Longitudinal stability of friendships in rhesus monkeys (Macaca mulatta): individual- and relationship-level effects.

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): (1) individual characteristics including sex, dominance rank, matriline size, and temperament; and (2) 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.