injection and scored for paternal and non-paternal behavior. Treatment groups were combined into control (unmanipulated + vehicle, n = 15) and CORT (30 mg/kg + 60 mg/kg, n = 16) for analysis based on resulting plasma CORT concentrations. CORT treatment did not alter paternal or non-paternal behaviors or any long-term measures (male body mass or temperature, pup growth rate, pup survival, interbirth interval, number or mass of pups born in the second litter). Fathers showed a significant rise in body mass at day 30 postpartum, followed by a decrease in body mass after the birth of the second litter; however, this pattern did not differ between the CORT and control groups. In summary, acute elevation of plasma CORT did not alter direct paternal behavior, body mass, or reproductive outcomes, suggesting that acute CORT elevation alone does not overtly disrupt paternal care in this biparental mammal.

Effects of reproductive status on behavioral and endocrine responses to acute stress in a biparental rodent, the California mouse (Peromyscus californicus).

In several mammalian species, lactating females show blunted neural, hormonal, and behavioral responses to stressors. It is not known whether new fathers also show stress hyporesponsiveness in species in which males provide infant care. To test this possibility, we determined the effects of male and female reproductive status on stress responsiveness in the biparental, monogamous California mouse (Peromyscus californicus). Breeding (N=8 females, 8 males), nonbreeding (N=10 females, 10 males) and virgin mice (N=12 females, 9 males) were exposed to a 5-min predator-urine stressor at two time points, corresponding to the early postpartum (5-7 days postpartum) and mid/late postpartum (19-21 days postpartum) phases, and blood samples were collected immediately afterwards. Baseline blood samples were obtained 2 days prior to each stress test. Baseline plasma corticosterone (CORT) concentrations did not differ among male or female groups. CORT responses to the stressor did not differ among female reproductive groups, and all three groups showed distinct behavioral responses to predator urine. Virgin males tended to increase their CORT response from the first to the second stress test, while breeding and nonbreeding males did not. Moreover, virgin and nonbreeding males showed significant behavioral changes in response to predator urine, whereas breeding males did not. These results suggest that adrenocortical responses to a repeated stressor in male California mice may be modulated by cohabitation with a female, whereas behavioral responses to stress may be blunted by parental status.