2. Endocrines and Populations 307 



high population densities. Furthermore, it is very likely that the adaptive 

 reactions to density and their effects may overlap food shortages, so that 

 their effects are mutually augmentive at critical densities. However, the 

 aim of the present discussion is to assemble the evidence that adaptive 

 reactions to density occur irrespective of whether or not the effects of in- 

 adequate food or other environmental factors are superimposed. The growth 

 of populations unquestionably can be limited by environmental factors 

 which may either act through the physiologic adaptive mechanisms or in- 

 dependently of them, especially in localized populations. However, the 

 basic concept of physiologic adaptation to population density is that these 

 mechanisms are always operative, and will regulate and can limit popula- 

 tion growth. Finally, limitations of enviroimiental factors, even of food, 

 may increase competition directly and therefore indirectly produce in- 

 creased activity of the physiologic adaptive mechanisms. Pitelka's (1957b) 

 statement that "the interest in the stress mechanisms has led some students 

 to overlook the point that such a mechanism does not evolve without 

 linkages to critical variables extrinsic to the population" bear repeating 

 and is emphasized and supported by an immense amount of work with rat 

 populations (Davis, 1953). Nevertheless, it is extremely miportant to dis- 

 tinguish between the direct action of environmental variables on popula- 

 tions and their indirect action through sociopsychologic and physiologic 

 mechanisms; and experimental evidence to date emphasizes the general 

 importance of the latter mode of action. Evidence of the direct action of 

 food shortages in depressing reproduction in natural populations of Pero- 

 myscus boylii and P. maniculatus is cited by Jameson (1953, 1955), and 

 population declines evidently followed the shortages as a result of con- 

 tinuing normal mortality in the presence of a lull in reproductive activity 

 with no recruitment into the populations. The evidence presented by Jame- 

 son, supported by the controlled experiments previously described, strongly 

 favors the interpretation that inadequate food supplies were directly re- 

 sponsible for the inhibition of reproduction. However, the possibility that 

 the shortages induced increased competition and increased activity of 

 adaptive responses were not ruled out completely. Davis (1951c), in a 

 study of Norway rats from natural populations, provided definitive evi- 

 dence that a deficient food supply can inhibit reproduction under natural 

 circumstances. However, w^hether the effects were direct or produced by 

 increasing competition with stimulation of generalized physiologic adaptive 

 response is unknown. It was determined later that the adrenals of these 

 rats were responsive to changes in population density (Christian, 1959b). 

 Other examples could be cited, but the majority could serve only to empha- 

 size the fact that discrimination between the effects of food and other factors 

 is usually not attempted. 



