Individual Differences in Infant Temperament Predict Social Relationships of Yearling Rhesus Monkeys (Macaca mulatta).

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.