Effects of social defeat on paternal behavior and pair bonding behavior in male California mice (Peromyscus californicus).

Male parental care is an important social behavior for several mammalian species. Psychosocial stress is usually found to inhibit maternal behavior, but effects on paternal behavior have been less consistent. We tested the effects of social defeat stress on pair bond formation and paternal behavior in the monogamous California mouse (Peromyscus californicus). Social defeat reduced time spent in a chamber with a stranger female during a partner preference test conducted 24h after pairing, but increased latency to the first litter. In 10min partner preference tests conducted after the birth of pups, both control and stressed males exhibited selective aggression towards stranger females. Unlike prairie voles, side by side contact was not observed in either partner preference test. Stressed male California mice engaged in more paternal behavior than controls and had reduced anxiety-like responses in the open-field test. Defeat stress enhanced prodynorphin and KOR expression in the medial preoptic area (MPOA) but not PVN. Increased KOR signaling has been linked to increased selective aggression in prairie voles. Together the results show that defeat stress enhances behaviors related to parental care and pair bonding in male California mice.

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

In biparental mammals, the factors facilitating the onset of male parental behavior are not well understood. While hormonal changes in fathers may play a role, prior experience with pups has also been implicated. We evaluated effects of prior exposure to pups on paternal responsiveness in the biparental California mouse (Peromyscus californicus). We analyzed behavioral, neural, and corticosterone responses to pups in adult virgin males that were interacting with a pup for the first time, adult virgin males that had been exposed to pups 3 times for 20min each in the previous week, and new fathers. Control groups of virgins were similarly tested with a novel object (marble). Previous exposure to pups decreased virgins' latency to approach pups and initiate paternal care, and increased time spent in paternal care. Responses to pups did not differ between virgins with repeated exposure to pups and new fathers. In contrast, repeated exposure to a marble had no effects. Neither basal corticosterone levels nor corticosterone levels following acute pup or marble exposure differed among groups. Finally, Fos expression in the medial preoptic area, ventral and dorsal bed nucleus of the stria terminalis was higher following exposure to a pup than to a marble. Fos expression was not, however, affected by previous exposure to these stimuli. These results suggest that previous experience with pups can facilitate the onset of parental behavior in male California mice, similar to findings in female rodents, and that this effect is not associated with a general reduction in neophobia.

Social and physical environments as a source of individual variation in the rewarding effects of testosterone in male California mice (Peromyscus californicus).

Despite extensive research revealing the occurrence of testosterone (T) pulses following social encounters, it is unclear how they lead to varied behavioral responses. We investigated the influence of residency (home versus unfamiliar environment) and social/sexual experience (pair-bonded, isolated or housed with siblings) on the plasticity of T's rewarding effects by measuring the development of conditioned place preferences (CPPs), a classical paradigm used to measure the rewarding properties of drugs. For pair-bonded males, T-induced CPPs were only produced in the environment wherein the social/sexual experience was accrued and residency status had been achieved. For isolated males, the T-induced CPPs only occurred when the environment was unfamiliar. For males housed with a male sibling, the T-induced CPPs were prevented in both the home and unfamiliar chambers. Our results reveal the plasticity of T's rewarding effects, and suggest that the behavioral functions of T-pulses can vary based on social/sexual experience and the environment in which residency was established. The formation of CPPs or reward-like properties of drugs and natural compounds can therefore exhibit malleability based on past experience and the current environment.

Naturally occurring variation in vasopressin immunoreactivity is associated with maternal behavior in female Peromyscus mice.

In many mammals, species-appropriate social behavior is necessary for an individual's ability to survive and reproduce. In the present study, we examined whether arginine-vasopressin (AVP) pathways that have been associated with social behavior differed between two closely related species of Peromyscus mice with different patterns of maternal behavior. We also tested whether individual levels of AVP-immunoreactive staining (AVP-ir) were associated with individual levels of maternal behavior as measured using a composite score consisting of huddling, nursing, grooming and time spent inside the nest (HNGI score). In addition, we examined whether these associations between vasopressin and behavior differed between species. Females from the highly biparental species, California mice, displayed higher AVP-ir in the bed nucleus of the stria terminalis (BNST), which corresponded with a higher level of nest building and a higher HNGI score than was found in the less parental white-footed mice. The HNGI score was positively associated with AVP-ir in the medial amygdala in female California mice but not white-footed mice. Finally, we examined whether AVP-ir in these pathways varied based on the species-specific rearing environments by reciprocally cross-fostering California mice and white-footed mice. In contrast to previous research with male California mice, cross-fostering itself had no effect on maternal behavior or any consistent effect on AVP-ir staining in brain areas such as the BNST and associated brain areas. This suggests that there is little plasticity in maternal behavior and that the underlying AVP system in females does not respond to the postnatal environment provided by the parents. The positive associations between maternal behavior and AVP-ir indicate that AVP may regulate maternal behavior despite the lack of plasticity in AVP and maternal behavior.

Sexual imprinting and speciation between two Peromyscus species.

Sexual isolation, a reproductive barrier, can prevent interbreeding between diverging populations or species. Sexual isolation can have a clear genetic basis; however, it may also result from learned mate preferences that form via sexual imprinting. Here, we demonstrate that two sympatric species of mice-the white-footed mouse (Peromyscus leucopus) and its sister species, the cotton mouse (P. gossypinus)-hybridize only rarely in the wild despite co-occurrence in the same habitat and lack of any measurable intrinsic postzygotic barriers in laboratory crosses. We present evidence that strong conspecific mating preferences in each species result in significant sexual isolation. We find that these preferences are learned in at least one species: P. gossypinus sexually imprints on its parents, but in P. leucopus, additional factors influence mating preferences. Our study demonstrates that sexual imprinting contributes to reproductive isolation that reduces hybridization between otherwise interfertile species, supporting the role for learning in mammalian speciation.

Back to basics: A methodological perspective on marble-burying behavior as a screening test for psychiatric illness.

Animal models of human psychiatric illness are valuable frameworks to investigate the etiology and neurobiology underlying the human conditions. Accurate behavioral measures that can be used to characterize animal behavior, thereby contributing to a model's validity, are crucial. One such measure, i.e. the rodent marble-burying test (MBT), is often applied as a measure of anxiety- and compulsive-like behaviors. However, the test is characterized by noteworthy between-laboratory methodological differences and demonstrates positive treatment responses to an array of pharmacotherapies that are often of little translational value. Therefore, using a naturalistic animal model of obsessive-compulsive disorder, i.e. the deer mouse (Peromyscus maniculatus bairdii), the current investigation attempted to illuminate the discrepancies reported in literature by means of a methodological approach to the MBT. Five key aspects of the test that vary between laboratories, viz. observer/scoring, burying substrate, optional avoidance, the use of repeated testing, and determinations of locomotor activity, have been investigated. Following repeated MB tests in four different burying substrates and in two zone configurations, we have demonstrated that 1) observer bias may contribute to the significant differences in findings reported, 2) MB seems to be a natural exploratory response to a novel environment, rather than being triggered by aberrant cognition, 3) burying substrates with a small particle size and higher density deliver the most accurate results with respect to the burying phenotype, and 4) to exclude the influence of normal exploratory behavior on the number of marbles being covered, assessments of marble-burying should be based on pre-occupation with the objects itself.

Metabolic and affective consequences of fatherhood in male California mice.

Physiological and affective condition can be modulated by the social environment and parental state in mammals. However, in species in which males assist with rearing offspring, the metabolic and affective effects of pair bonding and fatherhood on males have rarely been explored. In this study we tested the hypothesis that fathers, like mothers, experience energetic costs as well as behavioral and affective changes (e.g., depression, anxiety) associated with parenthood. We tested this hypothesis in the monogamous, biparental California mouse (Peromyscus californicus). Food intake, blood glucose and lipid levels, blood insulin and leptin levels, body composition, pain sensitivity, and depression-like behavior were compared in males from three reproductive groups: virgin males (VM, housed with another male), non-breeding males (NB, housed with a tubally ligated female), and breeding males (BM, housed with a female and their first litter). We found statistically significant (P<0.007, when modified for Adaptive False Discovery Rate) or nominally significant (0.007

Social behavior in deer mice as a novel interactive paradigm of relevance for obsessive-compulsive disorder (OCD).

Greater obsessive-compulsive (OC) symptom severity may be associated with poor social adjustment. Rather than possessing deficits in social skill per se, OCD patients may be more socially isolative in the presence of normal controls. We aimed to apply a novel social interaction challenge (SIC) to an established animal model of OCD, viz., the deer mouse, to assess complex social behavior in animals by investigating group sociability and its response to chronic escitalopram treatment (50 mg/kg/day × 28 days), both within and between non (N)- (viz., normal) and high (H)- (viz., OCD-like) stereotypical cohorts. Using automated screening, we scored approach behavior, episodes of proximity, duration of proximity, and relative net weighted movement. H animals socialized more with one another within cohort in all of the above parameters compared to the within-cohort behavior of N animals. Furthermore, the social behavior of H animals toward one another, both within and between cohort demonstrated significant improvements following chronic escitalopram treatment. However, the study also demonstrates that the social interaction between H and N animals remain poor even after chronic escitalopram treatment. To conclude, findings from the current investigation support clinical data demonstrating altered sociability in patients with OCD.

Prolonged separation delays wound healing in monogamous California mice, Peromyscus californicus, but not in polygynous white-footed mice, P. leucopus.

Social interactions are often stressful, but under certain circumstances, they may be beneficial for health and well-being. In a previous study, wound healing was slowed after mate separation (2 days) in monogamous California mice, Peromyscus californicus, but not polygynous white-footed mice, P. leucopus. Although these results indicate that positive social interaction is critical for immune activity in some species, the extent to which such social effects are enduring remains unspecified. The goal of the present experiments was to determine whether a period representing approximately 20% of expected adult lifespan of these species in the wild (8 weeks) would affect wound healing. Because our experimental design required that the same animals were wounded twice, we were also able to determine the extent to which wound healing is repeatable. Wound healing remained delayed after 8 weeks of separation in P. californicus, and healing scores were not correlated between first and second wounds within individuals. In P. leucopus however, housing conditions did not influence wound healing, but first and second wound healings were correlated indicating repeatability. In sum, our results suggest that positive social interactions may be important for promoting immune activity in some species.

Relative Preference and Localized Food Affect Predator Space Use and Consumption of Incidental Prey.

Abundant, localized foods can concentrate predators and their foraging efforts, thus altering both the spatial distribution of predation risk and predator preferences for prey that are encountered incidentally. However, few investigations have quantified the spatial scale over which localized foods affect predator foraging behavior and consumption of incidental prey. In spring 2010, we experimentally tested how point-source foods altered how generalist predators (white-footed mice, Peromyscus leucopus) utilized space and depredated two incidental prey items: almonds (Prunus dulcis; highly profitable) and maple seeds (Acer saccharum; less profitable). We estimated mouse population densities with trapping webs, quantified mouse consumption rates of these incidental prey items, and measured local mouse activity with track plates. We predicted that 1) mouse activity would be elevated near full feeders, but depressed at intermediate distances from the feeder, 2) consumption of both incidental prey would be high near feeders providing less-preferred food and, 3) consumption of incidental prey would be contingent on predator preference for prey relative to feeders providing more-preferred food. Mouse densities increased significantly from pre- to post-experiment. Mean mouse activity was unexpectedly greatest in control treatments, particularly <15 m from the control (empty) feeder. Feeders with highly preferred food (sunflower seeds) created localized refuges for incidental prey at intermediate distances (15 to 25m) from the feeder. Feeders with less-preferred food (corn) generated localized high risk for highly preferred almonds <10 m of the feeder. Our findings highlight the contingent but predictable effects of locally abundant food on risk experienced by incidental prey, which can be positive or negative depending on both spatial proximity and relative preference.

Age and temperature related changes in behavioral and physiological performance in the Peromyscus leucopus mouse.

Age-related and ambient temperature-related changes in motor activity, body temperature, body weight (b.w.), and food consumption were studied in the long-lived Peromyscus leucopus mouse at environmental temperatures of 29 and 21 degrees C. Major changes in physiological performance were observed between the young (6 months) and old (60-72 month) age groups. The number of daily activity episodes, and total activity output was significantly lower in old mice. Maximum, average and minimum daily body temperature was lower in the old mice and a significant ambient temperature-by-age interaction was found. Maximum, minimum, and average daily b.w. was higher in old mice. Motor activity was evenly distributed over the active (night) phase in young mice but in old mice activity was significantly greater in the late night partition of the active cycle than in the early night partition. Both groups were significantly more active at night than during the day. Most of the food consumption in both groups occurred at night, but young mice consumed significantly more during the late night partition than the early night partition, and the consumption rates for old mice were not significantly different between early and late night partitions. The percentage of activity episodes involved with food consumption in both groups was significantly higher during the night partition, but the percentage during the early night partition was significantly higher in old mice than in young mice. Significant episodes of circadian torpor occurred in a high percentage of old mice at 06:00, on consecutive days, at both environmental temperatures, but young mice expressed no evidence of torpor.

Testosterone promotes paternal behaviour in a monogamous mammal via conversion to oestrogen.

Although high testosterone (T) levels inhibit paternal behaviour in birds breeding in temperate zones many paternal mammals have a very different breeding biology, characterized by a post-partum oestrus. In species with post-partum oestrus, males may engage in T-dependent behaviours such as aggression and copulation simultaneously with paternal behaviour. We previously found that T promotes paternal behaviour in the California mouse, Peromyscus californicus. We examine whether this effect is mediated by the conversion of T to oestradiol (E(2)) by aromatase. In the first experiment, gonadectomized males treated with T or E(2) implants showed higher levels of huddling and pup grooming behaviour than gonadectomized males treated with dihydrotestosterone or empty implants. In the second experiment, we used an aromatase inhibitor (fadrozole) (FAD) to confirm these results. Gonadectomized males treated with T + vehicle or E(2) + FAD showed higher levels of huddling and pup grooming behaviour than gonadectomized males treated with T + FAD or empty implants. Although E(2) is known to promote the onset of maternal behaviour to our knowledge our results are the first to demonstrate that E(2) can promote paternal behaviour in a paternal mammal. These results may explain how mammals express paternal behaviour while T levels are elevated.

Medial preoptic lesions disrupt parental behavior in both male and female California mice (Peromyscus californicus).

California mice (Peromyscus californicus) are monogamous and naturally biparental, making them an ideal species in which to study the neural basis of paternal behavior. A male or female from each male-female pair was given an electrolytic or sham lesion in the medial preoptic area (MPOA), an area known to be critical for the expression of maternal behavior in rats, and retested for parental responsiveness. MPOA-lesioned males and females showed significantly longer latencies to show parental behavior and spent significantly less time near pups, sniffing pups, and licking pups than sham-lesioned mice. However, MPOA lesions did not reduce time spent hovering over pups. The results suggest that the neural mechanisms mediating paternal behavior are similar to those mediating maternal behavior in this species.

Tests of preferences of young deer mice (Peromyscus maniculatus bairdi) for siblings versus nonsiblings.

Three experiments were conducted to test the performance of young deer mice (Peromyscus maniculatus bairdi) in associating preferentially with siblings versus nonsiblings. In Experiment 1, neither males nor females preferred caged siblings versus nonsiblings. In Experiment 2, with full contact permitted, nonsibling males were in contact as much as were sibling males. In Experiment 3, with pairs of littermate pairs permitted full contact, there was no significant differential association. Although discrimination and preferences related to kinship have been demonstrated in some species, they cannot be assumed to be universal.

Tests of preferences of adult deer mice (Peromyscus maniculatus bairdi) for siblings versus nonsiblings.

Seven experiments were conducted to test the performance of adult deer mice (Peromyscus maniculatus bairdi) in discriminating and preferring siblings versus nonsiblings. In two experiments there was no indication of preferences based on odor alone. Further, there was no indication of kin discrimination in a seminatural enclosure. There was some indication that in a test apparatus, estrous females associate preferentially with male siblings rather than nonsiblings. The effect appears dependent on the presence of siblings during development. Although the finding of a preference for siblings in estrous females is counter to expectations from an inbreeding avoidance approach, this is one of a variety of findings to suggest that not all animals avoid inbreeding all of the time.

The effects of exogenous melatonin and melatonin receptor blockade on aggression and estrogen-dependent gene expression in male California mice (Peromyscus californicus).

Photoperiodic regulation of aggression has been well established in several vertebrate species, with rodents demonstrating increased aggression in short day photoperiods as compared to long day photoperiods. Previous work suggests that estrogens regulate aggression via rapid nongenomic pathways in short days and act more slowly in long days, most likely via genomic pathways. The current study therefore examines the role of melatonin in mediating aggression and estrogen-dependent gene transcription. In Experiment 1, male California mice were housed under long day photoperiods and were treated with either 0.3 µg/g of melatonin, 40 mg/kg of the melatonin receptor antagonist luzindole, or vehicle for 10 days. We found that melatonin administration significantly increased aggression as compared to mice receiving vehicle, but this phenotype was not completely ameliorated by luzindole. In Experiment 2, male California mice were injected with either 1mg/kg of the aromatase inhibitor letrozole or vehicle, and oxytocin receptor (OTR), estrogen receptor alpha (ERα), and c-fos gene expression was examined in the bed nucleus of the stria terminalis (BNST) and medial preoptic area (MPOA). In the BNST, but not MPOA, OTR mRNA was significantly downregulated following letrozole administration, indicating that OTR is an estrogen-dependent gene in the BNST. In contrast, ERα was not estrogen dependent in either brain region. In the MPOA, OTR mRNA was inhibited by melatonin, and luzindole suppressed this effect. C-fos and ERα did not differ between treatments in any brain region examined. These results suggest that it is unlikely that melatonin facilitates aggression via broad spectrum regulation of estrogen-dependent gene expression. Instead, melatonin may act via regulation of other transcription factors such as extracellular signal regulated kinase.

Evolution of monogamy, paternal investment, and female life history in Peromyscus.

The timing of reproductive development and associated trade-offs in quantity versus quality of offspring produced across the life span are well documented in a wide range of species. The relation of these aspects of maternal life history to monogamy and paternal investment in offspring is not well studied in mammals, due in part to the rarity of the latter. By using five large, captive-bred populations of Peromyscus species that range from promiscuous mating with little paternal investment (P. maniculatus bairdii) to social and genetic monogamy with substantial paternal investment (P. californicus insignis), we modeled the interaction between monogamy and female life history. Monogamy and high paternal investment were associated with smaller litter size, delayed maternal reproduction that extended over a longer reproductive life span, and larger, higher quality offspring. The results suggest monogamy and paternal investment can alter the evolution of female life-history trajectories in mammals.

Physiological and neuroendocrine responses to chronic variable stress in male California mice (Peromyscus californicus): Influence of social environment and paternal state.

Social environment and parental state affect stress responses in mammals, but their impact may depend on the social and reproductive strategy of the species. The influences of cohabitation with a male or female conspecific, and the birth of offspring, on the physiological and endocrine responses to chronic variable stress were studied in the monogamous and biparental California mouse (Peromyscus californicus). Adult male California mice were housed either with a male cage mate (virgin males, VM), a female cage mate (pair-bonded males, PBM), or a female cage mate and their first newborn litter (new fathers, NF). VM, PBM and NF underwent a 7-day chronic variable stress paradigm (CVS, three stressors per day at semi-random times, n=7-8 per housing condition). Compared to control males (CON, n=6-7 per housing condition), CVS caused loss of body mass, increased basal plasma corticosterone concentrations, and increased basal expression of arginine vasopressin (AVP) mRNA in the paraventricular nucleus of the hypothalamus (PVN). These effects were independent of housing condition. Neither CVS nor housing condition altered novel-stressor-induced corticosterone release, spleen or testis mass, or basal expression of corticotropin-releasing hormone (CRH) mRNA in the PVN. Although CVS appeared to increase adrenal mass and reduce thymus mass specifically in NF, these effects were explained by the lower adrenal mass and higher thymus mass of NF compared to PBM and VM under control conditions. These results suggest that neither engaging in a pair bond nor becoming a father attenuates typical responses to CVS, but that fatherhood may provide a buffer against transient mild stressors (i.e., weighing and blood sampling in the control groups) in this monogamous and biparental rodent.