302 /. J. Christian 



city. It should also be emphasized that food was available in excess of needs 

 for all populations and from numerous sources ; so that food cannot be con- 

 sidered to be an imprtant factor with respect to these populations. 



Some of the variability between urban populations of rats may be ex- 

 plained by the fact that the rat population in each block is divided into 

 several discrete colonics when the total population is at a relatively low 

 density (at high density these colonies coalesce and the population evidently 

 becomes a single unit throughout the block). The colonies comprising 

 several of the block-populations were evaluated individually with respect 

 to relative density in the same way as the populations for the entire blocks 

 (Christian and Davis, 1956). Relative density values were assigned and 

 compared with the adrenal weights of the rats from these colonies. A sug- 

 gestive correlation between adrenal weights and relative population den- 

 sities of these colonies was found, although the differences were not signifi- 

 cant, probably owing to the small number of samples. The sample size was 

 limited by the relatively few colonies from which a sufficient number of rats 

 were collected for comparative purposes. 



Although adrenal weight is greatest in naturally declining populations 

 (Christian and Davis, 1956) , it was shown in another study that artificially 

 reducing populations of rats produces an immediate and proportional reduc- 

 tion in adrenal weight (Christian and Davis, 1955). Adrenal weight in both 

 sexes of rats from three populations was reduced 38% by intensive trapping. 

 The adrenal weights were maintained at this level for 5 months by main- 

 taining the populations at the reduced level. 



In another study a rural population of Norway rats was followed by 

 monthly sampling for two years (Christian, 1959b). An index of the size 

 of the population was obtained each time a sample was collected by using 

 a standardized trapping procedure. The weights of the adrenals, pituitaries, 

 and thyroids were determined for each sample. There was a highly signifi- 

 cant correlation between adrenal weight and population index for both 

 sexes, and an even more significant correlation between pituitary and 

 adrenal weight for both sexes for the 24 monthly samples in the two-year 

 period. The functional significance of the pituitary changes is not known; 

 it can only be inferred from data on weight. Evidently changes in the rate 

 of ACTH secretion, as indicated by adrenal weight, are accompanied by 

 parallel changes in pituitary weight. Changes in thyroid weight and in 

 thymus weight were not correlated with changes in the population size, 

 adrenal weight, or pituitary weight. 



The results of the foregoing experiments indicate that adrenocortical 

 activity in Norway rats from natural urban or rural populations is related 

 directly to changes in population density and that both sexes respond in the 

 same way. Therefore it appears that physiologic adaptive mechanisms are 



