GONADAL HORMONES AND SOCIAL BEHAVIOR 



1261 



skin tumescence may also have antagonized 

 other actions of the estrogen (for a discus- 

 sion of this action of progesterone see the 

 chapters by Hisaw, Villee and Young on the 

 ovary). If progesterone had this effect in 

 these animals, the reduction in dominance 

 which followed its administration was not 

 necessarily related in any direct way to the 

 prevention of swelling. 



VIII. Social Stress and the 

 Endocrine System 



Heretofore, discussion has been limited to 

 the gonadal hormones and aggressiveness. 

 There remains another axis, i.e., the effect 

 of aggressiveness on reproduction. The na- 

 ture of the relationship does not appear to 

 l)e that of a feed-back, as our statement of 

 the problem suggests. Females as well as 

 males are affected, and much of what has 

 been done indicates that the pituitiary-ad- 

 renal axis is involved. Mammals with cyclic 

 populations seem most susceptible. 



Christian (1950) reviewed the symptoms 

 and conditions associated with the die-offs 

 in mammals having population cycles. These 

 were related to the general-adaptation syn- 

 drome of Selye (1947). A working hypothe- 

 sis was developed for the population crash 

 which terminates the cycle. "Exhaustion of 

 the adrenopituitary system resulting from 

 increased stresses inherent in a high popula- 

 tion, especially in winter, plus the late 

 winter demands of the reproductive system, 

 due to increased light and other factors, 

 precipitate population-wide death with the 

 symptoms of adrenal insufficiency and hy- 

 poglycemic convulsions." Elements of this 

 hypothesis are supported by rather impres- 

 sive evidence. 



Southwick, (1955a, bl, in a carefully con- 

 trolled experiment with house mice {Mus 

 tnuscuhis) , found that the amount of ag- 

 gressive activity increased as the density 

 of the population increased. Crowcraft and 

 Rowe (1957) in a similar experiment, but 

 with some modifications, found that re- 

 duced fecundity of the females was the most 

 important single factor limiting population 

 growth. They concluded that some factor 

 other than fighting or food shortage appears 

 to inhibit ovarian activity. Christian (1955) 

 showed that mice mantaincd in groups have 



heavier adrenal glands than mice kept in 

 isolation, and that the adrenals and repro- 

 ductive organs of wild house mice are more 

 responsive to stress than those of laboratory 

 albino mice. In a subsequent study of popu- 

 lations of different sizes and of isolated 

 controls, and with food and water in excess, 

 Christian (1956) analyzed the endocrine re- 

 sponses. The results were indicative of an 

 amount of stress proportional to population 

 density. The secretion of adrenocortico- 

 trophin increased in response to stress and 

 gonadotrophin decreased. Increased adrenal 

 size and low eosinophil counts were taken 

 as evidence for an increase in adrenocortical 

 activity in a dense population of the 

 meadow vole, Microtus pennsylv aniens 

 (Louch, 1958). The results from one study 

 of the chicken (Siegel, 1959) suggests that 

 the response of this species may perhaps be 

 similar to that of the mouse and meadow 

 vole. The adrenals were heavier in birds 

 under crowded conditions than in those with 

 more floor space per bird. With progressive 

 decrease in the density of the flocks, adrenal 

 weights declined. 



Three urban populations of Norway rats 

 {Rattus norvegicus) with histories of stable 

 high population densities were reduced by 

 trapping to an average of 32 per cent below 

 the maximal density. There was a mean de- 

 cline of 28.3 per cent in the adrenal weights 

 immediately following population reduction, 

 and 7 months later the adrenal weights were 

 22.4 per cent below the original values 

 (Christian and Davis, 1955). Richter (1954) 

 found that the cortexes of adrenal glands 

 were much smaller in the domesticated labo- 

 ratory rat than in the wild Norway rat. 

 Population cycles of the vole {Microtus 

 arvalis Pallus) in Germany were investi- 

 gated by Frank (1957). He concluded that 

 intraspecific social behavior was of great 

 importance for the events of population 

 dynamics. Crowding favored competition 

 and caused a state of psychologic excitement 

 which was transformed by the pituitary- 

 adrenocortical system into physical stress. 

 This, acutely combined with the stress of 

 food shortage, produces a "readiness" for a 

 crash in the vole population. The ultimate 

 trigger was an additional stress of meteoro- 

 logic events. Davis (1957b) stated that 



