A comparison of the effects of simple versus complex environmental enrichment on the behaviour of group-housed, subadult rhesus macaques

Enrichment of the environments of captive primates is currently of interest as both a basic and an applied research question, particularly when social and inanimate enhancements are used simultaneously. We measured the behavioural effects of two intensities of inanimate enrichment on 12 unimale-multifemale groups and 12 all-male groups from three cohorts of three to four-year-old rhesus macaques (Macaca mulatta). Half of the groups received a simple, inexpensive enrichment programme while the other groups received a more complex and costly combination of physical and feeding enhancements. Observations were conducted on 93 subadults of both sexes during their initial year of group housing. Intensity of enrichment did not differentially affect the amount of time subjects spent in any of the activities analysed. Subjects that received the more complex programme spent only 8.3 per cent of their time using the extra enhancements. Therefore, there was little demonstrated benefit of the more costly enrichment programme. The three cohorts differed in the amount of time that they spent inactive, behaving agonistically, playing and located near a group mate. A planned comparison of one cohort that had been single-housed without visual access to social groups, to the two cohorts that had visual access to social groups during single caging, revealed differences in play and socially-located behaviour, which may have been due to differences in extra-cage conditions two years prior to the present study. When primates are housed socially with conspecifics as 'social enhancements', the relatively simple innimate enrichment programme we used was as effective as the more costly programme. When enrichment resources are limited, inanimate enrichment efforts should be focused on monkeys that are not socially enriched.