2. Endocrines and Populations 301 



of Norway rats, both urban and rural, and voles {Microtus sp.), although 

 suggestive results have been obtained for a few other species. 



Adrenal weight in urban Norway rats of both sexes was shown to be 

 related to population "density" in a study of 21 populations in Baltimore, 

 Md. (Christian and Davis, 1956). Each population was confined to a city 

 block, the latter acting as an island (Davis, 1953). The capacities of the 

 blocks for rats varied considerably, as did the sizes of the populations them- 

 selves, which resembled confined freely growing populations of house mice 

 in this respect. Therefore, the populations were categorized by their posi- 

 tion on their own growth curves at each sampling. A hypothetic growth 

 curve was divided into low stationary, low increasing, high increasing, high 

 stationary, and decreasing, the progression from low increasing through 

 decreasing being a progressive increase in relative density, although the 

 "density" status of low stationary is equivocal (Christian and Davis, 1956) . 

 Each time a sample was collected, the population from which it came was 

 put in one of these categories. "Density" is obviously not strictly in terms 

 of area, but in relation to the carrying capacity of the block and the social 

 characteristics of the population. As populations increased in relative 

 density from low increasing to decreasing, the mean adrenal weight was 

 increased with each relative density; so that rats of both sexes from de- 

 creasing populations had 19% heavier adrenals than those from low, in- 

 creasing populations. Thymus weight was reciprocally related to adrenal 

 weight in female rats, but there was no apparent relationship between 

 thymus weight and adrenal weight, with respect to population density, in 

 male rats. The weights of the pituitary glands of male rats were positively 

 related to the adrenal weights. That is, changes in adrenal weight were re- 

 flected by changes in pituitary weight in the male rats (Christian and Davis, 

 1956) . However, the functional significance of these changes in the weight 

 of the pituitary glands is not known. The weights of the pituitaries of fe- 

 males and thyroid glands of both sexes bore no apparent relationship to 

 population density. The data from the low stationary populations are 

 difficult to evaluate. The adrenals of female rats were almost as light as 

 those from low increasing populations, and their thymus glands were the 

 heaviest found in any category. On the other hand, the adrenals of male 

 rats were heavier than in any other category. Low, stationary populations 

 are extremely difficult to evaluate, as the actual numbers of rats are so 

 small that even proportionately large changes are difficult to detect. How- 

 ever, these difficulties with their inherent errors in properly assigning 

 populations to a category do not explain the divergence in the male and 

 female adrenal weights, nor do they alter the fact that these populations 

 were at very low levels of density. It should be pointed out that the de- 

 creasing populations were at maximum "density" because they were de- 

 clining naturally and therefore were at or above the environmental capa- 



