2. Endocrines and Populations 273 



Although inanition and starvation may not be stimuli to increased 

 pituitary-adrenocortical activity in mice and rats, they may be in guinea 

 pigs (D'Angelo et al., 1948) and white-footed mice {Pero7nyscus leucopus) 

 (Sealander, 1950). However, a limited amount of food does depress repro- 

 ductive activity in house mice as well as in other species which have been 

 investigated (Lutwak-Mann, 1958), possibly as a resvilt of a protein de- 

 ficiency, as it has been shown that a dietary deficiency of protein diminishes 

 the secretion of gonadotropins (Srebnik et al., 1958; Lutwak-Mann, 1958). 

 The striking declines in the weights of the preputial glands, and especially 

 of the seminal vesicles, brought about by a limited amount of food (Chris- 

 tian, 1959c) indicate that the secretion of androgens by the testes was 

 markedly depressed. Whatever the mechanism by which a food deficiency 

 depresses the secretion of gonadotropins, it was not by eliciting a generalized 

 adaptive response involving the pituitary-adrenocortical system. These 

 results lead to several conclusions, at least as far as mice are concerned. 

 One is that when food is supplied ad libitum, competition for food is not a 

 factor in eliciting pituitary-adrenocortical responses in relation to popula- 

 tion. A second is that inanition and starvation per se are not stimuli to 

 increased pituitary adrenocortical activity in mice or rats. The third is 

 that food restriction depresses reproduction by dimmishing the secretion 

 of gonadotropins without eliciting a more widespread response, and there- 

 fore a limited supply of food can limit population growth specifically by 

 depressing reproduction without operating through the pituitary-adreno- 

 cortical system. However, we have seen that there is some indication that 

 these relationships may not be universally true. Perhaps food supplies and 

 social competition can act independently to limit population growth. Food 

 conceivably may not be important to natural populations as long as sub- 

 ordmate animals are free to move elsewhere. Calhoun (1949, 1950) has 

 indicated that social competition is a more important factor than food 

 supply per se for Norway rats. One fact is abundantly clear as a result of 

 these experiments, and that is that sociopsychologic factors stimulate in- 

 creased pituitary-adrenocortical function and depress reproductive func- 

 tion in proportion to population density ; this reaction system is therefore 

 active at all levels of population in the control of population growth, 

 whereas a limitation in the food supply will exert its effects on reproduction 

 only when it results in inanition in the members of a population. These 

 considerations are of obvious importance to the investigator interested in 

 the control and regulation of mammalian populations. 



The production of splenic hypertrophy in highly inbred albino mice by 

 increased population density is especially interesting in view of Chitty's 

 (1957) conclusion that the splenic hypertrophy in voles {Microtus agrestis 

 and M . orcadensis) was due to inherited genetic factors. This conclusion 



