2. Endocrines and Populations 269 



reducing reproductive performance. The preceding experiments usually- 

 used populations consisting entirely of male or mixed male and female mice 

 or rats (Crew and Mirskaia, 1931 ; Retzlaff, 1938; Barnett, 1958) . However, 

 a depression of reproductive function occurs when only female mice are 

 placed in groups (Andervont, 1944). In these latter experiments estrous 

 cycles began at an earlier age, were more frequent, and lasted until a 

 greater age in segregated female mice than in their littermates kept in 

 groups of 8 each. Bullough (1952) showed that "overcrowding" mice (16 to 

 a cage) for 3 weeks resulted in a 30% increase in the cross-sectional area of 

 the adrenal cortex and an 80% increase in medullary area. These changes 

 were accompanied by a 60% reduction in epidermal mitoses, which was 

 attributed to an increased secretion of adrenocortical steroids. Finally, 

 Chitty (1955) showed that liver glycogen was appreciably lower in voles 

 maintained in the laboratory in groups than in those maintained under 

 segregated conditions. 



2. Adrenocortical and Reproductive Responses to Population 

 Density 



The preceding experiments indicate that sociopsychologic factors in- 

 volved in social interactions between mammals can elicit physiologic 

 adaptive responses with increased pituitary-adrenocortical and decreased 

 reproductive function in voles, house mice, both wild and albino, Norway 

 rats, rhesus monkeys, and humans. However, with the exception of the 

 experiments of Crew and Mirskaia (1931) and Retzlaff (1938) there was 

 no indication that these functions were altered in relation to population 

 density. Although an inverse relation between population size and reproduc- 

 tive performance was shown in the experiments of Crew and Mirskaia 

 (1931) and Retzlaff (1938), these authors did not investigate adrenocorti- 

 cal function. If social competition, as a stimulus to increased pituitary- 

 adrenocortical and decreased reproductive activity, is responsible for regu- 

 lating the growth and decline of mammalian populations, then there must 

 be a relationship between population density and the magnitude of the 

 endocrine responses. The existence of such a relationship has been demon- 

 strated in a variety of experiments. 



If male mice which have been segregated since weaning are placed to- 

 gether in groups of 1, 4, 8, 16, or 32 per cage for a week, there is a hyper- 

 trophy of the adrenal glands and atrophy of the gonads and sex accessories 

 which progresses more or less linearly as the logarithm of the population 

 increases (Christian, 1959b) . There is a decline in thymus weight from that 

 of the isolated controls, but the decrease is not related to the population 

 size. The increase in adrenal weight was found to result primarily from 



