The Relationship between Adrenal Weight and Population Status of Urban Norway Rats

It has been proposed that changes in the size of animal populations exert density-dependent effects on the physiology of individuals in these populations (Christian, 1950). Theoretically, the magnitude of the physiological alterations would increase with increases in population size until there is a cessation of population growth followed by a decline or, in extreme cases, a population crash. Emphasis was placed on the adaptive reactions of the pituitary-adrenocortical and reproductive systems to density-dependent stimuli which were presumably sociopsychological in nature. Therefore these stimuli would be effective throughout the history of a population, changing only in magnitude with changes in population size. Environmental hardships, such as food shortages and disease, of necessity would be imposed on and additive to this basic response to density. A direct relationship between adrenal weight and population density was demonstrated in the laboratory with albino and wild house mice in populations of fixed size (Christian, 1955a, b). A similar relationship was demonstrated in freely growing populations of wild house mice maintained in the laboratory (Christian, 1956). Increases in adrenal weight resulted primarily from cortical hypertrophy. Subsequently it was shown that the adrenal weights of wild Norway rats decreased following an artificial reduction of population (Christian and Davis, 1955). Finally, the subordinate rats of a group, as determined by losing fights, exhibit increased adrenocortical activity with eventual cortical hypertrophy (Barnett, 1955). The latter work suggests a sociopsychological mechanism by which population density might be related to adrenal activity in rats.