262 /. /. Christian 



The mechanisms, hormonal effects, and general responses so far described 

 have been confined largely to the results of experiments in the laboratory 

 with the usual laboratory species. Very little work has been done on native 

 mammals in the laboratory, and comparative studies are certainly needed. 

 In addition, the work has been to a great extent limited to studies of the 

 effects of injected hormones or of subjecting laboratory animals to ex- 

 tremely severe conditions. 



The role that physiological adaptive mechanisms play under natural 

 conditions was not investigated in most of these studies. However, it was 

 postulated in 1950 that these same responses could be evoked by increased 

 population density and that these same physiological reactions could serve 

 as a feedback to regulate the growth of mammalian populations, their 

 declines, and the mass mortality which occasionally terminates the build-up 

 of a natural population to extremely high densities (Christian, 1950b). It 

 was first suggested that the intraspecific strife and social competition that 

 force animals into adverse circumstances, together with all of the other 

 adversities which become aggravated by high population densities, would 

 elicit adaptive responses such as those which have been described. There- 

 fore one would anticipate a direct relationship between adrenocortical 

 activity and a more or less reciprocal relationship between reproductive 

 function and population density. The hypothesis that physiological mecha- 

 nisms were active in all populations in response to changes in density and 

 that most environmental deficiencies acted through this mechanism was 

 inherent in the original postulate. This hypothesis implied that social 

 competition or pressure was the sole factor, always present in all popula- 

 tions, which could logically be expected to elicit the gamut of adaptive 

 responses in every population. However, these relationships had to be 

 demonstrated, and it was necessary to show that changes in population 

 alone could induce a proportional increase in pituitary- adrenocortical 

 activity, decrease in reproductive activity, decrease in resistance to disease, 

 or even death from shock, and all the other reactions and responses which 

 have been described in the preceding section. 



Since that time considerable evidence has been accumulated from the 

 laboratory and from natural populations which indicates that these re- 

 sponses to population density do occur and that they can regulate popula- 

 tion growth. The balance of this chapter will be devoted to a presentation 

 of the experimental evidence for the response of physiological adaptive 

 mechanisms to social competition and therefore population density, and 

 the evidence implicating these mechanisms, acting as a feedback system, in 

 the regulation of mammalian populations. Finally there will be a discussion 

 of the pertinence of this evidence to the regulation and control of natural 

 populations. 



