Enrichment effects on rhesus monkeys successively housed singly, in pairs, and in groups

Many studies have examined the short-term effects of inanimate environmental enrichment on the behavior of captive primates. Similarly, numerous studies have examined the behavioral effects of manipulations to the social environment. Few analyses have examined the long-term effects of inanimate environmental enrichment or compared the effects of inanimate and social enhancements. The behavior of control (n = 49) and enriched (n = 44) rhesus monkeys (Macaca mulatta) was observed from 1-4 years, as subjects spent successive years housed singly, in pairs, and then in small groups. Social housing condition and/or age of subject significantly affected time spent inactive, grooming, playing, exploring, and behaving abnormally. Frequency of vocalization was similarly affected. Social housing resulted in more species-typical behavior patterns than did single caging. Subjects were more socially oriented when pair-housed than when living in small groups. Inanimate enrichment did not affect behavior over a 3 year period, although enhancements were well-used by singly-caged yearlings and pair-housed juveniles. Subjects used enrichment less frequently when housed in groups. The data suggest that the presence of a social partner(s) led to more beneficial changes in behavior than did the provision of inanimate enhancements for rhesus monkeys in the studied age range. Therefore, it is concluded that social enrichment should be provided when possible and appropriate. Efforts to enrich the inanimate environment will be most beneficial when focused on socially-restricted primates.