280 J. J. Christian 



although there is considerable variation from population to population in 

 the numbers which die in populations of the same size. The more reactive 

 and aggressive wild-stock house mice are much more prone to succumb in 

 this fashion than the usual albino mouse in the laboratory (Christian, 

 1955b). The proximate cause of sudden death in these animals has not been 

 investigated, although its onset, behavior, and general symptomatology arc 

 highly suggestive of hypoglycemic shock and closely resembles the deaths 

 from "shock disease" observed in natural populations at peak densities 

 (Christian, 1950b). Frank (1953) observed similar deaths in voles {Micro- 

 tus arvalis) after placing them in groups, as well as in natural populations, 

 and was able to demonstrate that glucose could prevent the fatal termina- 

 tion of the syndrome, and also showed that the symptomatology of insulin 

 hypoglycemia was identical to that observed in voles dying naturally 

 following "crowding." Furthermore, he showed that the voles which were 

 on the verge of dying had markedly decreased levels of blood sugar and 

 liver glycogen. These results make it appear probable that the convulsions 

 and death which follow shortly on the social strife produced by grouping 

 mice are due to hypoglycemic shock. There is reason to suggest that a 

 massive discharge of the adrenal medulla, with its subsequent exhaustion 

 may lead to a failure to further mobilize glucose and the animals die in a 

 hypoglycemic episode. However, this hypothesis requires further testing. 



In any event, social strife and the physiologic responses to it may result 

 in mortality directly as well as by affecting host resistance. 



8. Summary of Results from Populations of Limited Size 



Experiments with populations of fixed size have demonstrated that socio- 

 psychologic interactions can and do stimulate increased pituitary-adreno- 

 cortical and decreased reproductive activity and growth in mice, voles, and 

 Norway rats. Increased adrenocortical activity is related to social domi- 

 nance-siibordinance relationships, the more subordinate animals exhibiting 

 a greater response than the dominant animals. These responses apparentl}' 

 are unrelated to fighting per se. Fighting is evidently a sign of social interac- 

 tion, just as is the endocrine response, and not a causative factor. There is 

 a progressive increase in the pituitary-adrenocortical response and de- 

 crease in reproductive function in male and female mice with increasing 

 population size. All phases of reproductive activity are depressed, including 

 fertility, implantation of blastulae, intra-uterine survival, estrus, lactation 

 (with persistent effects for at least two generations) , and the size and 

 activity of the male gonads and sex accessories. Inhibition of reproductive 

 fimction apparently stems from a depressed secretion of pituitary gonado- 

 tropins. Growth is also inhibited by increased population density. 



