2. Endocrines and Populations 279 



cate that the adrenocortical response to grouping is sufficient to depress 

 antibody formation as well as inflammation. From these results one can 

 easily sec that behavioral factors associated with increases in population 

 density, or grouping, can sufficiently increase adrenocortical activity to 

 profoundly affect host resistance to infectious disease and parasites. 



The effects of hormones on miu-me tuberculosis are somewhat complex. 

 Cortisone enhances tuberculosis infections m mice, especially by converthig 

 a smoldering chronic infection into a fulminating acute process (Hart and 

 Rees, 1950) . However, ACTH was without effect on the early development 

 of tuberculosis in mice (LeMaistre et al., 1953). From these experiments it 

 would appear that an increased secretion of endogenous adrenal corticoids 

 was ineffective in enhancing tuberculosis in mice whereas injected carbohy- 

 drate-active corticoids markedly enhanced the infection. Therefore, it is 

 especially interesting that Tobach and Block (1956) were able to show that 

 crowding significantly altered the courses of acute and chronic tubercu- 

 losis m mice. Crowding after mfection decreased the survival time of mice 

 of both sexes suffering from an acute tuberculous infection. Crowding after 

 infection had essentially no effect on the course of chronic tuberculosis in 

 female mice, whereas it enhanced the chronic disease in male mice. These 

 experiments also indicate that crowding prior to infection may enhance 

 host resistance. 



These experiments, although quite limited in number, all confirm the 

 fact that crowding (increased population density) can reduce host resist- 

 ance to disease. The evidence from experiments with increased popula- 

 tion density is completely in accord with the results of experiments with 

 injected corticoids. These results, considered in the light of the mass 

 of this evidence on the effects of corticoids and ACTH on experimental 

 infections, can only mean that increased population density sufficiently 

 stimulates the pituitary-adrenocortical system to lower resistance to disease 

 by inhibition of inflammation, granulation, and probably antibody forma- 

 tion by endogenous corticoids. 



7. Production of Mortality Directly 



In the preceding section the role of behavioral factors and increases in 

 population density in decreasing resistance to disease was discussed. It 

 therefore follows that grouping can increase mortality by this means and 

 the more subordinate animals will be the ones most often affected. However, 

 grouping is often followed in a very short tune by sudden death preceded by 

 alternate prostration and convulsions of many of the animals (Christian, 

 1955b), and the greater the number which are placed together, the greater 

 the number which succumb in this fashion. (J. J. Christian, unpublished). 



