WEIGHT-GAIN AMONG JUVENILE RHESUS MACAQUES - A COMPARISON OF ENRICHEDAND CONTROL-GROUPS

Environmental enrichment techniques for captive primates are aimed at improving their psychological well-being. While behavioral variables a re used to measure changes in psychological well-being, physiologic me asures (e.g., heart rate, cortisol response) are sometimes gathered in addition to the behavioral evidence. Some of these physiologic indice s measure acute changes in the animals' well-being, limiting their use fulness. Body weight, however, is a measure of physical well-being tha t may have meaning as a long-term indicator of psychological well-bein g. We therefore collected body weight data from two groups of rhesus m acaques (Macaca mulatta group 1: n = 34, group 2: n = 30) every 8 week s beginning at the age of 1 year, as they passed through various housi ng conditions as part of a program to develop a specific pathogen-free breeding colony. One-half of the subjects in each group received a va riety of environmental enhancements during all housing conditions; the other half received no enrichment and served as controls. At the begi nning of the study (age 1 year), control and enriched subjects did not differ in body weight. Among group-1 subjects, enriched animals weigh ed significantly more than controls after 4 months of enrichment, and the weight difference was maintained 24 months later. Enriched animals in group 2 never differed in weight from their controls. The order in which different types of enrichment were presented and the extra-cage environment of the two groups differed, which may account for this di screpancy. Group-1 enriched subjects were the only animals that weighe d as much as free-ranging rhesus monkeys, and rates of weight gain amo ng all groups of subjects were similar to several populations maintain ed under more naturalistic conditions.