Behavioral responses of unrelated adult male rhesus monkeys familiarized and paired for the purpose of environmental enrichment.

An attempt was made to pair ten unrelated, adult male rhesus monkeys that had been kept in single cages for several years. Potential companions were first given the opportunity to establish clear-cut rank relationships (unidirectional fear-grinning and/or withdrawing) during a 5-day period of noncontact familiarization. Only then were they paired in a different double cage. Rank relationships were confirmed within the first 6 minutes after pairing without the occurrence of any biting and fighting. A total of four physical aggressions (slapping) were observed in all five dyads during 5 hours of observations on the first 5 days after pairing. It was concluded that the establishment of clear-cut rank relationships prior to pairing was instrumental in this extremely low incidence of aggression.