264 /. J. Christian 



out other factors which are known to produce the observed changes. That 

 pure sociopsychologic factors can produce these effects is clearly demon- 

 strated by the following experiments. 



When a strange vole {Microtus agrestis) of either sex was placed daily for 

 27 days in a cage containing a resident pair of voles, and therefore in what 

 was essentially their private territory, vicious fighting ensued (Clarke, 

 1953). The introduced voles lost weight and exhibited a significant hyper- 

 trophy of the adrenals and spleens and atrophy of the thymus compared 

 to the residents. In similar experiments with Microtus orcadensis there were 

 significant increases in the size of the livers, adrenals, intervertebral discs, 

 and spleens, and decrease in the thymus in the "stressed" animals, those 

 introduced as strangers to resident pairs (Chitty et al., 1956). Clarke 

 (1953) attributed the changes in organ weight to the actual fighting, 

 stating that fighting is a very effective form of stress since it involves 

 vigorous muscular exercise in addition to the trauma of wounds (Selye, 

 1950) . However, the fact that the resident voles had to fight as much as 

 those which were introduced, and yet did not reflect this with increased 

 adrenocortical activity, apparentlj^ was overlooked. This problem may be 

 explored by data on albino mice (Christian, 1959d). An analysis of the 

 relationship between scars from fighting and adrenal weight from 280 male 

 albuio house mice from 55 populations of 4, 5, or 6 each showed that, al- 

 though the mean adrenal weight of every population increased appreciably 

 with respect to isolated controls, there was no relationship between the 

 amount of scarring, as an indication of the severity of fighting, and adrenal 

 weight. Fm'thermore, the presence or absence of scarring in a population 

 made no difference in adrenal weight. The results were similar irrespective 

 of whether the analysis involved only the differences between populations 

 or the difference between individuals. Adrenal weight increased the same 

 amount in populations in which there was no fighting, or so little that none 

 of the mice had injuries from biting, as it did in populations in which 

 fighting was severe enough that most of the animals were badly scarred. It 

 should be pointed out that these injiu'ies were superficial and for the most 

 part represented bites through the skin only. These results indicate quite 

 clearly that fighting or injury per se are not the stimuli responsible for 

 stimulating increased adrenocortical activity with thymic involution when 

 animals are placed together. Conclusive evidence that sociopsychologic 

 pressures alone are mainly responsible for evoking these physiologic adap- 

 tive responses lies in the following experimental results (Davis and Chris- 

 tian, 1957) . 



When house mice are placed together in groups of six, there is immediate 

 fighting which soon ceases with the establishment of a social hierarchy with 

 one mouse dominant over the others and another subordinate to all the 



