270 /• J- Christian 



cellular hyperplasia and hypertrophy of the zona f asciculata, although the 

 glands were not critically examined for medullary hypertrophy. The decline 

 in the weights of the testes was a reflection of the generalized reduction in 

 body weight in one series of experiments (Christian, 1955a), but in another 

 series the testes declined in relative testicular weight as well as in absolute 

 weight with increasing population size (Christian, 1955b). Body weight 

 was significantly less in populations of 4, 8, and 16. 



These experiments were repeated using wild house mice raised in the 

 laboratory in populations of 1, 3, 4, 6, 8, 9, and 17 with similar though 

 more pronounced results (Christian, 1955b). The thymus weight of these 

 mice decreased markedly as population size increased. The adrenocortical 

 response was also much more pronounced in these mice than in the albino 

 mice, although the mean weights of the adrenals of segregated albino and 

 brown male mice were identical. The increase in adrenal weight of albino 

 mice reached a maximum of 8% above the control levels in populations of 

 16, whereas that of the wild stock attained a maximum increase of 21% 

 above the control levels in populations of 9. In populations of 32 albino 

 mice and 17 wild mice the mean adrenal weight was less than in the next 

 smaller population size in each case. This was found to be due to a loss of 

 lipid and a marked decrease in the size of cells in the zona fasciculata al- 

 though the degree of hyperplasia was greater than in the adrenals of mice 

 from the preceding population size (Christian, 1959b). The wild mice are 

 much more alert, reactive, and aggressive than albino mice, and the 

 differences in adrenal reactivity probably reflect such behavioral differences. 



The decline in the weights of the accessory reproductive organs, seminal 

 vesicles, and preputial glands indicated a diminished secretion of androgens 

 from the testes with increasing population size, based on the assumption 

 that these organs accurately reflect androgen levels (Burrows, 1949; Ren- 

 nels ct al, 1953) . This conclusion is strengthened by the fact that relative 

 testicular weight also declined in one group of experiments (Christian, 

 1955b) and absolute weight declined in all experiments with increased 

 population size. Since all indications point to a decline in the secretion of 

 androgens, the decline in thymus weight must therefore represent involu- 

 tion by increased amounts of circulating corticoids. 



The progressive hypertrophy of the adrenals and atrophy of the thymus 

 and reproductive organs do not reflect a diminished space per mouse per 

 se, as the results were essentially identical where populations of 1, 4, 6, 8, 

 and 17 each of male wild mice were placed in cages with 42 times the area 

 of the cages used in the preceding experiments (Christian, 1959b). The 

 similarity of the results from these two series of experiments with vastly 

 different amounts of area per mouse also suggests that the amount of 

 exercise or activity was not a factor in the observed endocrine responses. 



