Ontogeny of the transition from killer to caregiver in dwarf hamsters (Phodopus campbelli) with biparental care

African striped mice (Rhabdomys pumilio) as a neurobehavioral model for male parental care

Parental care is diversely demonstrated across the animal kingdom, such that active practitioners and repertoires of parental behavior vary dramatically between and within taxa. For mammals, maternal care is ubiquitous while paternal and alloparental care are rare. The African striped mouse, a rodent species in the family Muridae, demonstrates maternal, paternal, and alloparental care. Because socio-environmental factors can considerably influence the development of their social behavior, including that of paternal and alloparental care, African striped mice are considered socially flexible. Here, we highlight African striped mice as a new model for the neurobiological study of male parental care. We first provide essential background information on the species' natural ecological setting and reproductive behavior, as well as the species-relevant interaction between ecology and reproduction. We then introduce the nature of maternal, paternal, and alloparental care in the species. Lastly, we provide a review of existing developmental and neurobiological perspectives and highlight potential avenues for future research.

Involvement of the dopamine system in paternal behavior induced by repeated pup exposure in virgin male ICR mice

Like mothers, fathers play a vital role in the development of the brain and behavior of offspring in mammals with biparental care. Unlike mothers, fathers do not experience the physiological processes of pregnancy, parturition, or lactation before their first contact with offspring. Whether pup exposure can induce the onset of paternal behavior and the underlying neural mechanisms remains unclear. By using Slc:ICR male mice exhibiting maternal-like parental care, the present study found that repeated exposure to pups for six days significantly increased the total duration of paternal behavior and shortened the latency to retrieve and care for pups. Repeated pup exposure increased c-Fos-positive neurons and the levels of dopamine- and TH-positive neurons in the nucleus accumbens (NAc). In addition, inhibition of dopamine projections from the ventral tegmental area to the NAc using chemogenetic methods reduced paternal care induced by repeated pup exposure. In conclusion, paternal behavior in virgin male ICR mice can be initiated by repeated pup exposure via sensitization, and the dopamine system may be involved in this process.

Consumption of Maternal Placenta in Humans and Nonhuman Mammals: Beneficial and Adverse Effects

Placentophagia is a common mammalian behavior, and the first scientific study of the potential effects of human maternal placentophagia on lactation was in 1917. More recently, in the 1970s, human placentophagia was reported in North America with a trend toward increased consumption. There are different hypotheses about the women and nonhuman mammals' motivation towards placentophagia, but few have been subject to hypotheses testing. In women, the controversy continues; on the one hand, researchers attribute benefits like increased breast milk, weight gain in newborns, decreased postpartum depression and fatigue, and improved mothers' mood. In contrast, bacterial or viral infections, hormonal, or trace elements that could become toxic for both the mother and baby are reported as possible health risks. Other reports argue a lack of scientific rigor to support the self-reported benefits of placentophagia. Also, the way the placenta is prepared (raw, cooked, dehydrated, processed, or encapsulated) alters its components, and thus the desired effects. This review provides relevant information and the different hypotheses and points of view around placentophagia. However, there are still questions to be resolved, and more studies are needed to confirm or reject the data generated so far about placentophagia in humans and nonhuman mammals.

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.

Data quality and the comparative method: the case of pregnancy failure in rodents

In mammalian species where infanticide by males is likely, females exhibit counterstrategies to prevent or mitigate the costs of infanticide. One putative mitigation strategy is the “Bruce effect,” in which pregnant or inseminated females exposed to an unfamiliar male experience pregnancy block or failure. Females then mate with the new male, thus shifting investment from a “doomed” pregnancy to a more fruitful one. However, the Bruce effect may be an adaptive response to other factors besides infanticide. For example, if paternal care is necessary for offspring survival, and an unfamiliar male replacing the original mate is unlikely to provide such care to offspring of a litter it did not sire, then a female may terminate a pregnancy to initiate a new one. The infanticide and paternal care hypotheses have not been rigorously tested because comparative data on the Bruce effect across mammals are scarce. We compiled data on the Bruce effect, infanticide, and paternal care from one particularly rich source of information, rodents, but found the data set to be less rich than expected. The Bruce effect, infanticide, and paternal care were common among rodent species, but we found no clear relationship among the traits. However, this was likely due to 1) a bias toward positive results, 2) missing data, and 3) a reliance on studies of captive animals. These are common problems in comparative research, and we outline standards that should be implemented to successfully answer questions of importance in the field.

Neuronal activation associated with paternal and aversive interactions toward pups in the Mongolian gerbils (Meriones unguiculatus)

Approach/avoid model is used to analyze the neural regulation of maternal behavior in the laboratory rat. This model proposes that the medial preoptic area (mPOA) and bed nucleus of stria terminalis (BNST) are brain regions involved in facilitating mechanisms. By contrast, anterior hypothalamic nucleus (AHN), ventromedial hypothalamic nucleus (VMH), and periaqueductal gray participate in the inhibiting mechanisms of neural regulation of maternal behavior. We hypothesized that there are also facilitating and inhibiting mechanisms in the neural regulation of paternal behavior. Here, we determined which neural areas are activated during paternal and aversive interactions with pups in the Mongolian gerbils (Meriones unguiculatus). By testing paternal behavior, we selected 40 males aggressive toward pups and 20 paternal males. These males were organized into six groups of 10 animals in each group: aggressive males that interacted with pups (AGG-pups) or candy (AGG-candy), paternal males that interacted with pups (PAT-pups) or candy (PAT-candy), and males with testosterone (T)-induced paternal behavior that interacted with pups (IPAT-pups) or candy (IPAT-candy). After interacting with pups or candy, the brains were extracted and analyzed for immunoreactivity (ir) with c-fos. Males that interacted with pups had significantly higher c-fos-ir in the mPOA/BNST than males that interacted with candy. Males that displayed aggression had significantly higher c-fos-ir in the AHN, VMH, and periaqueductal gray than aggressive males that interacted with candy. These results suggest that in the neural regulation of paternal behavior in the Mongolian gerbil underlie positive and negative mechanisms as occurs in maternal behavior.

An increase in estradiol facilitates the onset of paternal behavior in the dwarf hamster (Phodopus campbelli)

In the dwarf hamster (Phodopus campbelli), activational effects of testosterone (T) and estradiol (E₂) in the regulation of paternal behavior have been repeatedly rejected because peripheral concentrations of E₂ do not change across the reproductive cycle of males. Further, castration no affected paternal behavior despite that both T and E₂ concentrations decreased significantly. However, the role of these hormones has not been evaluated in models of castration and hormonal replacement in virgin males. Here, we analysed the effects of E₂ and T in paternal behavior in virgin male dwarf hamster (Phodopus campbelli). Thirty paternal (PAT) males were bilaterally castrated; of them, 10 were implanted with T, 10 with E₂ and 10 males received no treatment. Other 10 PAT males underwent sham-castration. Seventeen aggressive (AGG) males were also bilaterally castrated; of these, 10 AGG received E₂ replacement, 7 were not treated. Other 7 AGG males were submitted to sham-castration. Following treatments, paternal behavior tests were conducted again. T and E₂ levels in plasma were quantified by radioimmunoassay (RIA). The results showed that the treatments did not affect the paternal behavior of males that were initially paternal. Neither castration nor sham-castration surgery affected the behavior of AGG males. However, when these males were treated with E₂ and the concentrations of this hormone increase significantly they became paternal. Our data suggest that an increase in E₂ levels shifted infanticidal behavior to paternal behavior in dwarf hamster.

Paternal Care in Biparental Rodents: Intra- and Inter-individual Variation

Parental care by fathers, although rare among mmmals, can be essential for the survival and normal development of offspring in biparental species. A growing body of research on biparental rodents has identified several developmental and experiential influences on paternal responsiveness. Some of these factors, such as pubertal maturation, interactions with pups, and cues from a pregnant mate, contribute to pronounced changes in paternal responsiveness across the course of the lifetime in individual males. Others, particularly intrauterine position during gestation and parental care received during postnatal development, can have long-term effects on paternal behavior and contribute to stable differences among individuals within a species. Focusing on five well-studied, biparental rodent species, we review the developmental and experiential factors that have been shown to influence paternal responsiveness, and consider their roles in generating both intra- and inter-individual variation. We also review hormones and neuropeptides that have been shown to modulate paternal care and discuss their potential contributions to behavioral differences within and between males. Finally, we discuss the possibility that vasopressinergic and possibly oxytocinergic signaling within the brain, modulated by gonadal steroid hormones, may represent the "final common pathway" mediating effects of developmental and experiential variables on intra- and inter-individual variation in paternal care.

Development-dependent behavioral change toward pups and synaptic transmission in the rhomboid nucleus of the bed nucleus of the stria terminalis

Sexually naïve male C57BL/6 mice aggressively bite unfamiliar pups. This behavior, called infanticide, is considered an adaptive reproductive strategy of males of polygamous species. We recently found that the rhomboid nucleus of the bed nucleus of the stria terminalis (BSTrh) is activated during infanticide and that the bilateral excitotoxic lesions of BSTrh suppress infanticidal behavior. Here we show that 3-week-old male C57BL/6 mice rarely engaged in infanticide and instead, provided parental care toward unfamiliar pups, consistent with observations in rats and other rodent species. This inhibition of infanticide at the periweaning period is functional because the next litter will be born at approximately the time of weaning of the previous litter through maternal postpartum ovulation. However, the mechanism of this age-dependent behavioral change is unknown. Therefore, we performed whole-cell patch clamp recordings of BSTrh and compared evoked neurotransmission in response to the stimulation of the stria terminalis of adult and 3-week-old male mice. Although we were unable to detect a significant difference in the amplitudes of inhibitory neurotransmission, the amplitudes and the paired-pulse ratio of evoked excitatory postsynaptic currents differed between adult and 3-week-old mice. These data suggest that maturation of the synaptic terminal in BSTrh that occurred later than 3 weeks after birth may mediate by the adaptive change from parental to infanticidal behavior in male mice.

Comparative analysis of oxytocin receptor density in the nucleus accumbens: an adaptation for female and male alloparental care?

Parental behavior is commonly displayed by progenitors. However, other individuals, genetically related (e.g. siblings, aunts, uncles) or not with the newborns, also display parental behavior (commonly called alloparental, or adoptive behavior). I hypothesize that species that live in family or social groups where other non-reproductive members (males and females) take care of infants, have brain adaptations to promote or facilitate that behavioral response. The present work revises the evidence supporting the hypothesis that high density of oxytocin receptors (OXTR) in the nucleus accumbens (NA) is one of those adaptations. All species known to have high NA OXTR show not only female, but also male alloparental care. Therefore, I predict that high NA OXTR could be present in all species in which juvenile and adult male alloparental behavior have been observed. Strategies to test this and other alternative working hypothesis and its predictions are presented.

Placentophagia in weanling female laboratory rats

Placentophagia is common in parturient mammals and offers physiological and behavioral advantages for mothers. In natural environments, weanlings are often present during the birth of younger siblings, but it is unknown if weanling rats are placentophagic or prefer placenta over other substances. To examine this, primiparous rats were remated during the postpartum estrus and their weanling daughters remained in the natal nest during their mother's next parturition. Continuous observation revealed that 58% of weanlings were placentophagic. To determine if this placentophagia occurs away from parturient mothers, weanling females still living in their natal nest were offered placenta, liver, or cake frosting in a novel chamber. They ingested more placenta and liver than frosting. Thus, many weanling female laboratory rats are placentophagic during the birth of younger siblings but do not selectively prefer placenta when tested outside their natal nest. Consequences of placentophagia by weanling female rats are unknown, but it may promote their alloparenting or later postpartum mothering.

Effects of siblings on reproductive maturation and infanticidal behavior in cooperatively breeding Mongolian gerbils

Mongolian gerbils living with their natal families undergo delayed reproductive maturation while helping to rear their younger siblings, whereas those housed away from their natal families may mature earlier but often respond aggressively to unfamiliar pups. We tested whether cohabitation with pups contributes to reproductive suppression and inhibition of infanticidal behavior, using young males and females housed with (1) their parents and younger siblings (pups), (2) parents without pups, (3) mixed-sex littermate groups, or (4) mixed-sex groups of unrelated peers. Maturation in males was inhibited by cohabitation with the parents, while maturation in females was further suppressed in the presence of pups. Males in all housing conditions showed little aggression towards unfamiliar pups, whereas females were usually infanticidal unless housed with pups. Aggression toward pups was especially pronounced in females that were pregnant or undergoing ovulatory cycles. Thus, cohabitation with younger siblings may intensify reproductive suppression and inhibit infanticidal behavior in female gerbils, whereas male gerbils exhibit parentally induced reproductive suppression and low rates of infanticide even in the absence of younger siblings.

In three brain regions central to maternal behaviour, neither male nor female Phodopus dwarf hamsters show changes in oestrogen receptor alpha distribution with mating or parenthood

Oestrogen receptor (ER)alpha immunoreactivity in three brain regions relevant to maternal behaviour (medial preoptic area, bed nucleus of the stria terminalis and medial amygdala) was measured in two species of dwarf hamster that both mate during a postpartum oestrous but differ in expression of paternal behaviour. Male and female Phodopus campbelli and Phodopus sungorus were sampled as sexually naive adults, following mating to satiety, and as new parents. In all brain regions, females expressed higher levels of ER alpha than males. Species did not have an effect on ER alpha distribution except in the medial amygdala, where P. sungorus females had higher expression levels than all other groups. Behavioural status was not associated with altered ER alpha expression. These results were not expected for females and suggest that a primary activational role for oestrogen, acting through ER alpha in these regions, does not generalize to maternal behaviour in Phodopus. In males, these results are consistent with previous manipulations of the ER alpha ligand, oestrogen, and suggest that paternal behaviour in P. campbelli is likely to be regulated by developmental effects of oestrogen on the brain during early life (similar to Microtus ochrogaster), rather than through activation by oestrogen at the time of fatherhood (similar to Peromyscus californicus).

Paternal care in rodents: weakening support for hormonal regulation of the transition to behavioral fatherhood in rodent animal models of biparental care

Male rodents that are naturally paternal, like all females, must inhibit infanticide and activate direct parental behavior as they become parents. Males, however, alter their behavior in the absence of parturition, postpartum ovulation and lactation, and therefore do not experience the hormone dynamics associated with such conditions. Paternal males might nevertheless use the same hormones to activate pre-existing maternal behavior pathways in the brain. Positive and inverse associations between prolactin, sex steroids (estradiol, testosterone, progesterone), glucocorticoids, oxytocin and vasopressin and paternal behavior are reviewed. Across biparental rodents (Phodopus campbelli, Peromyscus californicus, Microtus ochrogaster, and Meriones unguiculatus), as well as non-human primates and men, hormone-behavior associations are broadly supported. However, experimental manipulations (largely restricted to P. campbelli) suggest that the co-variation of hormones and paternal behavior is not causal in paternal behavior. Perhaps the hormone-behavior associations shared by P. campbelli and other paternal males are important for other challenges at the same time as fatherhood (e.g., mating during the postpartum estrus). On the other hand, each paternal species might, instead, have unique neuroendocrine pathways to parental behavior. In the latter case, future comparisons might reveal extraordinary plasticity in how the brain forms social bonds and alters behavior in family groups.

Phodopus campbelli detect reduced photoperiod during development but, unlike Phodopus sungorus, retain functional reproductive physiology

Golden (Mesocricetus auratus) and Siberian (Phodopus sungorus) hamsters are widely used as animal models for seasonal reproduction; butM. auratusshows no developmental delay in short days until after sexual maturity, whereasP. sungorusjuveniles delay development in short days. As the photoperiodic response ofPhodopus campbelliis not well established, litters of the twoPhodopusspecies were gestated and reared under long days (14 h light:10 h darkness) or short days (10 h light:14 h darkness) until 70 days of age. As expected, under short photoperiodP. sungorusshowed reduced body, testes, epididymides, uterus, and ovary weight; antral follicles and corpora lutea were absent and vaginae remained closed. Animals moulted to winter pelage, and low concentrations of each of leptin, testosterone, and prolactin were present in male serum.Phodopus campbellijuveniles also responded to the short photoperiod as measured by reduced body, testes, epididymides, and ovary weight. The summer pelage persisted. However, both sexes ofP. campbellideveloped functional reproduction under 10 h light:14 h darkness. All females had a patent vagina by 10 weeks; ovaries contained antral follicles and corpora lutea, and uteri were not reduced in weight. In males, the concentrations of testosterone, leptin, and prolactin were not reduced by short photoperiod. Developmental patterns in the three species of hamster, therefore, differ and are not predicted by relatedness or latitude of origin. Other ecological traits, such as predictability of summer rainfall, ambient temperature, and differential responses to social cues might be important.

Paternal responsiveness in biparental dwarf hamsters (Phodopus campbelli) does not require estradiol

Males of the biparental hamster species Phodopus campbelli act as midwives and are responsive to an experimentally displaced pup. Males also have peripheral estradiol concentrations that are similar to conspecific females. Castration reduces peripheral estradiol, yet does not affect paternal responsiveness despite the known role of estradiol in maternal behavior. Synthesis of estradiol within the central nervous system, however, might not be affected by castration. Males received implants of osmotic pumps containing the aromatase inhibitor letrozole to reduce both peripheral and central estradiol concentrations. Though estradiol was effectively reduced, it had no effect on paternal responsiveness or reproductive success. Neither testosterone nor aggression directed towards an intruder was altered. Results support the emerging conclusion that estradiol is not required for the exceptional paternal behavior of male P. campbelli.

In uniparentalPhodopus sungorus, new mothers, and fathers present during the birth of their offspring, are the only hamsters that readily consume fresh placenta

Placentophagia is common among parturient female mammals but non-parturient females generally refuse placenta. Biparental male dwarf hamsters (Phodopus campbelli) readily consume placenta. The present study quantified placentophagia and liver acceptance in the closely related Siberian hamster P. sungorus in which males do not participate in the birth and are not responsive to a displaced pup. Sexually naïve P. sungorus males and females refused both placenta and liver (all groups <10%). Reproductive females specifically consumed placenta on the day before (G17), and the day of, parturition (G18) (>80%). Males rejected both tissues on G17 and accepted placenta soon after the birth (G18) (80%) only if they were present during the birth. Palatability of the placenta was not responsible for the species difference as P. campbelli accepted P. sungorus placenta. Results are consistent with a neophobic reaction to both placenta (conspecific or heterospecific) and liver as P. sungorus also rejected P. campbelli placenta.

Castration reduces male testosterone, estradiol, and territorial aggression, but not paternal behavior in biparental dwarf hamsters (Phodopus campbelli)

Biparental male hamsters, Phodopus campbelli, act as midwives during the birth of their litter and are highly responsive to an experimentally displaced pup. They also have high peripheral concentrations of estradiol, a hormone with known roles in maternal behavior. Surgical castration during the gestation of their first litter was used to investigate the source of that estradiol and the functional role of testicular sex steroids in paternal responsiveness. In Experiment I, castration reduced both testosterone and estradiol concentrations, confirming that the testes were the primary source of estradiol. However, neither paternal responsiveness nor multiple measures of reproductive success were altered by the castration. Aggression directed towards an intruder, however, was reduced by castration. In Experiment II, removal of prior experience with birth or pups also failed to alter paternal responsiveness in castrated males. Although the present results do not preclude a role for local estradiol synthesis in the brain, results do not support an association between high circulating estradiol in males and their paternal behavior.

Dopamine agonist treatment before and after the birth reduces prolactin concentration but does not impair paternal responsiveness in Djungarian hamsters, Phodopus campbelli

Male Djungarian hamsters, Phodopus campbelli, are highly parental and experience a late-afternoon prolactin surge before the birth that is not seen in a closely related species, P. sungorus, which lacks paternal care. At the same stage, female prolactin is needed for later maternal behavior. Male prolactin was suppressed in first-time fathers before the birth of the litter using two different dopamine agonists, bromocriptine mesylate and cabergoline. Plasma prolactin concentration confirmed the efficacy of each treatment. Paternal responsiveness was quantified using three variations on a pup-displacement paradigm. No adverse effects of either treatment were seen. Across four experiments, there was no decrease in paternal retrieval or in retrieval latency in response to male prolactin suppression. In addition, there was no decrease in litter growth or survival, nor was there an increase in maternal investment to compensate for a deficit in paternal care. As cabergoline suppression of prolactin persisted after the birth without behavioral deficits, prolactin after the birth was also not required for the expression of paternal behavior. In spite of an extensive literature supporting an association between prolactin and natural paternal behavior, we conclude that dopamine-mediated prolactin release into peripheral plasma is not essential for paternal responsiveness in P. campbelli.

Placentophagia in naïve adults, new fathers, and new mothers in the biparental dwarf hamster,Phodopus campbelli

Placentophagia in mammals typically occurs only in females during the birth. Male hamsters, Phodopus campbelli, with an extensive paternal behavior repertoire eat placenta during the birth and as alloparental juveniles. Two fresh placentae were presented to sexually naïve males and females covering the developmental range from puberty through reproductive maturity and into senescence. Expectant parents and new mothers were also tested. Placentophagia occurred in both sexes at all developmental stages and was higher in reproductive than in naïve hamsters. Placentophagia declined with increasing age in females, but not males. Liver was readily accepted, but acceptance did not decline with age in females, and was not low in juvenile males, confirming that animals distinguished between the two tissues. Senescent females consumed both tissues willingly. In these paternal males, which do not experience pregnancy or parturition, and in naïve females that selectively refuse placenta, the stimuli influencing placentophagia remain unknown.