Fathering in rodents: Neurobiological substrates and consequences for offspring

Sex and Age Differences in Ontogeny of Alloparenting: A Relation to Forebrain DRD1, DRD2, and HTR2A mRNA Expression?

Alloparenting refers to the practice of caring for the young by individuals other than their biological parents. The relationship between the dynamic changes in psychological functions underlying alloparenting and the development of specific neuroreceptors remains unclear. Using a classic 10-day pup sensitization procedure, together with a pup preference and pup retrieval test on the EPM (elevated plus maze), we showed that both male and female adolescent rats (24 days old) had significantly shorter latency than adult rats (65 days old) to be alloparental, and their motivation levels for pups and objects were also significantly higher. In contrast, adult rats retrieved more pups than adolescent rats even though they appeared to be more anxious on the EPM. Analysis of mRNA expression using real-time-PCR revealed a higher dopamine D2 receptor (DRD2) receptor expression in adult hippocampus, amygdala, and ventral striatum, along with higher dopamine D1 receptor (DRD1) receptor expression in ventral striatum compared to adolescent rats. Adult rats also showed significantly higher levels of 5-hydroxytryptamine receptor 2A (HTR2A) receptor expression in the medial prefrontal cortex, amygdala, ventral striatum, and hypothalamus. These results suggest that the faster onset of alloparenting in adolescent rats compared to adult rats, along with the psychological functions involved, may be mediated by varying levels of dopamine DRD1, DRD2, and HTR2A in different forebrain regions.

Paternal absence and increased caregiving independently and interactively shape the development of male prairie voles at subadult and adult life stages

The influence of maternal caregiving is a powerful force on offspring development. The absence of a father during early life in biparental species also has profound implications for offspring development, although it is far less studied than maternal influences. Moreover, we have limited understanding of the interactive forces that maternal and paternal caregiving impart on offspring. We investigated if behaviorally upregulating maternal care compensates for paternal absence on prairie vole (Microtus ochrogaster) pup development. We used an established handling manipulation to increase levels of caregiving in father-absent and biparental families, and later measured male offspring behavioral outcomes at sub-adulthood and adulthood. Male offspring raised without fathers were more prosocial (or possibly less socially anxious) than those raised biparentally. Defensive behavior and responses to contextual novelty were also influenced by the absence of fathers, but only in adulthood. Offensive aggression and movement in the open field test changed as a function of life-stage but not parental exposure. Notably, adult pair bonding was not impacted by our manipulations. Boosting parental care produced males that moved more in the open field test. Parental handling also increased oxytocin immunoreactive cells within the supraoptic nucleus of the hypothalamus (SON), and in the paraventricular nucleus (PVN) of biparentally-reared males. We found no differences in vasopressinergic cell groups. We conclude that male prairie voles are contextually sensitive to the absence of fathers and caregiving intensity. Our study highlights the importance of considering the ways early experiences synergistically shape offspring behavioral and neural phenotypes across the lifespan.

Conserved and differing functions of the endocrine system across different social systems - oxytocin as a case study

A key goal of the field of endocrinology has been to understand the hormonal mechanisms that drive social behavior and influence reactions to others, such as oxytocin. However, it has sometimes been challenging to understand which aspects and influences of hormonal action are conserved and common among mammalian species, and which effects differ based on features of these species, such as social system. This challenge has been exacerbated by a focus on a relatively small number of traditional model species. In this review, we first demonstrate the benefits of using non-traditional models for the study of hormones, with a focus on oxytocin as a case study in adding species with diverse social systems. We then expand our discussion to explore differing effects of oxytocin (and its response to behavior) within a species, with a particular focus on relationship context and social environment among primate species. Finally, we suggest key areas for future exploration of oxytocin's action centrally and peripherally, and how non-traditional models can be an important resource for understanding the breadth of oxytocin's potential effects.

Reproductive function and behaviors: an update on the role of neural estrogen receptors alpha and beta

Infertility is becoming a major public health problem, with increasing frequency due to medical, environmental and societal causes. The increasingly late age of childbearing, growing exposure to endocrine disruptors and other reprotoxic products, and increasing number of medical reproductive dysfunctions (endometriosis, polycystic ovary syndrome, etc.) are among the most common causes. Fertility relies on fine-tuned control of both neuroendocrine function and reproductive behaviors, those are critically regulated by sex steroid hormones. Testosterone and estradiol exert organizational and activational effects throughout life to establish and activate the neural circuits underlying reproductive function. This regulation is mediated through estrogen receptors (ERs) and androgen receptor (AR). Estradiol acts mainly via nuclear estrogen receptors ERα and ERβ. The aim of this review is to summarize the genetic studies that have been undertaken to comprehend the specific contribution of ERα and ERβ in the neural circuits underlying the regulation of the hypothalamic-pituitary-gonadal axis and the expression of reproductive behaviors, including sexual and parental behavior. Particular emphasis will be placed on the neural role of these receptors and the underlying sex differences.

Father's Absence in the Mongolian gerbil (Meriones unguiculatus) is associated with alterations in paternal behavior, T, cort, presence of ERα, and AR in mPOA/ BNST

Testosterone (T), estrogen receptor alpha (ERα), and androgen receptor (AR) play a significant role in the regulation of paternal behavior. We determined the effects of deprivation of paternal care on alterations in paternal behavior, T concentrations in plasma, and the presence of ERα and AR in the medial preoptic area (mPOA), bed nucleus of the stria terminalis (BNST), medial amygdala (MeA), and olfactory bulb (OB), as well as the corticosterone (CORT) concentrations in plasma caused by deprivation of paternal care in the Mongolian gerbil (Meriones unguiculatus). Twenty pairs of gerbils were formed; the pups were deprived of paternal care (DPC) in 10 pairs. In another 10 pairs, the pups received paternal care (PC). Ten males raised in DPC condition and 10 males raised in PC conditions were mated with virgin females. When they became fathers, each DPC male and PC male was subjected to tests of paternal behavior on day three postpartum. Blood samples were obtained to quantify T and CORT concentrations, and the brains were removed for ERα and AR immunohistochemistry analyses. DPC males gave less care to their pups than PC males, and they had significantly lower T concentrations and levels of ERα and AR in the mPOA and BNST than PC males. DPC males also had higher CORT concentrations than PC males. These results suggest that in the Mongolian gerbil father's absence causes a decrease in paternal care in the offspring, which is associated with alterations in the neuroendocrine mechanisms that regulate it.

Defence against the intergenerational cost of reproduction in males: oxidative shielding of the germline

Reproduction is expected to carry an oxidative cost, yet in many species breeders appear to sustain lower levels of oxidative damage compared to non-breeders. This paradox may be explained by considering the intergenerational costs of reproduction. Specifically, a reduction in oxidative damage upon transitioning to a reproductive state may represent a pre-emptive shielding strategy to protect the next generation from intergenerational oxidative damage (IOD) - known as the oxidative shielding hypothesis. Males may be particularly likely to transmit IOD, because sperm are highly susceptible to oxidative damage. Yet, the possibility of male-mediated IOD remains largely uninvestigated. Here, we present a conceptual and methodological framework to assess intergenerational costs of reproduction and oxidative shielding of the germline in males. We discuss variance in reproductive costs and expected payoffs of oxidative shielding according to species' life histories, and the expected impact on offspring fitness. Oxidative shielding presents an opportunity to incorporate intergenerational effects into the advancing field of life-history evolution.

Shaping the development of complex social behavior

Early life experiences can have an enduring impact on the brain and behavior, with implications for stress reactivity, cognition, and social behavior. In particular, the neural systems that contribute to the expression of social behavior are altered by early life social environments. However, paradigms that have been used to alter the social environment during development have typically focused on exposure to stress, adversity, and deprivation of species-typical social stimulation. Here, we explore whether complex social environments can shape the development of complex social behavior. We describe lab-based paradigms for studying early life social complexity in rodents that are generally focused on enriching the social and sensory experiences of the neonatal and juvenile periods of development. The impact of these experiences on social behavior and neuroplasticity is highlighted. Finally, we discuss the degree to which our current approaches for studying social behavior outcomes give insight into "complex" social behavior and how social complexity can be better integrated into lab-based methodologies.

Father contribution to human resilience

Fathers have been an important source of child endurance and prosperity since the dawn of civilization, promoting adaptation to social rules, defining cultural meaning systems, teaching daily living skills, and providing the material background against which children developed; still, the recent reformulation in the role of the father requires theory-building. Paternal caregiving is rare in mammals, occurring in 3-5% of species, expresses in multiple formats, and involves flexible neurobiological accommodations to ecological conditions and active caregiving. Here, we discuss father contribution to resilience across development. Our model proposes three tenets of resilience - plasticity, sociality, and meaning - and discussion focuses on father-specific contributions to each tenet at different developmental stages; newborn, infant, preschooler, child, and adolescent. Father's style of high arousal, energetic physicality, guided participation in daily skills, joint adventure, and conflict resolution promotes children's flexible approach and social competence within intimate bonds and social groups. By expanding children's interests, sharpening cognitions, tuning affect regulation, encouraging exploration, and accompanying the search for identity, fathers support the sense of meaning, enhancing the human-specific dimension of resilience. We end by highlighting pitfalls to paternal contribution, including absence, abuse, rigidity, expectations, and gender typing, and the need to formulate novel theories to accommodate the "involved dad."

Paternal deprivation induces vigilance-avoidant behavior and accompanies sex-specific alterations in stress reactivity and central proinflammatory cytokine response in California mice (Peromyscus californicus)

RATIONALE

Early-life stress (ELS) can increase anxiety, reduce prosocial behaviors, and impair brain regions that facilitate emotional and social development. This knowledge greatly stems from assessing disrupted mother-child relationships, while studies investigating the long-term effects of father-child relationships on behavioral development in children are scarce. However, available evidence suggests that fathers may uniquely influence a child's behavioral development in a sex-specific manner. Rodent models examining mother-offspring interaction demonstrate relationships among ELS, neuroinflammatory mediators, and behavioral development; yet, the role paternal care may play in neuroimmune functioning remains unreported.

OBJECTIVES

Using the biparental California mouse (Peromyscus californicus), we examined to what extent paternal deprivation impairs social and anxiety-like behaviors, augments peripheral corticosterone (CORT) response, and alters central proinflammatory cytokine production following an acute stressor in adulthood.

METHODS

Biparentally reared and paternally deprived (permanent removal of the sire 24 h post-birth) adult mice were assessed for sociability, preference for social novelty, social vigilance, and social avoidance behaviors, followed by novelty-suppressed feeding (NSF) testing for general anxiety-like behavior. Following an acute stressor, circulating CORT concentrations and region-specific proinflammatory cytokine concentrations were determined via radioimmunoassay and Luminex multianalyte analysis, respectively.

RESULTS

In response to a novel same-sex conspecific, social vigilance behavior was associated with reduced sociability and increased avoidance in paternally deprived mice-an effect not observed in biparentally reared counterparts. Yet, in response to a familiar same-sex conspecific, social vigilance persisted but only in paternally deprived females. The latency to consume during NSF testing was not significantly altered by paternal deprivation. In response to an acute physical stressor, lower circulating CORT concentrations were observed in paternally deprived females. Compared to control-reared males, paternal deprivation increased hypothalamic interleukin-1β, but decreased hippocampal IL-6 protein concentration.

CONCLUSION

Greater social vigilance behavior was demonstrated in paternally deprived mice while they avoided social interaction with a novel same-sex conspecific; however, in response to a familiar same-sex conspecific, paternal deprivation increased social vigilance behavior but only in females. It is possible that different neurobiological mechanisms underlie these observed behavioral outcomes as sex-specific central proinflammatory cytokine and stress responsivity were observed in paternally deprived offspring.

Compensatory enhancement of paternal care in maternally neglected mice family

Parental care strategies, ranging from biparental to uniparental, evolve based on factors affecting sexual conflict over care. Plasticity in how parents respond to reduction in each other's care effort is thus proposed to be important in the evolution of parental care behaviors. Models predict that 'obligate' biparental care is stable when a parent responds to reduced partner effort with 'partial' compensation, trading-off current and future reproduction. A meta-analysis of experimental studies on biparental birds also revealed partial compensation, supporting coevolution of parental care type and plasticity pattern. However, few studies have addressed this issue across different taxa and different parental care types. In laboratory mice, a female-biased 'facultative' biparental species, fathers paired with a competent mother rarely provide care. We show that, when mated with a pup-neglecting mutant mother, fathers increased care effort to 'fully' compensate for the lost maternal care in both pup survival rate and total care amount. Pup retrieval latency was significantly shorter, and neural activity in relevant brain regions twice as high, suggesting enhanced motivation. This study with mice not only opens a road to explore the neural correlates of paternal plasticity but will also help understand how behavioral plasticity contributes to adaptive evolution of parental care behaviors.

Mating-induced prolactin surge is not required for subsequent neurogenesis in male mice

Parenting involves major behavioral transitions that are supported by coordinated neuroendocrine and physiological changes to promote the onset of novel offspring-directed behaviors. In comparison to maternal care, however, the mechanisms underlying the transition to paternal care are less understood. Male laboratory mice are predominantly infanticidal as virgins but show paternal responses 2 weeks after mating. Interestingly, males show a mating-induced surge of prolactin, which we hypothesized may be involved in initiating this behavioral transition. During pregnancy, prolactin stimulates olfactory bulb neurogenesis, which is essential for maternal behavior. Mating induces olfactory bulb neurogenesis in males, but it is unknown whether this is driven by prolactin or is important for subsequent paternal care. New olfactory neurons are generated from cells in the subventricular zone (SVZ) and take about 2 weeks to migrate to the olfactory bulb, which may account for the delayed behavioral change in mated males. We investigated whether mating increases cell proliferation at the SVZ. Males were either mated, exposed to receptive female cues, or left alone (control) and injected with Bromodeoxyuridine (BrdU, a marker of cell division). Contrary to our hypothesis, we found that mating decreased cell proliferation in the caudal lateral portion of the SVZ. Next, we tested whether prolactin itself mediates cell proliferation in the SVZ and/or new cell survival in the olfactory bulb by administering bromocriptine (prolactin inhibitor), vehicle, or bromocriptine + prolactin prior to mating. While suppressing prolactin had no effect on cell proliferation in the SVZ, administering exogenous prolactin resulted in significantly higher BrdU-labeled cells in mated but not virgin male mice. No effects of prolactin were observed on new olfactory cell survival. Taken together, prolactin may have context-dependent effects on new cell division in the SVZ, while other unknown mechanisms may be driving the effects on new olfactory cell survival following mating.

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.

Contributory Parenting: A "Priceless Shift" from Indirect to Direct Parenting

Parenting is a valuable investment that determines the quality of future independent life. From an evolutionary aspect, it has been well ingrained in the minds of humans as to how much resource each parent should contribute to this energy and time-consuming task. To encourage father's contribution towards parenting and reduce the stress on mother, the concept of paid paternal leave has been implemented. Mere presence of the father in terms of the quantity of time spent without much qualitative value has no benefit, but the assumption that fathers are less competent based on their lower performance might also not be acceptable. An intriguing finding has demonstrated that prolonged contact with the infant triggers a change in previously absent male parenting behavior. With incentives on one hand and associated societal stigma on the other hand, it is to be analyzed whether the purpose of true parenting by fathers is being achieved. Hence, in the concept of contributory parenting it is necessary to recognize and respect each parenting style with the ultimate benefit being passed on to the child.

Functional relationships between estradiol and paternal care in male red-bellied lemurs, Eulemur rubriventer

Fathers contribute substantially to infant care, yet the mechanisms facilitating paternal bonding and interactions with infants are not as well understood as they are in mothers. Several hormonal changes occur as males transition into parenthood, first in response to a partner's pregnancy, and next in response to interacting with the newborn. These changes may prepare fathers for parenting and help facilitate and maintain paternal care. Experimental studies with monkeys and rodents suggest that paternal care requires elevated estradiol levels, which increase when a male's partner is pregnant and are higher in fathers than non-fathers, but its role in the expression of paternal behaviors throughout infant development is unknown. To assess estradiol's role in paternal care, we analyzed the relationship between paternal estradiol metabolites and 1) offspring age, and 2) paternal care behavior (holding, carrying, huddling, playing, grooming), in wild, red-bellied lemurs (Eulemur rubriventer). We collected 146 fecal samples and 1597 h of behavioral data on 10 adult males who had newborn infants during the study. Estradiol metabolites increased four-fold in expectant males, and in new fathers they fluctuated and gradually decreased with time. Infant age, not paternal behavior, best predicted hormone levels in new fathers. These results suggest that hormonal changes occur in expectant males with facultative paternal care, but they do not support the hypothesis that estradiol is directly associated with the day-to-day expression of paternal care. Future research should explore estradiol's role in facilitating behaviors, including infant-directed attention and responsiveness, or preparing fathers for infant care generally.

Plasticity in auditory cortex during parenthood

Most animals display robust parental behaviors that support the survival and well-being of their offspring. The manifestation of parental behaviors is accompanied by physiological and hormonal changes, which affect both the body and the brain for better care giving. Rodents exhibit a behavior called pup retrieval - a stereotyped sequence of perception and action - used to identify and retrieve their newborn pups back to the nest. Pup retrieval consists of a significant auditory component, which depends on plasticity in the auditory cortex (ACx). We review the evidence of neural changes taking place in the ACx of rodents during the transition to parenthood. We discuss how the plastic changes both in and out of the ACx support the encoding of pup vocalizations. Key players in the mechanism of this plasticity are hormones and experience, both of which have a clear dynamic signature during the transition to parenthood. Mothers, co caring females, and fathers have been used as models to understand parental plasticity at disparate levels of organization. Yet, common principles of cortical plasticity and the biological mechanisms underlying its involvement in parental behavior are just beginning to be unpacked.

Development of the Paternal Brain in Humans throughout Pregnancy

Previous studies have demonstrated that paternal caregiving behaviors are reliant on neural pathways similar to those supporting maternal care. Interestingly, a greater variability exists in parental phenotypes in men than in women among individuals and mammalian species. However, less is known about when or how such variability emerges in men. We investigated the longitudinal changes in the neural, hormonal, and psychological bases of expression of paternal caregiving in humans throughout pregnancy and the first 4 months of the postnatal period. We measured oxytocin and testosterone, paternity-related psychological traits, and neural response to infant-interaction videos using fMRI in first-time fathers and childless men at three time points (early to mid-pregnancy, late pregnancy, and postnatal). We found that paternal-specific brain activity in prefrontal areas distinctly develops during middle-to-late pregnancy and is enhanced in the postnatal period. In addition, among fathers, the timing of the development of prefrontal brain activity was associated with specific parenting phenotypes.

Proximate and Ultimate Mechanisms of Human Father-child Rough-and-tumble Play

The aim of this contribution is to attempt to understand the adaptive functions of father-child rough-and-tumble play (RTP) in humans. We first present a synthesis of the known proximate and ultimate mechanisms of peer-peer RTP in mammals and compare human parent-child RTP with peer-peer RTP. Next, we examine the possible biological adaptive functions of father-child RTP in humans, by comparing paternal behavior in humans versus biparental animal species, in light of the activation relationship theory and the neurobiological basis of fathering. Analysis of analogies reveals that the endocrine profile of fathers is highly variable across species, compared to that of mothers. This can be interpreted as fathers' evolutionary adjustment to specific environmental conditions affecting the care of the young. Given the high unpredictability and risk-taking features of RTP, we conclude that human adult-child RTP appears to have a biological adaptive function, one of 'opening to the world'.

Neural activation associated with maternal and aversive interactions with pups in the Mongolian gerbil (Meriones unguiculatus)

According to approach-avoidance model, virgin female laboratory rats display maternal behaviour when the tendency to approach and interact with the pup is stronger than avoiding it. A positive neural mechanism that includes the medial preoptic area (mPOA)/bed nucleus of the stria terminalis (BNST) and a negative mechanism that involves the anterior hypothalamic nucleus (AHN)/ventromedial nucleus (VMN)/ periaqueductal grey (PAG) underlie to these behaviours. Unlike virgin rats, which avoid the pups, virgin females Mongolian gerbils (Meriones unguiculatus) can be immediately either maternal or aggressive with the pups. Furthermore, the Mongolian gerbil is monogamous and biparental species. Despite these difference, we hypothesised that maternal and aggressive interaction with the pups could activate mPOA/BNST and AHN/VMH/PAG, respectively, and that maternal response could be associated with high concentrations of estradiol (E₂). Twenty virgin maternal females and 20 aggressive toward the pups were selected. Ten maternal females interacted with the pups (MAT-pups) and 10 with candy (MAT-candy). Of the 20 aggressive females, 10 interacted with the pups (AGG-pups) and 10 with candy (AGG-candy). Immediately after the test, blood samples were taken to quantify E₂. The brains were dissected for c-Fos immunohistochemistry. MAT-pups females had significantly higher activation in mPOA/BNST than MAT-candy females, while AGG-pups showed significant activation in AHN/VMH/PAG compared with AGG-candy females. The maternal response was associated with high concentrations of E₂. These results suggested a positive and a negative mechanism in the regulation of maternal behaviour in the Mongolian gerbil, and that the immediate maternal response could be due to high E₂ concentrations.

Plasticity in parental behavior and vasopressin: responses to co-parenting, pup age, and an acute stressor are experience-dependent

Introduction

The impact of variation in parental caregiving has lasting implications for the development of offspring. However, the ways in which parents impact each other in the context of caregiving is comparatively less understood, but can account for much of the variation observed in the postnatal environment. Prairie voles (Microtus ochrogaster) demonstrate a range of postnatal social groups, including pups raised by biparental pairs and by their mothers alone. In addition to the challenges of providing parental care, prairie vole parents often experience acute natural stressors (e.g., predation, foraging demands, and thermoregulation) that could alter the way co-parents interact.

Methods

We investigated how variation in the experience of raising offspring impacts parental behavior and neurobiology by administering an acute handling stressor on prairie vole families of single mothers and biparental parents over the course of offspring postnatal development.

Results

Mothers and fathers exhibited robust behavioral plasticity in response to the age of their pups, but in sex-dependent ways. Pup-directed care from mothers did not vary as a function of their partner's presence, but did covary with the number of hypothalamic vasopressin neurons in experience-dependent ways. The relationship between vasopressin neuron numbers and fathers' behaviors was also contingent upon the stress handling manipulation, suggesting that brain-behavior associations exhibit stress-induced plasticity.

Conclusion

These results demonstrate that the behavioral and neuroendocrine profiles of adults are sensitive to distinct and interacting experiences as a parent, and extend our knowledge of the neural mechanisms that may facilitate parental behavioral plasticity.

Insights into Parental Care from Studies on Non-mammalian Vertebrates

Parental care has attracted attention from both proximate and ultimate perspectives. While understanding the adaptive significance of care has been the focus of work in diverse organisms in behavioral ecology, most of what we know about the proximate mechanisms underlying parental care behavior comes from studies in mammals. Although studies on mammals have greatly improved our understanding of care, viewing parental care solely through a mammalian lens can limit our understanding. Here, we draw upon examples from non-mammalian vertebrate systems to show that in many ways mammals are the exception rather than the rule for caregiving: across vertebrates, maternal care is often not the ancestral or the most common mode of care and fathering is not derivative of mothering. Embracing the diversity of parental care can improve our understanding of both the proximate basis and adaptive significance of parental care and the affective processes involved in caregiving.

Prolactin Action Is Necessary for Parental Behavior in Male Mice

Parental care is critical for successful reproduction in mammals. Recent work has implicated the hormone prolactin in regulating male parental behavior, similar to its established role in females. Male laboratory mice show a mating-induced suppression of infanticide (normally observed in virgins) and onset of paternal behavior 2 weeks after mating. Using this model, we sought to investigate how prolactin acts in the forebrain to regulate paternal behavior. First, using c-fos immunoreactivity in prolactin receptor (Prlr) Prlr-IRES-Cre-tdtomato reporter mouse sires, we show that the circuitry activated during paternal interactions contains prolactin-responsive neurons in multiple sites, including the medial preoptic nucleus, bed nucleus of the stria terminalis, and medial amygdala. Next, we deleted Prlr from three prominent cell types found in these regions: glutamatergic, GABAergic, and CaMKIIα. Prlr deletion from CaMKIIα, but not glutamatergic or GABAergic cells, had a profound effect on paternal behavior as none of these KO males completed the pup-retrieval task. Prolactin was increased during mating, but not in response to pups, suggesting that the mating-induced secretion of prolactin is important for establishing the switch from infanticidal to paternal behavior. Pharmacological blockade of prolactin secretion at mating, however, had no effect on paternal behavior. In contrast, suppressing prolactin secretion at the time of pup exposure resulted in failure to retrieve pups, with exogenous prolactin administration rescuing this behavior. Together, our data show that paternal behavior in sires is dependent on basal levels of circulating prolactin acting at the time of interaction with pups, mediated through Prlr on CaMKIIα-expressing neurons.SIGNIFICANCE STATEMENT Parental care is critical for offspring survival. Compared with maternal care, however, the neurobiology of paternal care is less well understood. Here we show that the hormone prolactin, which is most well known for its female-specific role in lactation, has a role in the male brain to promote paternal behavior. In the absence of prolactin signaling specifically during interactions with pups, father mice fail to show normal retrieval behavior of pups. These data demonstrate that prolactin has a similar action in both males and females to promote parental care.

Oxytocin and testosterone administration amplify viewing preferences for sexual images in male rhesus macaques

Social stimuli, like faces, and sexual stimuli, like genitalia, spontaneously attract visual attention in both human and non-human primates. Social orienting behaviour is thought to be modulated by neuropeptides as well as sex hormones. Using a free viewing task in which paired images of monkey faces and anogenital regions were presented simultaneously, we found that male rhesus macaques overwhelmingly preferred to view images of anogenital regions over faces. They were more likely to make an initial gaze shift towards, and spent more time viewing, anogenital regions compared with faces, and this preference was accompanied by relatively constricted pupils. On face images, monkeys mostly fixated on the forehead and eyes. These viewing preferences were found for images of both males and females. Both oxytocin (OT), a neuropeptide linked to social bonding and affiliation, and testosterone (TE), a sex hormone implicated in mating and aggression, amplified the pre-existing orienting bias for female genitalia over female faces; neither treatment altered the viewing preference for male anogenital regions over male faces. Testosterone but not OT increased the probability of monkeys making the first gaze shift towards female anogenital rather than face pictures, with the strongest effects on anogenital images of young and unfamiliar females. Finally, both OT and TE promoted viewing of the forehead region of both female and male faces, which display sexual skins, but decreased the relative salience of the eyes of older males. Together, these results invite the hypothesis that both OT and TE regulate reproductive behaviours by acting as a gain control on the visual orienting network to increase attention to mating-relevant signals in the environment.

This article is part of the theme issue ‘Interplays between oxytocin and other neuromodulators in shaping complex social behaviours’.

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 10minutes 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.

Oxytocin via oxytocin receptor excites neurons in the endopiriform nucleus of juvenile mice

The neuropeptide oxytocin (OXT) modulates social behaviors across species and may play a developmental role for these behaviors and their mediating neural pathways. Despite having high, stable levels of OXT receptor (OXTR) ligand binding from birth, endopiriform nucleus (EPN) remains understudied. EPN integrates olfactory and gustatory input and has reciprocal connections with several limbic areas. Because the role of OXTR signaling in EPN is unknown, we sought to provide anatomical and electrophysiological information about OXTR signaling in mouse EPN neurons. Using in situ hybridization, we found that most EPN neurons co-express Oxtr mRNA and the marker for VGLUT1, a marker for glutamatergic cells. Based on high levels of OXTR ligand binding in EPN, we hypothesized that oxytocin application would modulate activity in these cells as measured by whole-cell patch-clamp electrophysiology. Bath application of OXT and an OXTR specific ligand (TGOT) increased the excitability of EPN neurons in wild-type, but not in OXTR-knockout (KO) tissue. These results show an effect of OXT on a mainly VGLUT1+ cell population within EPN. Given the robust, relatively stable OXTR expression in EPN throughout life, OXTR in this multi-sensory and limbic integration area may be important for modulating activity in response to an array of social or other salient stimuli throughout the lifespan and warrants further study.

Comparative anatomical analysis of dopamine systems in Mus musculus and Peromyscus californicus

Dopamine plays important roles in motivational and social behaviors in mammals, and it has been implicated in several human neurological and psychiatric disorders. Rodents are used extensively as experimental models to study dopamine function in health and disease. However, interspecies differences of dopamine systems remain incompletely characterized. Here, we assessed whether the commonly referenced anatomical organization of dopamine systems in Mus musculus differs from another rodent species, Peromyscus californicus, which exhibits unique social behaviors such as biparental care. We applied tyrosine hydroxylase immunofluorescence labeling and high-throughput microscopy to establish whole-brain maps of dopamine systems in P. californicus. By comparing these maps to those from M. musculus, we identified unexpected anatomical similarity and difference between these two species. A sex difference in dopamine neurons at the anteroventral periventricular nucleus of hypothalamus, which has been implicated in regulating the maternal behaviors of the uniparental M. musculus, is similarly present in the biparental P. californicus. In contrast, major interspecies differences from M. musculus are found in the ventral midbrain and striatum of P. californicus, including the expansion of midbrain dopamine neurons into the ventral substantia nigra and the presence of an internal capsule-like white matter tract that demarcates a dorsomedial area from the rest of the striatum. These features identified in P. californicus resemble the anatomical organization of the primate brain more closely compared to those in M. musculus. Our findings suggest that P. californicus is a unique model organism for studying the evolution of dopamine systems in mammals and the disorders of dopamine systems.

The elusive role of prolactin in the sociality of the naked mole-rat

Despite decades of research into the evolutionary drivers of sociality, we know relatively little about the underlying proximate mechanisms. Here we investigate the potential role of prolactin in the highly social naked mole-rat. Naked mole-rats live in large social groups but, only a small number of individuals reproduce. The remaining non-breeders are reproductively suppressed and contribute to burrow maintenance, foraging, and allo-parental care. Prolactin has well-documented links with reproductive timing and parental behaviour, and the discovery that non-breeding naked mole-rats have unusually high prolactin levels has led to the suggestion that prolactin may help maintain naked mole-rat sociality. To test this idea, we investigated whether urinary prolactin was correlated with cooperative behaviour and aggression. We then administered the prolactin-suppressing drug Cabergoline to eight female non-breeders for eight weeks and assessed the physiology and behaviour of the animals relative to controls. Contrary to the mammalian norm, and supporting previous findings for plasma, we found non-breeders had elevated urinary prolactin concentrations that were similar to breeding females. Further, prolactin levels were higher in heavier, socially dominant non-breeders. Urinary prolactin concentrations did not explain variation in working behaviour or patterns of aggression. Furthermore, females receiving Cabergoline did not show any behavioural or hormonal (progesterone) differences, and urinary prolactin did not appear to be suppressed in individuals receiving Cabergoline. While the results add to the relatively limited literature experimentally manipulating prolactin to investigate its role in reproduction and behaviour, they fail to explain why prolactin levels are high in non-breeding naked mole-rats, or how female non-breeding phenotypes are maintained.

Early Life Adversity and Males: Biology, Behavior, and Implications for Fathers' Parenting

Fathers have an important and unique influence on child development, but influences on fathers' parenting have been vastly understudied in the scientific literature. In particular, very little empirical research exists on the effects of early life adversity (ELA; e.g. childhood maltreatment, parental separation) on later parenting among fathers. In this review, we draw from both the human and non-human animal literature to examine the effects of ELA, specifically among males, in the following areas: 1) neurobiology and neurocognitive functioning, 2) hormones and hormone receptors, 3) gene-environment interactions and epigenetics, and 4) behavior and development. Based on these findings, we present a conceptual model to describe the biological and behavioral pathways through which exposure to ELA may influence parenting among males, with a goal of guiding future research and intervention development in this area. Empirical studies are needed to improve understanding of the relationship between ELA and father's parenting, inform the development of paternal and biparental interventions, and prevent intergenerational transmission of ELA.

Parental Behavior in Rodents

Members of the order Rodentia are among the best-studied mammals for understanding the patterns, outcomes, and biological determinants of maternal and paternal caregiving. This research has provided a wealth of information but has historically focused on just a few rodents, mostly members of the two Myomorpha families that easily breed and can be studied within a laboratory setting (including laboratory rats, mice, hamsters, voles, gerbils). It is unclear how well this small collection of animals represents the over 2000 species of extant rodents. This chapter provides an overview of the hormonal and neurobiological systems involved in parental care in rodents, with a purposeful eye on providing information known or could be gleaned about parenting in various less-traditional members of Rodentia. We conclude from this analysis that the few commonly studied rodents are not necessarily even representative of the highly diverse members of Myomorpha, let alone other rodent suborders, and that additional laboratory and field studies of members of this order more broadly would surely provide invaluable information toward revealing a more representative picture of the rich diversity in rodent parenting.

Love and longevity: A Social Dependency Hypothesis

Mammals, including humans, are reliant for survival and reproduction on adaptations associated with sociality and physiological investment, which can be linked to interactions with their parents or other bonded adult conspecifics. A wide range of evidence in human and non-human mammal species links social behaviors and relationships - including those characterized by what humans call "love" - to positive health and longevity. In light of this evidence, we propose a Social Dependency Hypothesis of Longevity, suggesting that natural selection has favored longer and healthier adult lives in species or in individuals exhibiting enhanced caregiver responsibilities contributing to the success of the next generation. In highlighting cellular, physiological, and behavioral effects of mammalian reproductive hormones, we examine the specific hypothesis that the neuropeptide oxytocin links longevity to the benefits of parental investment and associated relationships. Oxytocin is a pleiotropic molecule with anti-oxidant and anti-inflammatory properties, capable of regulating the hypothalamic-pituitary-adrenal axis, the parasympathetic nervous system and other systems associated with the management of various challenges, including chronic diseases and therefore may be crucial to establishing the maximum longevity potential of a species or an individual.

PATERNAL AND INFANTICIDAL BEHAVIOR IN THE MONGOLIAN GERBIL (Meriones unguiculatus): AN APPROACH TO NEUROENDOCRINE REGULATION

This study aimed to provide evidence on estrogen and androgen pathways regulating the Mongolian gerbil's paternal and infanticidal behaviors (Meriones unguiculatus). We analyzed estrogen receptor alpha (ERα) and androgen receptor (AR) distribution in the medial preoptic area (mPOA), the bed nucleus of stria terminalis (BNST), as well as the anterior hypothalamic nucleus (AHN), the ventromedial hypothalamus nucleus (VMH), and the periaqueductal gray area (PAG) nuclei activated when males interact paternally or aggressively with the pups, respectively. Twenty aggressive males towards the pups and 10 paternal were selected through a screen paternal behavior test. Three groups of 10 males each were formed: paternal males (PAT), males with testosterone (T)-induced paternal behavior (T-PAT), and aggressive males (AGG). Male gerbils could interact with a pup for a few minutes, and their brains were removed and dissected for ERα and AR immunoreactivity (ir). The results showed that in T-PAT and PAT males, the number of ERα-ir and AR-ir cells in the mPOA/BNST was significantly higher than in AGG males. In AGG males, the number of ERα-ir and AR-ir cells in the AHN/VMH/PAG was significantly higher than PAT and T-PAT males. This difference in the presence of ERα and AR in nuclei activated in paternal interactions in the Mongolian gerbil supports the idea that these receptors participate in regulating paternal behavior. Also, these results suggest, for the first time, that they could be involved in the infanticidal behavior in this rodent.

Transmission of paternal retrieval behavior from fathers to sons in a biparental rodent

Transmission of maternal behavior across generations occurs, but less is known about paternal behavior. In biparental species like the California mouse (Peromyscus californicus), paternal care contributes to the well-being of offspring with lasting consequences on the brain and behavior. Paternal huddling/grooming behavior can be passed on to future generations, but whether paternal retrieval, which removes young from potential harm, is transmitted independently is unclear. We manipulated paternal retrieval experience through pup displacement manipulations, then examined whether males exposed to higher levels of paternal retrieval in development altered their adult retrieval behavior with their offspring. Males exposed to heightened paternal retrievals, as compared to reduced retrievals, retrieved their offspring more often but huddled/groomed offspring less during undisturbed natural observations. No differences were observed following a pup displacement challenge. The high paternal retrieval group also exhibited more physical activity and stereotypy. Our results are consistent with the hypothesis that paternal retrieval levels are transmitted across generations and may function via mechanisms separate from huddling/grooming. One modifying factor may be anxiety because increased activity and stereotypy occurred in the high retrieval group. We speculate how the transmission of paternal retrievals may inform a protective parenting style.

Compositional variation in early-life parenting structures alters oxytocin and vasopressin 1a receptor development in prairie voles (Microtus ochrogaster)

Paternal absence can significantly alter bio-behavioural development in many biparental species. This effect has generally been demonstrated by comparing the development of offspring reared under biparental care with those reared by a single mother. However, studies employing this design conflate two significant modifications to early-life experience: removal of father-specific qualities and the general reduction of offspring-directed care. In the socially monogamous prairie vole (Microtus ochrogaster), the experience of paternal absence without substitution during development inhibits partner preference formation in adulthood, a hallmark of social monogamy, in females and males. Employing alloparents as substitutes for fathers, our previous work demonstrated that paternal absence affects pair-bond formation in female offspring via reduced quantity of care, although it affects pair-bond formation in male offspring by means of a missing paternal quality (or qualities). Here, we present evidence that paternal absence (with and without alloparental substitution) may alter the ontogeny of neural oxytocin receptor (OXTR) and/or vasopressin 1a receptor (AVPR1a) distribution in male and female prairie voles. Compared to biparentally reared controls (BPC), male offspring reared in mother only (MON) and maternal-plus-alloparental (MPA) conditions show lower densities of OXTR in the central amygdala; and MPA males show lower densities of OXTR in the caudate putamen and nucleus accumbens. Early-life experience was not associated with differences in AVPR1a density in males. However, MON and MPA females show greater densities of AVPR1a in the medial amygdala than BPC; and MPA females show greater densities of AVPR1a in the ventromedial nucleus of the hypothalamus. We also demonstrate with corticosterone concentrations that MON and MPA offspring are not differentially susceptible to a stressor (ie, social isolation) than BPC offspring. These findings suggest that paternal absence, although likely not a salient early-life stressor, has neuroendocrine consequences for offspring, some of which may affect partner preference formation.

Neurotranscriptomic changes associated with chick-directed parental care in adult non-reproductive Japanese quail

For many species, parental care critically affects offspring survival. But what drives animals to display parental behaviours towards young? In mammals, pregnancy-induced physiological transformations seem key in preparing the neural circuits that lead towards attraction (and reduced-aggression) to young. Beyond mammalian maternal behaviour, knowledge of the neural mechanisms that underlie young-directed parental care is severely lacking. We took advantage of a domesticated bird species, the Japanese quail, for which parental behaviour towards chicks can be induced in virgin non-reproductive adults through a sensitization procedure, a process that is not effective in all animals. We used the variation in parental responses to study neural transcriptomic changes associated with the sensitization procedure itself and with the outcome of the procedure (i.e., presence of parental behaviours). We found differences in gene expression in the hypothalamus and bed nucleus of the stria terminalis, but not the nucleus taeniae. Two genes identified are of particular interest. One is neurotensin, previously only demonstrated to be causally associated with maternal care in mammals. The other one is urocortin 3, causally demonstrated to affect young-directed neglect and aggression in mammals. Because our studies were conducted in animals that were reproductively quiescent, our results reflect core neural changes that may be associated with avian young-directed care independently of extensive hormonal stimulation. Our work opens new avenues of research into understanding the neural basis of parental care in non-placental species.

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.

The Paternal Brain in Action: A Review of Human Fathers' fMRI Brain Responses to Child-Related Stimuli

As fathers are increasingly involved in childcare, understanding the neurological underpinnings of fathering has become a key research issue in developmental psychobiology research. This systematic review specifically focused on (1) highlighting methodological issues of paternal brain research using functional magnetic resonance imaging (fMRI) and (2) summarizing findings related to paternal brain responses to auditory and visual infant stimuli. Sixteen papers were included from 157 retrieved records. Sample characteristics (e.g., fathers’ and infant’s age, number of kids, and time spent caregiving), neuroimaging information (e.g., technique, task, stimuli, and processing), and main findings were synthesized by two independent authors. Most of the reviewed works used different stimuli and tasks to test fathers’ responses to child visual and/or auditory stimuli. Pre-processing and first-level analyses were performed with standard pipelines. Greater heterogeneity emerged in second-level analyses. Three main cortical networks (mentalization, embodied simulation, and emotion regulation) and a subcortical network emerged linked with fathers’ responses to infants’ stimuli, but additional areas (e.g., frontal gyrus, posterior cingulate cortex) were also responsive to infants’ visual or auditory stimuli. This review suggests that a distributed and complex brain network may be involved in facilitating fathers’ sensitivity and responses to infant-related stimuli. Nonetheless, specific methodological caveats, the exploratory nature of large parts of the literature to date, and the presence of heterogeneous tasks and measures also demonstrate that systematic improvements in study designs are needed to further advance the field.

Major depression, temperament, and social support as psychosocial mechanisms of the intergenerational transmission of parenting styles

In this three-generation longitudinal study of familial depression, we investigated the continuity of parenting styles, and major depressive disorder (MDD), temperament, and social support during childrearing as potential mechanisms. Each generation independently completed the Parental Bonding Instrument (PBI), measuring individuals' experiences of care and overprotection received from parents during childhood. MDD was assessed prospectively, up to 38 years, using the semi-structured Schedule for Affective Disorders and Schizophrenia (SADS). Social support and temperament were assessed using the Social Adjustment Scale - Self-Report (SAS-SR) and Dimensions of Temperament Scales - Revised, respectively. We first assessed transmission of parenting styles in the generation 1 to generation 2 cycle (G1→G2), including 133 G1 and their 229 G2 children (367 pairs), and found continuity of both care and overprotection. G1 MDD accounted for the association between G1→G2 experiences of care, and G1 social support and temperament moderated the transmission of overprotection. The findings were largely similar when examining these psychosocial mechanisms in 111 G2 and their spouses (G2+S) and their 136 children (G3) (a total of 223 pairs). Finally, in a subsample of families with three successive generations (G1→G2→G3), G2 experiences of overprotection accounted for the association between G1→G3 experiences of overprotection. The results of this study highlight the roles of MDD, temperament, and social support in the intergenerational continuity of parenting, which should be considered in interventions to "break the cycle" of poor parenting practices across generations.

A systematic review of human paternal oxytocin: Insights into the methodology and what we know so far

With the consolidation of fathers' engagement in caregiving, understanding the neuroendocrine and hormonal mechanisms underlying fatherhood becomes a relevant topic. Oxytocin (OT) has been linked with maternal bonding and caregiving, but less is known about the role of OT in human fatherhood and paternal caregiving. A systematic review of methods and findings of previous OT research in human fathers was carried. The literature search on PubMed and Scopus yielded 133 records. Twenty-four studies were included and analyzed. Significant variability emerged in OT methodology, including laboratory tasks, assessment methods, and outcome measures. Fathers' OT levels appear to increase after childbirth. OT was significantly correlated with less hostility and with the quality of paternal physical stimulation in play interactions, but not with paternal sensitivity. Fathers' and children's OT levels were significantly correlated in a limited subset of studies, intriguingly suggesting that cross-generational OT regulation may occur during the early years of life. This study highlights relevant issues and limitations of peripheral OT assessment in human subjects, especially in fathers. Although the study of paternal neuroendocrinology appears promising, coping with these issues requires dedicated efforts and methodological suggestions are provided to guide future advances in this field.

Maternal Motivation: Exploring the Roles of Prolactin and Pup Stimuli

Motherhood entails increased motivation for pups, which become strong reinforcers and guide maternal behaviours. This depends on steroids and lactogens acting on the brain of females during pregnancy and postpartum. Since virgin female mice exposed to pups are nearly spontaneously maternal, the specific roles of endocrine and pup-derived signals in the induction of maternal motivation remain unclear. This work investigates maternal motivation in dams and virgin female mice, using a novel variant of the pup retrieval paradigm, the motivated pup retrieval test. We also analyse the role of prolactin (PRL) and of stimuli derived from a litter of pups and its mother, in the acquisition of maternal motivation. Experimental design included female mice in 3 conditions: lactating dams, comothers (virgins housed and sharing pup care with dams) and pup-naïve virgins. Females underwent 3 motivated-pup-retrieval trials, with pups displaced behind a 10-cm-high wire-mesh barrier. Dams retrieved with significantly lower latencies than comothers or virgins, indicating that full maternal motivation appears only after pregnancy. Although initially comothers and virgins showed no retrieval, comothers significantly improved throughout the experiment, suggesting an induced sensitization process. Lengthening exposure of comothers to the dyad pups-dam (from 2 to 5 days at the beginning of testing) had no strong effects on maternal sensitization. PRL responsiveness was analysed in these animals using immunohistochemical detection of phosphorylated signal transducer and activator of transcription 5 (pSTAT5, PRL-derived signalling marker). As expected, dams showed significantly higher pSTAT5 expression in most of the analysed nuclei. Moreover, comothers displayed significantly higher PRL responsiveness than pup-naïve virgins in the medial preoptic nucleus, even if they display similar circulating PRL levels, which are significantly lower than those of dams. Given the instrumental role of this nucleus in the relay and integration of pup-derived stimuli to facilitate proactive maternal responses, this increase in PRL responsiveness likely reflects the mechanism underlying the maternal sensitization process reported in this work. Since the analyses of maternal motivation and PRL signalling in the brain were performed in the same animals, we were able to explore correlation between both set of data. The results shed light on the neuroendocrine mechanisms underlying maternal motivation and other aspects of maternal behaviour.

Exploring the hormonal and neural correlates of paternal protective behavior to their infants

Infant protection is an important but largely neglected aspect of parental care. Available theory and research suggest that endocrine levels and neural responses might be biological correlates of protective behavior. However, no research to date examined associations between these neurobiological and behavioral aspects. This study, preregistered on https://osf.io/2acxd, explored the psychobiology of paternal protection in 77 new fathers by combining neural responses to infant-threatening situations, self-reported protective behavior, behavioral observations in a newly developed experimental set-up (Auditory Startling Task), and measurements of testosterone and vasopressin. fMRI analyses validated the role of several brain networks in the processing of infant-threatening situations and indicated replicable findings with the infant-threat paradigm. We found little overlap between observed and reported protective behavior. Robust associations between endocrine levels, neural responses, and paternal protective behavior were absent.

Development of the paternal brain in expectant fathers during early pregnancy

The human parenting brain network mediates caregiving behaviors. When exposed to the stimuli of their infants, compared with non-parents, both fathers and mothers exhibit distinct patterns of neural activation. As human males, relative to females, do not undergo robust physiological changes during pregnancy, when and how the paternal brain networks begin to form remains unclear. Thus, using functional MRI, we examined brain activation in response to infant-interaction videos in two groups, childless males and first-time expectant fathers during their partners' early pregnancy before remarkable changes in their partners' appearances commenced. Multivoxel pattern analysis revealed that expectant fathers' left anterior insula and inferior frontal gyrus showed incipient changes in response to parenthood during early pregnancy. Furthermore, these changes were associated with several paternal traits, such as a negative image toward parenting. Such external factors might influence the paternal brain's development during early pregnancy.

Sharing and caring: Testosterone, fathering, and generosity among BaYaka foragers of the Congo Basin

Humans are rare among mammals in exhibiting paternal care and the capacity for broad hyper-cooperation, which were likely critical to the evolutionary emergence of human life history. In humans and other species, testosterone is often a mediator of life history trade-offs between mating/competition and parenting. There is also evidence that lower testosterone men may often engage in greater prosocial behavior compared to higher testosterone men. Given the evolutionary importance of paternal care and heightened cooperation to human life history, human fathers' testosterone may be linked to these two behavioral domains, but they have not been studied together. We conducted research among highly egalitarian Congolese BaYaka foragers and compared them with their more hierarchical Bondongo fisher-farmer neighbors. Testing whether BaYaka men's testosterone was linked to locally-valued fathering roles, we found that fathers who were seen as better community sharers had lower testosterone than less generous men. BaYaka fathers who were better providers also tended to have lower testosterone. In both BaYaka and Bondongo communities, men in marriages with greater conflict had higher testosterone. The current findings from BaYaka fathers point to testosterone as a psychobiological correlate of cooperative behavior under ecological conditions with evolutionarily-relevant features in which mutual aid and sharing of resources help ensure survival and community health.

Paternal Deprivation and Female Biparental Family Rearing Induce Dendritic and Synaptic Changes in Octodon degus: I. Medial Prefrontal Cortex

In most mammalian species parent-offspring interactions during early life periods primarily comprise social contacts with the mother, whereas the role of males in parental care is one of the most overlooked and understudied topics. The present study addressed the hypothesis that the complete deprivation of paternal care delays or permanently retards synaptic connectivity in the brain, particularly in the medial prefrontal cortex (mPFC) of the offspring in a sex-specific manner. Another aim of this study was to address the question whether and in which way replacing the father with a female caregiver (in our experiments the “aunt”) can “buffer” the detrimental effects of paternal deprivation on neuronal development. The comparison of: (a) single mother rearing; (b) biparental rearing by father and mother; and (c) biparental rearing by two female caregivers revealed that: (i) paternal care represents a critical environmental factor for synaptic and dendritic development of pyramidal neurons in the vmPFC of their offspring; (ii) a second female caregiver (“aunt”) does not “buffer” the neuronal consequences of paternal deprivation; and that (iii) neuronal development in the vmPFC is differentially affected in male and female offspring in response to different family constellations.

Long-Term Effects of Fatherhood on Morphology, Energetics, and Exercise Performance in California Mice (Peromyscus californicus)

In male mammals that provide care for their offspring, fatherhood can lead to changes in behavioral, morphological, and physiological traits, some of which might constitute trade-offs. However, relatively little is known about these changes, especially across multiple reproductive bouts, which are expected to magnify differences between fathers and nonreproductive males. We evaluated consequences of fatherhood in the monogamous, biparental California mouse (Peromsycus californicus) across seven consecutive reproductive bouts. We compared breeding adult males (housed with sham-ovariectomized females) with two control groups: nonbreeding males (housed with ovariectomized females treated with estrogen and progesterone to induce estrous behavior) and virgin males (housed with untreated ovariectomized females). At five time points (before pairing, early postpartum of the first litter, late postpartum of the second litter, early postpartum of the sixth litter, and late postpartum of the seventh litter or comparable time points for nonbreeding and virgin males), we measured males' body composition, hematocrit, predatory aggression, resting metabolic rate, maximal oxygen consumption (V˙O2 max⁡), grip strength, and sprint speed. We also weighed organs at the final time point. We predicted that fathers would have lower relative body fat and lower performance abilities compared with control groups and that these effects would become more pronounced with increasing parity. Contrary to predictions, breeding and control males differed in surprisingly few measures, and the number and magnitude of differences did not increase with parity. Thus, our expectations regarding trade-offs were not met. As reported in studies of single reproductive events, these results suggest that fatherhood has few costs in this species when housed under standard laboratory conditions, even across multiple reproductive bouts.

Flipping the parental switch: from killing to caring in male mammals

Killing of unrelated young by sexually naïve male mammals is taxonomically widespread, but in many species, males subsequently show paternal care or at least do not harm their own young. This dramatic and important change is due to a shift in paternal state rather than to recognition of young, the mother or the location in which mating occurred. This transition from infanticidal to paternal behaviour is timed so that the inhibition of infanticide is synchronized with the birth of their own young. Ejaculation followed by cohabitation with the pregnant female causes this transition, but the precise stimuli from the female remain elusive. However, changes in social status also cause changes in infanticide. The switch from infanticide is accompanied by physiological change in the male that can be detected by both females and pups. Hormonal changes have been implicated in the switch but establishing causal links has been difficult. Recent neuroanatomical studies show that pup odours activate the vomeronasal organ and its efferent projections to induce infanticide. The emergence of paternal care depends on the inability of the vomeronasal organ to detect pup odours. In the absence of vomeronasal input, pup odours activate a conserved parental circuit and induce caregiving behaviour. An emerging picture is of complex, antagonistic circuits competing for behavioural expression, which allow for males to commit infanticide when they may benefit from such activity but ensure that they do not damage their fitness by killing their own young. However, we stress the need for more work on the neural mechanisms that mediate this process.

Effects of sex and age on parental motivation in adult virgin California mice

Female mammals often demonstrate a rapid initiation of maternal responsiveness immediately after giving birth, as a result of neuroendocrine changes that occur during pregnancy and parturition. However, fathers and virgins of some species may display infant care similar to that performed by mothers but without experiencing these physiological events. In biparental species, in which both mothers and fathers care for their offspring, both sex and age may affect parental motivation, even in adult virgins. We examined the effects of sex and age on parental motivation in the California mouse, a monogamous, biparental rodent. We compared parental motivation of male and female virgins in both mid- and old adulthood using two new tests - a T-maze test and a rain test - as well as in standard parental-behavior tests. Adult virgin males were more parentally motivated than adult virgin females in both the T-maze test and the parental-behavior test, but parental motivation did not differ markedly between middle-aged and older adults of either sex. These findings suggest that sex differences in parental motivation in adult virgins are similar to those observed in other biparental rodents, and indicate that the T-maze test may be useful for evaluating parental motivation in this species.

Estradiol and estrogen receptor α in the mPOA and MeA in dwarf hamster (Phodopus campbelli) fathers

E₂ and its alpha receptor (ERα) have an essential role in the regulation of maternal behavior. In dwarf hamster (Phodopus campbelli), E₂ facilitates the display of paternal care, and it is possible that ERα is part of the neuroendocrine mechanisms that regulate this behavior. The aim of this study was to analyze the influence of copulation, cohabitation with the pregnant mate and the presence of the pups on paternal behavior, circulating E₂ levels and the presence of ERα in the medial preoptic area (mPOA) and medial amygdala (MeA) in dwarf hamsters. Eight males were mated with intact females (IFs), 8 with tubally ligated females (TLFs) and 8 with ovariectomized females (OFs). In males mated with IFs, paternal behavior tests were performed after copulation, halfway through pregnancy and 24 h after the birth of their pups. Males mated with TLFs were subjected to paternal behavior tests at equivalent periods as the males mated with IFs. In males mated with OFs, paternal behavior tests were performed on days 1, 5 and 10 of cohabitation. After the last paternal behavior tests, blood samples were taken for quantification of E₂ by radioimmunoassay (RIA), and the brains were dissected to determine ERα immunoreactivity (ir) in the mPOA and MeA. Fathers mated with IFs had higher serum E₂ concentrations and more ERα-ir cells in the mPOA than those of males mated with TLFs and OFs. These results suggest that E₂ and its ERα may be associated with paternity in the dwarf hamster.

Social Stimuli Induce Activation of Oxytocin Neurons Within the Paraventricular Nucleus of the Hypothalamus to Promote Social Behavior in Male Mice

Oxytocin (OT) is critical for the expression of social behavior across a wide array of species; however, the role of this system in the encoding of socially relevant information is not well understood. In the present study, we show that chemogenetic activation of OT neurons within the paraventricular nucleus of the hypothalamus (PVH) of male mice (OT-Ires-Cre) enhanced social investigation during a social choice test, while chemogenetic inhibition of these neurons abolished typical social preferences. These data suggest that activation of the OT system is necessary to direct behavior preferentially toward social stimuli. To determine whether the presence of a social stimulus is sufficient to induce activation of PVH-OT neurons, we performed the first definitive recording of OT neurons in awake mice using two-photon calcium imaging. These recordings demonstrate that social stimuli activate PVH-OT neurons and that these neurons differentially encode social and nonsocial stimuli, suggesting that PVH-OT neurons may act to convey social salience of environmental stimuli. Finally, an attenuation of social salience is associated with social disorders, such as autism. We therefore also examined possible OT system dysfunction in a mouse model of autism, Shank3b knock-out (KO) mice. Male Shank3b KO mice showed a marked reduction in PVH-OT neuron number and administration of an OT receptor agonist improved social deficits. Overall, these data suggest that the presence of a social stimulus induces activation of the PVH-OT neurons to promote adaptive social behavior responses.SIGNIFICANCE STATEMENT Although the oxytocin (OT) system is well known to regulate a diverse array of social behaviors, the mechanism in which OT acts to promote the appropriate social response is poorly understood. One hypothesis is that the presence of social conspecifics activates the OT system to generate an adaptive social response. Here, we selectively recorded from OT neurons in the paraventricular hypothalamic nucleus (PVH) to show that social stimulus exposure indeed induces activation of the OT system. We also show that activation of the OT system is necessary to promote social behavior and that mice with abnormal social behavior have reduced numbers of PVH-OT neurons. Finally, aberrant social behavior in these mice was rescued by administration of an OT receptor agonist.