306 J- J- Christian 



various factors indicates that adrenocortical physiology in this species, as 

 in others, reflects environmental changes, and the adrenal was therefore 

 said to be an indicator of conditions as in any other species. (Hughes and 

 Mall, 1958). However, sika deer (Cervus nippon) respond to changes in 

 population density in a manner quite similar to that seen in rats (Christian 

 et al., 1960) . A 60% decline in the population of these deer was accompanied 

 by a proportional decline in adrenal weight. The decline of this population 

 was brought about by a mass mortality which apparently was due to 

 metabolic disturbances resulting from the prolonged adrenocortical hyper- 

 activity associated with a high density of the population. There is evidence 

 that potassium deficiency resulting from cortical hyperactivity may have 

 contributed directly to the mortality. Marked stunting of growth was also 

 seen during the period of high density and especially during the year of 

 die-off. Other possible causes of the die-off, such as malnutrition, could be 

 ruled out. 



More experimental work is needed to relate adrenal function with popula- 

 tion density for a number of species in natural populations. The problems 

 are numerous and not the least of these is being able to rule out extraneous 

 factors or else to assess their role in producing the measured effects. 



2. Reproductive Function 



A large number of studies suggest that reproductive function is depressed 

 with increasing density of natural populations, but studies sufficiently 

 discriminative to attribute changes in reproductive function to changes 

 in density per se without additional complications are few. Therefore a 

 great deal of the evidence is circumstantial and tentative at best. The 

 present discussion is limited to those studies which have been conducted 

 with suflficient care and attention to a variety of details so that one is 

 confident that the factors have been properly considered which possibly 

 might affect reproduction. However, a brief discussion of various problems 

 involved in evaluating reproductive function is in order before considering 

 the evidence implicating physiologic responses to population density in 

 the suppression of reproductive function. 



First there is the problem of the food supply of the experimental popula- 

 tion. As we have seen, there is ample evidence that food deficiencies can 

 curtail reproductive function independently of other adaptive mechanisms. 

 However, the effects of food and of increased density on the reproductive 

 system and growth are so similar that unless careful assessments are made 

 of the food supply (preferably these should be accompanied by an evalua- 

 tion of adrenocortical and other adaptive fvmctions, no separation can be 

 made between the effects of food and of density, especially at relatively 



