Rearing environment and hypothalamic-pituitary-adrenal regulation in young rhesus monkeys (Macaca mulatta)

A mammal's early social environment has important regulatory effects on its behavior and physiology, and this is especially true for regulation of the hypothalamic-pituitary-adrenal (HPA) system. The present study was designed to test hypotheses that various aspects of the social environment are important influences on HPA regulation. Seven hundred seventy eight, 3- to 4-month-old rhesus monkeys were studied as part of a standardized, 24-hr biobehavioral assessment program, which included blood sampling to determine plasma cortisol concentrations. Results indicate that nursery-rearing results in a reduced cortisol set-point for the HPA system, and, for nursery-reared (NR) animals, more peer exposure during infancy is associated with a higher set-point. Age and sex differences during this period were evident but small in magnitude. These data demonstrate the important regulatory role of the social environment on nonhuman primate physiology and suggest caution in assuming that differences between individuals' cortisol levels reflect only differences in perceptions of the "stressfulness" of events.