Weight gain among juvenile rhesus macaques: a comparison of enriched and control groups

Environmental enrichment techniques for captive primates are aimed at improving their psychological well-being. While behavioral variables are used to measure changes in psychological well-being, physiologic measures (e.g., heart rate, cortisol response) are sometimes gathered in addition to the behavioral evidence. Some of these physiologic indices measure acute changes in the animals' well-being, limiting their usefulness. Body weight, however, is a measure of physical well-being that may have meaning as a long-term indicator of psychological well-being. We therefore collected body weight data from two groups of rhesus macaques (Macaca mulatta group 1: n = 34, group 2: n = 30) every 8 weeks beginning at the age of 1 year, as they passed through various housing conditions as part of a program to develop a specific pathogen-free breeding colony. One-half of the subjects in each group received a variety of environmental enhancements during all housing conditions; the other half received no enrichment and served as controls. At the beginning of the study (age 1 year), control and enriched subjects did not differ in body weight. Among group-1 subjects, enriched animals weighed 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 discrepancy.(ABSTRACT TRUNCATED AT 250 WORDS)