Mismatches in resident and stranger serotonin transporter genotypes lead to escalated aggression, and the target for aggression is mediated by sex differences in male and female rhesus monkeys (Macaca mulatta).

A variety of studies show that the s-allele of the serotonin transporter genotype (5-HTT) is related to aggression. However, influences of sex and 5-HTT genotype of both subject and opponent have not received as much attention in aggression research. Using a nonhuman primate model, the present study explores differences in rates of aggression exhibited by 201 group-housed male and female rhesus monkeys (Macaca mulatta; 122 females; 79 males) exposed to an unfamiliar age- and sex-matched stranger while in the presence of other same-sex members of their social group. The study also assesses whether the rates of aggression increase when the home-cage resident, the unfamiliar stimulus animal, or both possess the short (s) allele of the 5-HTT. Results showed that, when compared to females, males exhibited higher rates of physical aggression toward the stranger, and when both the male resident and the male stranger possessed the s-allele, rates of physical aggression toward the stranger increased five-fold. Resident females also engaged in higher rates of physical aggression when they possessed the s-allele, although unlike the males, their physical aggression was directed toward familiar same-sex members of their social group. The findings of this study indicate that rates of physical aggression are modulated by 5-HTT resident and stranger suggest a role of sexual competition in the phenotype of the 5-HTT genotype. Importantly, when two males with impulse deficits, as a function of the s-allele, are placed together, rates of violence exhibited by the dyad escalate substantially.

Central nervous system monoamine metabolite response to alcohol exposure is associated with future alcohol intake in a nonhuman primate model (Macaca mulatta).

It is widely held that the central monoamine neurotransmitters modulate alcohol intake. Few studies, however, directly assess the relationship between baseline and alcohol-induced monoamine turnover, as well as the change from baseline, as predictors of alcohol intake. Using a nonhuman primate model, this study investigates baseline, alcohol-induced and alcohol-induced change in monoamine activity and their relationship with alcohol intake. Alcohol-naïve, adolescent rhesus macaques (Macaca mulatta, N = 114) were administered a standardized intravenous bolus of alcohol solution (16.8%, v/v) on two occasions, approximately 1 month apart. One month prior to and 1 h following each alcohol infusion, cisternal cerebrospinal fluid (CSF) was obtained and assayed for monoamine metabolite concentrations. Approximately 6-7 months later, subjects were allowed unfettered access to an aspartame-sweetened alcohol solution (8.4%, v/v) for 1 h/day, 5 days/week, over 5-7 weeks. Results showed strong positive correlations between baseline and post-infusion CSF monoamine metabolite concentrations, indicating a trait-like response. Low baseline and post-infusion serotonin and dopamine metabolite concentrations and a smaller change in serotonin and dopamine metabolites from one infusion to the next were associated with higher alcohol intake. Low baseline and post-infusion norepinephrine metabolite concentrations predicted high alcohol intake, but unlike the other monoamines, a greater change in norepinephrine metabolite concentrations from one infusion to the next was associated with higher alcohol intake. These findings suggest that individual differences in naturally occurring and alcohol-induced monoamine activity, as well as the change between exposures, are important modulators of initial alcohol consumption and may play a role in the risk for excessive alcohol intake.

Multi-group multi-time point confirmatory factor analysis of the triadic structure of temperament: A nonhuman primate model.

Attempts to describe the latent structure of human infant temperament have led some to suggest the existence of three major dimensions. An earlier exploratory factor analysis (EFA) supported a triadic structure of temperament in week-old rhesus monkey infants, paralleling the structure in human infants. This study sought to confirm the latent triadic structure of temperament across the first month of life in a larger sample of rhesus monkey infants (N = 668), reared by their mothers or in a neonatal nursery. A weekly behavioral assessment was obtained during the first month of life using a subset of items from the widely utilized Infant Behavioral Assessment Scale (IBAS), an instrument designed to measure temperament in infant monkeys. Using the latent constructs proposed by the earlier EFA (Orienting/Regulation, Negative Affectivity, Surgency/Extraversion), multi-group, multi-time point confirmatory factor analyses were conducted to confirm the latent temperament structure across rearing groups at each time point (weeks 1-4). Results confirm and extend those of the earlier EFA: latent Orienting/Regulation,  Negative Affectivity, and Surgency/Extraversion constructs were present across the rearing groups at each time point, with the IBAS items consistently loading onto the latent factors to a similar degree across rearing groups at each time point. These findings suggest foundational evolutionary roots for the triadic structure of human infant temperament, but that its behavioral manifestations vary across maturation and rearing condition. Similarities in latent temperament structure in humans and a representative nonhuman primate highlights the potential for utilizing translational nonhuman primate models to increase understanding of human temperament.

Parental genetic contributions to neonatal temperament in a nonhuman primate (Macaca mulatta) model.

Temperament is an individual's nature and is widely believed to have a heritable foundation. Few studies, however, have evaluated paternal and maternal contributions to the triadic dimensions of temperament. Rhesus monkeys are widely utilized to model genetic contributions to human development due to their close genetic-relatedness and common temperament structure, providing a powerful translational model for investigating paternal and maternal genetic influences on temperament. The temperament of rhesus monkey infants born to 19 different sires and 50 different dams was assessed during the first month of life by comparing the temperament of paternal or maternal half-siblings reared with their mothers in species-normative conditions or reared in a neonatal nursery. Factor scores from three dimensions of temperament were obtained (Orienting/Regulation, Negative Affectivity, and Surgency/Extraversion) and ANOVAs were used to assess genetic effects. For paternal half-siblings, results showed a statistically significant paternal contribution to Orienting/Regulation, Negative Affectivity, and Surgency/Extraversion factor scores. For maternal half-siblings, results showed a statistically significant contribution to Orienting/Regulation factor scores. When parsed by early rearing condition, results showed a paternal contribution Orienting/Regulation, Negative Affectivity, and Surgency/Extraversion scores for paternal half-siblings reared in the neonatal nursery, while there was only a paternal contribution to Surgency/Extraversion for paternal half-siblings reared by their mothers. There was only a maternal contribution to Orienting/Regulation for maternal half-siblings reared by their mothers. These results show that paternal and maternal contributions to temperament vary by environmental context, and that mothers may environmentally buffer their infants from paternal contributions to their temperament.

Maternal neglect and the serotonin system are associated with daytime sleep in infant rhesus monkeys.

Environmental and biological factors contribute to sleep development during infancy. Parenting plays a particularly important role in modulating infant sleep, potentially via the serotonin system, which is itself involved in regulating infant sleep. We hypothesized that maternal neglect and serotonin system dysregulation would be associated with daytime sleep in infant rhesus monkeys. Subjects were nursery-reared infant rhesus macaques (n = 287). During the first month of life, daytime sleep-wake states were rated bihourly (0800-2100). Infants were considered neglected (n = 16) if before nursery-rearing, their mother repeatedly failed to retrieve them. Serotonin transporter genotype and concentrations of cerebrospinal fluid 5-hydroxyindoleacetic acid (5-HIAA) were used as markers of central serotonin system functioning. t tests showed that neglected infants were observed sleeping less frequently, weighed less, and had higher 5-HIAA than non-neglected nursery-reared infants. Regression revealed that serotonin transporter genotype moderated the relationship between 5-HIAA and daytime sleep: in subjects possessing the Ls genotype, there was a positive correlation between 5-HIAA and daytime sleep, whereas in subjects possessing the LL genotype there was no association. These results highlight the pivotal roles that parents and the serotonin system play in sleep development. Daytime sleep alterations observed in neglected infants may partially derive from serotonin system dysregulation.

Early life temperamental anxiety is associated with excessive alcohol intake in adolescence: A rhesus monkey (Macaca mulatta) model.

Teenage alcohol abuse is a major health concern, particularly because the majority of alcohol consumed by teenagers is via binge drinking, a known risk factor for increasing the likelihood for the development of future alcohol use disorders (AUDs). Identifying individuals at risk for excessive alcohol intake in adolescence is a step toward developing effective preventative measures and intervention programs. As adults with AUDs tend to self-medicate their anxiety with alcohol, this longitudinal study assesses the role of infant anxiety-like temperament in the development of adolescent alcohol abuse using a nonhuman primate model. From birth until they were 5 months of age, behaviors of 64 rhesus monkeys (Macaca mulatta) were coded twice a week using an objective mother-infant scoring system that included behaviors traditionally used to assess anxiety and fearfulness in rhesus monkeys. When subjects were four months old, plasma cortisol was obtained. When subjects were adolescents (Mage = 44.88 months), another plasma cortisol sample was obtained about one month prior to allowing them unfettered access to an 8.4% (v/v) aspartame-sweetened alcohol solution for one hour a day over five-to-seven weeks. Results showed that behavioral indications of anxiety-like temperament in infancy, including high levels of mother-infant mutual ventral contact, low levels of environmental exploration, and low levels of interactions with peers were predictive of high adolescent alcohol intake (ie, drinking to intoxication). Plasma cortisol levels in infancy were positively correlated with plasma cortisol in adolescence, and both were positively correlated with high adolescent alcohol intake. Our findings indicate that high levels of traditional anxiety-like behaviors measured in the context of mother-infant interactions, coupled with high infant and adolescent plasma cortisol, are associated with binge-like high alcohol intake in adolescence, suggesting that individuals at risk for developing an AUD later in life may be determined, at least in part, by assessing their physiological and behavioral propensity for anxiety early in life.

Clitoral length in immature and mature captive tufted capuchin (Sapajus spp.) females: A cross-sectional study.

Quantitative data on female external genital morphology are sporadic in the primate literature, and the intraspecific and interfemale variation is especially under investigated (e.g., external clitoris length). Since in most anthropoid primate species female external genitals are relatively small and often hidden, for those species whose external clitoris is described as hypertrophic, external genital resemblance may represent a source of confusion in distinguishing the sexes at a distance. This is the case of both captive and wild tufted capuchin (Sapajus spp.) infants. We provided data on external clitoral length and investigated differences in this trait at different ages in a captive female tufted capuchin population. Since likely allometric growth describes changes in relative dimensions of parts of the body that are correlated with changes in overall size, clitoris length has been analyzed by using body weight as a covariate. We measured clitoral length by adapting a technique developed for spotted hyenas (Crocuta crocuta). Our results suggest that the small body size may be only in part responsible of the perception of long clitoris in female infants, since the clitoris is actually longer in immature females compared to adult ones and its size is inversely related to body weight. While the cross-sectional nature of these data does not allow for conclusive interpretation of the results, we tentatively suggest this phenomenon as a transient male-mimicry by immature females. Our study contributed to the description of normative data in a clitoral trait, thus providing foundation for future studies about causal mechanisms and possible adaptive function(s).

Neonatal temperament and neuromotor differences are predictive of adolescent alcohol intake in rhesus monkeys (Macaca mulatta).

Identifying predictors of teenage alcohol use disorder (AUDs) is a major health initiative, with studies suggesting that there are distinct personality-related traits that underlie patterns of alcohol intake. As temperament is biologically based, identifiable early in life, and stable across time, it is considered the foundation of personality. As such, we hypothesized that neonatal temperament traits would predict anxiety-mediated adolescent alcohol consumption. To test this, N = 145 rhesus macaque (Macaca mulatta) infants (14 days of age), reared in a neonatal nursery (n = 82) or in a control condition with their mothers (n = 63) were assessed with a widely used standardized nonhuman primate testing battery, the Infant Behavioral Assessment Scale (IBAS), modeled after the Brazelton Neonatal Assessment Scale, evaluating visual orienting, temperament, motor maturity and, more recently, sensory sensitivity. As adolescents (3-4 years of age), these same subjects were allowed unfettered access to a sweetened-alcohol solution for 1 hr/day, 4 days/week, over 5-7 weeks. Subjects were allowed to self-administer alcohol while housed alone (n = 70) or socially in their home cage (n = 55). Linear regressions showed that alcohol intake was predicted by neonatal orienting ability (ß = -.35; p = .01), state control (ß = -.19; p = .04), and motor maturity (ß = -.24; p = .01). Poor neonatal orienting, state control (ease of consolability), and motor maturity were associated with higher adolescent alcohol intake in rhesus monkeys. These findings suggest that neonatal temperament is predictive of patterns of adolescent alcohol intake. To the extent that these results generalize to humans, they provide evidence that early-life temperament and neurodevelopment may be important risk factors for adolescent AUDs and that the IBAS may be used as an assessment tool for identifying such risk.

Early rearing history influences oxytocin receptor epigenetic regulation in rhesus macaques.

Adaptations to stress can occur through epigenetic processes and may be a conduit for informing offspring of environmental challenge. We employed ChIP-sequencing for H3K4me3 to examine effects of early maternal deprivation (peer-rearing, PR) in archived rhesus macaque hippocampal samples (male, n = 13). Focusing on genes with roles in stress response and behavior, we assessed the effects of rearing on H3K4me3 binding by ANOVA. We found decreased H3K4me3 binding at genes critical to behavioral stress response, the most robust being the oxytocin receptor gene OXTR, for which we observed a corresponding decrease in RNA expression. Based on this finding, we performed behavioral analyses to determine whether a gain-of-function nonsynonymous OXTR SNP interacted with early stress to influence relevant behavioral stress reactivity phenotypes (n = 194), revealing that this SNP partially rescued the PR phenotype. PR infants exhibited higher levels of separation anxiety and arousal in response to social separation, but infants carrying the alternative OXTR allele did not exhibit as great a separation response. These data indicate that the oxytocin system is involved in social-separation response and suggest that epigenetic down-modulation of OXTR could contribute to behavioral differences observed in PR animals. Epigenetic changes at OXTR may represent predictive adaptive responses that could impart readiness to respond to environmental challenge or maintain proximity to a caregiver but also contribute to behavioral pathology. Our data also demonstrate that OXTR polymorphism can permit animals to partially overcome the detrimental effects of early maternal deprivation, which could have translational implications for human psychiatric disorders.

Intergenerational effects of mother's early rearing experience on offspring treatment and socioemotional development.

This longitudinal study spans two generations of rhesus monkeys. First, the study investigates the effects of early rearing experiences on the maternal behavior of first-generation mothers (rates of premature infant rejection) and, second, the study investigates the effects of maternal rejection on the behavior of second-generation infants. Rhesus macaque mother-infant dyads (Macaca mulatta-N = 176) were observed twice weekly, with each session lasting 300 s. First-generation mothers were raised in one of three conditions: as mother-reared controls (MR; [n = 95]), in peer groups (PR; raised without adults but with constant access to three same-aged peers [n = 49]), or with an inanimate surrogate (SPR; raised with an inanimate fleece-covered, surrogate mother and limited daily peer-group interactions [n = 32]). Second-generation infants were all raised by their differentially reared mothers and statistically grouped into one of two groups: those that were rejected by their mothers beginning at a more-typical weaning age (controls), starting in the third month of life (n = 108), and those that were prematurely rejected, with mothers showing rejections before the third month of infant life (n = 68). Overall, PR mothers exhibited the highest rates of premature infant rejection, except for month 1 of infant life, when SPR mothers exhibited the highest rates of rejection. Intriguingly, after month 1, SPR mothers showed high rates of infant cradling and seldom rejected their infants. Independent of their mothers' early rearing environment, prematurely rejected infants displayed more aggression and passive vigilance, and were cradled and groomed less by their mothers, and there was evidence that the overall rates of rejection after the first 2 months of life had a cumulative negative effect on the developing infant. Post hoc analyses of plasma cortisol levels showed that the prematurely rejected infants had higher cortisol concentrations, suggesting a high level of stress in the prematurely rejected infants. These results suggest that maternal presence during infancy has long-term effects on a female's future maternal skills which, in turn, have intergenerational consequences for the socioemotional development of second-generation infants.

Low Inherent Sensitivity to the Intoxicating Effects of Ethanol in Rhesus Monkeys with Low CSF Concentrations of the Serotonin Metabolite 5-Hydroxyindoleacetic Acid.

BACKGROUND: Type 2 alcoholism is characterized by low serotonin system functioning and has a high degree of heritability, with offspring of alcoholics often showing a reduced response to the intoxicating effects of ethanol (EtOH), which is thought to be marker for future alcohol use disorders (AUDs). As such, an important aim of studies investigating the origins of AUDs is to understand the relationship between serotonin system functioning and level of intoxication. A nonhuman primate model was used to evaluate observational ratings of sensitivity to EtOH and to further investigate the relationship between central serotonin activity and behavioral response to EtOH. METHODS: Cerebrospinal fluid (CSF) concentrations of 5-hydroxyindoleacetic acid (5-HIAA) were obtained from 4 cohorts of alcohol-naïve, adolescent rhesus macaques (N = 82, 45 females, 37 males). One to 3 months after the CSF sample, subjects were administered a standardized intravenous EtOH bolus (males: 2.1 g/kg body weight, females: 2.0 g/kg body weight), placed into an open-top, clear plexiglass chamber suspended from the ceiling, and their latency to escape was recorded as a measure of the degree of intoxication. Thereafter, subjects were rated using a Likert scale for the degree of intoxication during a 30-minute observation period. RESULTS: Our results indicate that latency to escape from the chamber was associated with intoxication ratings (p = 0.0009) following the standardized intravenous administration of EtOH. Low CSF 5-HIAA concentrations predicted short escape latency (p = 0.007) and were associated with low intoxication ratings (p = 0.02), indicating that low central nervous system (CNS) serotonin functioning is related to relative insensitivity to the intoxicating effects of alcohol. CONCLUSIONS: Our study shows that, in monkeys exposed to alcohol for the first time, objective measures of intoxication are associated with subjective ratings for intoxication, and both were associated with CSF 5-HIAA concentrations. Our data confirm and extend the finding that low CNS serotonin functioning is predictive of intrinsic low sensitivity to the intoxicating effects of EtOH.

Who's my little monkey? Effects of infant-directed speech on visual retention in infant rhesus macaques.

Both human and nonhuman primate adults use infant-directed facial and vocal expressions across many contexts when interacting with infants (e.g., feeding, playing). This infant-oriented style of communication, known as infant-directed speech (IDS), seems to benefit human infants in numerous ways, including facilitating language acquisition. Given the variety of contexts in which adults use IDS, we hypothesized that IDS supports learning beyond the linguistic domain and that these benefits may extend to nonhuman primates. We exposed 2.5-month-old rhesus macaque infants (N = 15) to IDS, adult-directed speech (ADS), and a non-social control (CTR) during a video presentation of unrelated stimuli. After a 5- or 60-minute delay, infants were shown the familiar video side-by-side with a novel video. Infants exhibited a novelty preference after the 5-minute delay, but not after the 60-minute delay, in the ADS and CTR conditions, and a novelty preference in the IDS condition only after the 60-minute delay. These results are the first to suggest that exposure to IDS affects infants' long-term memory, even in non-linguistic animals.

Cortisol in Neonatal Mother's Milk Predicts Later Infant Social and Cognitive Functioning in Rhesus Monkeys.

Milk provides not only the building blocks for somatic development but also the hormonal signals that contribute to the biopsychological organization of the infant. Among mammals, glucocorticoids (GCs) in mother's milk have been associated with infant temperament. This study extended prior work to investigate rhesus monkey (Macaca mulatta) mother-infant dyads (N = 34) from birth through 8 months postpartum. Regression analysis revealed that cortisol concentrations in milk during the neonatal period predicted impulsivity on a cognitive task, but not global social behaviors, months later. During this time period, sex-differentiated social behavior emerged. For female infants, milk cortisol concentrations predicted total frequency of play. Collectively, these findings support and extend the "lactational programming" hypothesis on the impact of maternal-origin hormones ingested via milk.

Face Detection and the Development of Own-Species Bias in Infant Macaques.

In visually complex environments, numerous items compete for attention. Infants may exhibit attentional efficiency-privileged detection, attention capture, and holding-for face-like stimuli. However, it remains unknown when these biases develop and what role, if any, experience plays in this emerging skill. Here, nursery-reared infant macaques' (Macaca mulatta; n = 10) attention to faces in 10-item arrays of nonfaces was measured using eye tracking. With limited face experience, 3-week-old monkeys were more likely to detect faces and looked longer at faces compared to nonfaces, suggesting a robust face detection system. By 3 months, after peer exposure, infants looked faster to conspecific faces but not heterospecific faces, suggesting an own-species bias in face attention capture, consistent with perceptual attunement.

The Signature of Maternal Social Rank in Placenta Deoxyribonucleic Acid Methylation Profiles in Rhesus Monkeys.

The effects of social status on human health can be modeled in captive cohorts of nonhuman primates. This study shows that maternal social rank is associated with broad changes in DNA methylation in placentae of rhesus monkeys (N = 10). Differentially methylated genes between social ranks are enriched in signaling pathways playing major roles in placenta physiology. Moreover, the authors found significant overlaps with genes whose expression was previously associated with social rank in adult rhesus monkeys (Tung et al., 2012) and whose methylation was associated with perinatal stress in newborn humans and rhesus monkeys (Nieratschker et al., 2014). These results are consistent with the hypothesis that system-wide epigenetic changes in multiple tissues are involved in long-term adaptations to the social environment.

OPRM1 genotype interacts with serotonin system dysfunction to predict alcohol-heightened aggression in primates.

Although the notion that alcohol promotes violence is widespread, not all individuals are aggressive while intoxicated. Genetic variation could be a contributing factor to individual differences in alcohol-heightened aggression. The present study examines the effects of OPRM1C77G genotype on responses to threat in rhesus macaques under normal conditions and following alcohol administration. Prior studies have shown that a low CSF level of 5-HIAA is a trait marker for individuals prone to escalated aggression. We wanted to examine whether the predictive value for this marker on aggression was moderated by OPRM1 genotype. Animals were administered alcohol (BAC 100-200 mg%), were provoked by a human intruder, and aggressive responses were recorded. Factor analysis was performed to generate aggressive response factors, which were then used as dependent variables for ANOVA, with OPRM1 genotype and CSF 5-HIAA as independent variables. Factor analysis generated three factors ('Threatening', 'Distance Decreasing' and 'High Intensity'). We found that High Intensity aggression was increased among carriers of the OPRM1 G allele, especially among individuals with low CSF levels of 5-HIAA. Aggression in the non-intoxicated state was predicted by 5-HIAA, but not by genotype. This study demonstrates that OPRM1 genotype predicts alcohol-heightened aggression in rhesus macaques with low CSF levels of 5-HIAA. Because OPRM1 variation predicts similar effects on alcohol response and behavior in humans and macaques, this study could suggest a role for OPRM1 genotype in alcohol-heightened aggression in humans. If so, it may be that compounds that block this receptor could reduce alcohol-associated violence in selected patient populations.

Differential relationships between chronic hormone profiles in pregnancy and maternal investment in rhesus monkey mothers with hair loss in the neonatal period.

Hair loss is commonly used as an indicator of well being in primate facilities, yet it has been shown to also occur in otherwise healthy pregnant and postpartum females. There is significant variability in the incidence of hair loss during these important developmental periods, reasons for which remain unclear. We studied female rhesus monkeys (Macaca mulatta, n = 47) with and without hair loss in pregnancy/postpartum. We hypothesized that, similar to previously published reports, pregnancy would result in an increased likelihood of hair loss, and that hair loss would be correlated with higher hair cortisol concentrations (HCCs). We further hypothesized that hair loss among pregnant females is related to differential maternal investment. We studied a subset of monkeys (n = 26) from mid-to-late pregnancy through peak lactation, some of which exhibited hair loss in the perinatal period (n = 15), and some of which did not (n = 11). We examined fetal measurements, infant birth weight, infant growth rate, and milk yield volume (MYV) in the first 30 days as indices of investment. We found that pregnant monkeys showed a greater incidence of hair loss across the study year (χ2(2) = 6.55, P = 0.038), and that mothers with hair loss had significantly higher HCCs in pregnancy than those without (F(2,28) = 3.8, P = 0.017, ηp2 = 0.21). HCCs in pregnancy were correlated with severity of hair loss in the neonatal period (r(37) = 0.42, P = 0.008). Moreover, HCCs in pregnancy were positively correlated with infant birth weight (r(12) = 0.56, P = 0.038), infant growth rate (r(12) = 0.64, P = 0.014), and MYV (r(11) = 0.85, P < 0.001) for alopecic but not non-alopecic mothers. These mothers did not differ in fetal measurements, infant birth weight/growth rate, or MYV. Our results suggest that hair loss in some monkeys, especially during the birthing season, may be a signal of greater physiological stress during pregnancy and differential investment by mothers to their offspring. Am. J. Primatol. 79:e22489, 2017. © 2015 Wiley Periodicals, Inc.

Inhaled oxytocin increases positive social behaviors in newborn macaques.

Early caregiver-infant interactions are critical for infants' socioemotional and cognitive development. Several hormones and neuromodulators, including oxytocin, affect these interactions. Exogenous oxytocin promotes social behaviors in several species, including human and nonhuman primates. Although exogenous oxytocin increases social function in adults--including expression recognition and affiliation--it is unknown whether oxytocin can increase social interactions in infants. We hypothesized that nebulized oxytocin would increase affiliative social behaviors and such effects would be modulated by infants' social skills, measured earlier in development. We also hypothesized that oxytocin's effects on social behaviors may be due to its anxiolytic effects. We tested these hypotheses in a blind study by nebulizing 7- to 14-d-old macaques (n = 28) with oxytocin or saline. Following oxytocin administration, infants' facial gesturing at a human caregiver increased, and infants' salivary oxytocin was positively correlated with the time spent in close proximity to a caregiver. Infants' imitative skill (measured earlier in development: 1-7 d of age) predicted oxytocin-associated increases in affiliative behaviors--lip smacking, visual attention to a caregiver, and time in close proximity to a caregiver--suggesting that infants with higher propensities for positive social interactions are more sensitive to exogenous oxytocin. Oxytocin also decreased salivary cortisol, but not stress-related behaviors (e.g., scratching), suggesting the possibility of some anxiolytic effects. To our knowledge, this study provides the first evidence that oxytocin increases positive social behaviors in newborns. This information is of critical importance for potential interventions aimed at ameliorating inadequate social behaviors in infants with higher likelihood of developing neurodevelopmental disorder.

Rank acquisition in rhesus macaque yearlings following permanent maternal separation: The importance of the social and physical environment.

Rank acquisition is a developmental milestone for young primates, but the processes by which primate yearlings attain social rank in the absence of the mother remain unclear. We studied 18 maternally reared yearling rhesus macaques (Macaca mulatta) that differed in their social and physical rearing environments. We found that early social experience and maternal rank, but not individual traits (weight, sex, age), predicted dominance acquisition in the new peer-only social group. Yearlings also used coalitions to reinforce the hierarchy, and social affiliation (play and grooming) was likely a product, rather than a determinant, of rank acquisition. Following relocation to a familiar environment, significant rank changes occurred indicating that familiarity with a physical environment was salient in rank acquisition. Our results add to the growing body of literature emphasizing the role of the social and physical environment on behavioral development, namely social asymmetries among peers.

Animal studies help clarify misunderstandings about neonatal imitation.

Empirical studies are incompatible with the proposal that neonatal imitation is arousal driven or declining with age. Nonhuman primate studies reveal a functioning brain mirror system from birth, developmental continuity in imitation and later sociability, and the malleability of neonatal imitation, shaped by the early environment. A narrow focus on arousal effects and reflexes may grossly underestimate neonatal capacities.