2. Endocrines and Populations 281 



In addition to these primary effects of physiologic adaptation to the 

 social pressures associated with increased population density, there is a 

 decrease in resistance to parasitism and infection. It was shown that in- 

 creased density produced an inhibition of inflammation, granulation, and 

 probably antibody formation. A decrease in host resistance to trichinosis 

 and tuberculosis was observed which apparently was due to these factors. 



It was also shown that food was not a factor in these effects, either in- 

 directly by increasing social competition or directly by inanition in the 

 subordinate animals. A deficiency of food is not a stimulus to increased 

 adrenocortical activity in mice, or rats, although it may be in other species. 

 A food deficiency, however, does result in a marked suppression of reproduc- 

 tive function, probably by decreasing the production of pituitary gonado- 

 tropins. 



B. Freely Growing Populations 



1. Introduction 



Experiments with populations of limited size, although indicative, do not 

 establish that these same responses occur in freely growing populations. In 

 the first place, experiments with limited populations were conducted for 

 limited periods of time, usually too brief to permit adaptation to the situa- 

 tion. Furthermore, in every case the experimental approach involved sud- 

 denly placing strange animals together under rather artificial circum- 

 stances. Therefore, these experiments provided no evidence that mammals 

 which have been more or less in contact with each other from birth would 

 respond similarly. House mice {Mus musculus) and voles {Microtus penn- 

 sylvanicus, Microtus agrestis) from free growing populations show the same 

 physiologic responses to increased population density as do mice from 

 populations of fixed size but to an even greater degree. 



2. Population Density and Adrenocortical Function 



Increased pituitary-adrenocortical activity has been related to increased 

 population density in a number of experiments with freely growing popula- 

 tions of house mice and voles (Christian, 1956: Louch, 1956; Christian, 

 1959a, b). A number of experimenters have shown that the growth of 

 freely growing confined populations is self-limited in spite of the fact that 

 food, water, nesting material, and nesting space were provided ad libitum 

 and well scattered (Strecker and Emlen, 1953; Clarke, 1955; Southwick, 

 1955a; Christian, 1956; Louch, 1956; Crowcroft and Rowe, 1957; Christian, 

 1959b) . Competition for food or a lack of availability of food by particular 

 individuals was not a factor (Christian, 1956; Crowcroft and Rowe, 1957). 



