Environmental enrichment and crowding: Behavioral and hormonal effects

Abstract

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.