2. Endocrines and Populations 283 



testosterone) have been effective, although Delost (1954) has reported that 

 cortisone involutes the X-zone in voles. This delay in androgen production 

 presumably indicated an inhibited secretion of gonadotropins from the 

 anterior pituitary, although there apparently were sufficient gonadotropins 

 to maintain the X-zone, as luteinizing hormone presumably is responsible 

 for maintaining this zone (Jones, 1949b, 1950, 1952). 



Thymic involution is effected by carbohydrate-active corticoids, estro- 

 gens, and androgens, with variations in the mode of involution (cf . above) . 

 Therefore thymic involution, as a measure of increased adrenocortical 

 secretion, must be interpreted with caution if there is reason to suspect 

 differences in the levels of circulating sex steroids. Such was the case in 

 the experiments under discussion, but additional information makes it 

 possible to state with reasonable certainty that the weights of the thymus 

 reflect increased corticoid secretion. The thymuses of 13-19-gm. male mice 

 from high populations weighed more than their segregated controls, whereas 

 the mean thymus weight of 19-23-gram mice from the high populations was 

 less than that of the segregated mice. The greater thymus weight coincides 

 with the greater width of the adrenal X-zone in the experimental mice and 

 probably, as in the case of the X-zone, represents inhibition of androgenic 

 activity which is not overridden by the increased amounts of circulating 

 corticoids. It has been shown that the natural adrenal secretory products of 

 mice will produce thymic involution and lymphopenia (Bahn ct al, 1957; 

 Wilson ct al, 1958) . However, depression of the thymus weight to below 

 the control levels in the larger mice from high populations can only reflect 

 increased adrenocortical activity. The mean thymus weights of mice from 

 the intermediate populations were greater than those of the controls or 

 experimental mice from high populations in the 16-19-gm. body weight 

 range. Data from the X-zone indicate that the secretion of androgen (or 

 at least its activity) was inhibited to the same degree in the intermediate 

 and high populations, whereas adrenal weights and width of the zona fas- 

 ciculata were less in the intermediate than in the high populations. There- 

 fore the greater thymus weights in mice weighing 16-19 grams from the 

 intermediate populations may have resulted from a less marked increase in 

 adrenocortical activity than occurred in the high-density populations, 

 whereas androgen secretion was depressed equally in populations of both 

 sizes. These results might be interpreted to mean that at increased popula- 

 tion densities the younger, and presumably subordinate, animals are the 

 ones predominantly affected by increased density. Since these results are 

 obtained by sacrificing an entire population at one time, such a conclusion 

 would be valid if it were not for the fact that the evidence indicates that the 

 increase in pituitary-adrenocortical activity involved all weight groups and 

 therefore all ages. Nevertheless, as we shall see below, a few of the heaviest 



