268 /. /• Christian 



groups for prolonged periods of time there is a significant increase in their 

 urinary tetrahydroxycorticoids, the metabolites of the carbohydrate-active 

 corticoids, excreted by grouped monkeys is greater than their combined 

 daily production of corticoids when individually caged. Furthermore, the 

 production of corticoids remains high for the entire period of grouping. It 

 has also been observed that when human bomber crews are housed as a 

 group, their production of urinary corticoids is increased over the combined 

 individual production of corticoids by the same men, paralleling the findings 

 for monkeys (Mason, 1959) . In the case of monkeys and men there can be 

 no question that the stimulus to increased corticosteroid production is 

 psychological, resulting from social interactions. 



There can be little doubt that social pressures can increase pituitary- 

 adrenocortical activity. We have also suggested that there is a depression of 

 reproductive function in male mice, as indicated by the weights of the sex 

 accessories, coinciding with increased adrenocortical function in relation to 

 social factors. These results are in agreement with the earlier work of Crew 

 and Mirskaia ( 1931 ) and Retzlaff ( 1938) , who showed that increased popu- 

 lation density depresses reproduction in female mice. The reproductive 

 performance of female mice was inversely related to population sizes in 

 populations of 1, 4, 8, or 12 pairs. Retzlaff (1938) also indicated that repro- 

 ductive performance was best in the socially dominant females in each 

 population. He made several additional observations of interest in these 

 experiments. He noted that there were aggressively dominant females that 

 attacked and viciously fought introduced females or females which had 

 been removed and were later replaced. The subordinate females suffered 

 death or injury, and, of particular interest, any infections that they had 

 were greatly exaggerated. This is one of the early experunental indications 

 of decreased resistance to disease following social stress, and is further 

 evidence that resistance is decreased primarily in the subordinate animals. 

 He also noted that in the largest populations, 12 pairs of mice, there was a 

 sufficient confusion among the mice to offer the subordinate animals partial 

 protection from attack. Similar effects were later noted in populations of 

 32 male mice (Christian, 1955a, b). Finally, it was found that a reduction 

 in environmental temperature of approximately 16° C. resulted in a signifi- 

 cant decline in mean litter size for mice from populations of 1, 2, or 8 pairs, 

 but not from populations of 4 or 12. The protection against a reduction in 

 litter size by decreased temperature probably reflects huddling as a means 

 of maintaining body heat and therefore diminishing the need for increasing 

 thyroid activity. Retzlaff could not explain the reduction in populations of 

 8, but evidently it was due to severe social strife rather than the reduction 

 in temperature. It is conceivable that severe strife prevented huddling due 

 to mutual intolerance; so that both factors could play a causative role in 



