300 J' J- Christian 



weights, eosinophil counts, and histologic studies. Thymus weights re- 

 flected these changes. Increasing infant mortality with increasing density 

 may be due to diminished lactation, but may also result from social and 

 behavioral factors. Food limitation may restrict population growth by 

 completely inhibiting reproduction, but is without any other apparent 

 effects. 



Finally, it is evident that the growth of confined populations of mice and 

 voles is regulated and limited by density-dependent physiologic responses 

 which are activated by sociopsychologic pressures. Whether or not these 

 relationships hold for all species remains to be determined. 



C. Natural Populations 



Evidence gathered in the laboratory relating physiologic adaptive mecha- 

 nisms to changes in population density cannot prove that these responses 

 occur in natural populations. Even for freely growing confined populations 

 in the laboratory, the environmental conditions are quite altered by the 

 removal of the effects of climatic and seasonal variability from those found 

 in natural situations. Preventing egress is another artificially imposed 

 condition; although it seems more likely that confinement parallels the 

 situation when populations reach densities where egress into neighboring 

 areas becomes possible. Clearly, evidence from natural populations is neces- 

 sary before any conclusions are justified regarding the role of physiologic 

 adaptive mechanisms in the regulation of mammalian populations under 

 natural conditions. Conclusive results are much more difficult to obtain from 

 natural populations than from confined populations, as might be expected, 

 due to the complexity and variability of these populations and techincal 

 difficulties in collecting the required data. Adequate controls and proper 

 assessment of environmental factors are often severe obstacles, and ob- 

 taining samples of adequate size from populations at very low densities is 

 intrinsically very difficult. Nevertheless, evidence has been obtained to 

 indicate that the density-dependent physiologic responses in natural popu- 

 lations are similar to those seen in experimental populations. Finally, social 

 strife has been implicated (Kalela, 1957) as an important factor in producing 

 the effects associated with increased density ; territorial mutual intolerance 

 increases greatly with sexual maturity in Clethrionomys rufocanus, which 

 serves to increase tension even with decreased density (Kalela, 1957). 

 Therefore behavioral factors again appear to be unportant. 



1. Adrenocortical Activity 



The available evidence relating adrenocortical activity to changes in 

 population density has been obtained largely from studies on populations 



