Environmental enrichment and crowding: behavioral and hormonal effects.

Environmental enrichment and crowding: behavioral and hormonal effects. PHYSIOL. BEHAV. 17(5) 735-741, 1976.-Male and female gerbils lived from weaning to adulthood in Enriched Crowded, Plain Crowded, Dense Crowded, or Paired conditions. Daily observations, social interaction tests with one animal from each of the 4 groups, and anatomical measures were made, and levels of cortisol and testosterone were determined by radioimmunoassay. In the social interaction tests Paired gerbils marked and fought the most. Home cage observations estabished that Enriched animals failed to pair bond or establish nests in the small cubicles provided. They also reproduced no more successfully than did other crowded groups, while all crowded groups reproduced less than Paired animals. All groups of crowded animals had lighter testes and adrenal glands than Paired animals, although their cortisol levels were consisderably higher than Paired animals'. This inverse relationship of cortisol and adrenal weight was interpreted as a sign of extreme adrenal activity on the part of crowded animals. Enriched animals had cortisol levels intermediate between those of Paired and of Dense and Plain crowded animals, and their testosterone levels were as high as those of Paired males, indicating some diminution of hormonal effects of crowding. There was, however, little evidence of diminished behavioral and reproductive effects of crowding by environmental enrichment. Environmental crowding Hormonal effects Aggression Reproduction IT has been frequently observed that crowding animals leads to decreased reproduction, increased adrenal activity, and behavioral changes, including decreased aggression in social interaction tests and in the home cage if a sufficient period of adjustment is In gerbils we have observed depressed ventral gland marking, aggression, and nonaggressive contacts, as well as diminished reproductive success, resulting from crowding normal animals [17,18]. We have also seen smaller ventral glands (used by gerbils to deposit a sebaceous substance on low-lying objects in the environment) and in some experiments smaller testes and/or enlarged adrenal glands in crowded animals [ 17,18]. In a previous experiment in this lab [ 17], one group of crowded gerbils was placed into individual, physically enriched boxes for 2 hr per day for 10 weeks in order to determine whether this intermittent isolation would diminish the physiological and/or behavioral effects of crowding. However, the crowded animals subjected to intermittent isolation became more aggressive both in the home cage and in social interaction tests, and reproduced less than either paired or continuously crowded animals. There was no indication of adrenal hyperactivity, although …