Affiliative relationship formation in nonhuman primates is known to be influenced by kinship, rank, and sex, but such factors do not fully explain observed variation in primate social relations. Individual differences in temperament have a number of important behavioural and physiological correlates that might influence relationship formation. We observed 57 yearling rhesus macaques at the California National Primate Research Center for 10 weeks to determine whether individual differences in temperament relate to the number and quality of affiliative relationships formed with peers. Subjects’ temperament characteristics had previously been quantified during a colony-wide biobehavioural assessment at 90–120 days of age. Yearlings that had scored high on Equability (demonstrating calmness and low levels of physical activity) as infants had fewer peer relationships compared to animals low on this dimension. Additionally, yearlings preferentially affiliated with peers that were similar to themselves in Equability as well as in Adaptability (reflecting the degree of behavioural flexibility that subjects displayed during the biobehavioural assessment). Although kinship, rank, and sex influenced relationship formation as expected, temperament remained a significant predictor of affiliative preferences even after controlling for these variables. Our findings suggest that temperament is a proximate determinant of variation in affiliative relationship formation in group-living primates.

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The longevity of children’s friendships is influenced by a multitude of individual- and relationship-level attributes, but little is known about the factors that impact friendship maintenance in nonhuman primate juveniles. We investigated whether the following predicted the longitudinal stability of friendships in juvenile rhesus monkeys (Macaca mulatta): (a) individual characteristics including sex, dominance rank, matriline size, and temperament; and (b) relationship characteristics including kinship, reciprocity, complexity, and similarity between friends in sex, rank, and temperament. We recorded affiliative interactions of 29 two-year-old rhesus monkeys, previously observed as yearlings, at the California National Primate Research Center. Friends were defined as peers with whom subjects spent more time affiliating than expected by chance. Temperament had been assessed at 3-4 months of age. Sex was the only individual characteristic predicting friendship stability: males maintained more friendships from age one to two than did females. Relationship characteristics predicting friendship stability included similarity between individuals in temperament, kinship, and sex. In addition, reciprocated friendships, rather than unidirectional friendships, were significantly more likely to persist over time. Our findings suggest that the factors influencing friendship maintenance in rhesus monkeys are similar to those impacting human friendship longevity.

The neuropeptides oxytocin (OT) and arginine vasopressin (AVP) are involved in social bonding in attachment relationships, but their role in friendship is poorly understood. We investigated whether rhesus macaques’ (Macaca mulatta) friendships at age one predicted plasma OT and AVP at two later time points. Subjects were 54 rhesus macaques at the California National Primate Research Center (CNPRC). Blood was drawn during a brief capture-and-release in the home cage, and plasma assayed for OT and AVP using an enzyme immunoassay (EIA). Separate linear mixed models for each sex tested the effects of dominance rank, age, sampling time point, housing condition, parturition status, two blood draw timing measures, and five friendship types: proximity friendships, play friendships, reciprocal friendships (a preference for a peer that also preferred the subject), multiplex friendships (friendships displayed in more than one behavioral domain), and total number of friendships. Females’ number of reciprocal and play friendships at age one significantly predicted later OT; additionally, these two friendship types interacted with rank, such that high-ranking females with the fewest friendships had the highest OT concentrations. Friendship did not predict later OT levels in males, however proximity, play, reciprocal, and total number of friendships predicted males’ plasma AVP. Play and total number of friendships also tended to predict AVP in females. Our results show that peripheral measures of neuroendocrine functioning in juvenile rhesus monkeys are influenced by early involvement in friendships. Friendships have an especially strong impact on an individual’s psychosocial development, and our data suggest OT and AVP as potential underlying mechanisms. Moreover, sex differences in the functioning of the OT and AVP systems, and their relation to friendship, may have important clinical implications for the use of OT as a therapeutic, as well as informing the social context in which it is administered.

In socially monogamous titi monkeys, involuntary separation from a pair mate can produce behavioral distress and increased cortisol production. The neuropeptide oxytocin (OXT) is thought to play an important role in the separation response of pair-bonded species. Previous studies from our lab have shown that chronic intranasal oxytocin (IN OXT) during development can have long-term effects on adult social behavior. In the current study, we examined the chronic and acute effects of IN OXT or Saline (SAL) on the subjects’ response to a brief separation from their pair mates. Subjects with a history of chronic IN OXT or SAL treatment during development received a single dose of OXT or SAL as adults 30 min before being separated from their pair mate. Chronic treatment consisted of a daily dose of IN OXT (0.8 IU/kg) or SAL (control) from 12 to 18 months of age. Subjects (N = 29) were introduced to a pair mate at 30 months of age. After the pairs had cohabitated for 5 months, pairs underwent two “Brief Separation” (OXT and SAL) and two “Non-Separation” (OXT and SAL) test sessions. Vocalizations and locomotion were measured as behavioral indices of agitation or distress during the Brief Separation and Non-Separation periods (30 min each). We collected blood samples after the Brief Separation and Non-Separation periods to measure cortisol levels. Our results showed subjects treated with chronic OXT had a reduction in long call and peep vocalizations compared to subjects treated with chronic SAL. Subjects treated with chronic SAL and acute OXT produced more peeps and long calls compared to animals treated with acute SAL; however, patterns in this response depended on sex. Cortisol and locomotion were significantly higher during the Brief Separation period compared to the Non-Separation period; however, we did not find any treatment or sex effects. We conclude that chronic IN OXT given during development blunts the separation response, while acute OXT in chronic SAL subjects had sexually dimorphic effects, which could reflect increased partner seeking behaviors in males and increased anxiety in females.