326 J- f- Christiau 



iMiuill}', what has been presented here only represents a selected stopping 

 point in a very rapidly advancing field of research and must, perforce, be 

 used as such and modified in the light of newer developments. 



The second part of this chapter attempted to review critically the evi- 

 dence from the laboratory and from the field relating physiologic adaptive 

 responses to changes in population density, largely in response to poorly 

 defined sociopsychologic factors. There seems to be little doubt that endo- 

 crine adaptive responses to sociopsychologic pressures are of basic import- 

 ance in the regulation of population growth, at least for a limited number 

 of species. It also is obvious that a great many gaps remain to be filled 

 before the evidence for the physiologic regulation of natural populations 

 can be considered conclusive one way or the other except for a few species 

 of rodent. These gaps have been pomted out in the appropriate portions 

 of the foregoing discussions. Nevertheless, considerable support has been 

 derived in the last ten years for the original hypothesis that factors inti- 

 mately related to population density are stimuli to physiologic feedback 

 mechanisms and that population growth and decline are largely controlled 

 by changes in density. It has also become evident that "density" in terms 

 of mammalian populations is related only indirectly to numbers of mammals 

 per unit of area, being more directly related to intraspecific competition, 

 social strife, sociopsychologic pressures, or whatever other comparable 

 designation one may choose to use for the interactions between mammals 

 in the same population. It is obvious that these factors require a great deal 

 more study in order to define them precisely. 



Figure 1 is presented as a schematic summary of the physiologic feedback 

 n^gulation of population growth as it is envisioned today in the light of the 

 available experimental evidence. It should be re-emphasized that this is a 

 dynamic and flexible system and that the importance of various compo- 

 nents may vary with respect to each other. The broad ascending arrows on 

 the left of the figure indicate that as population size increases, social pres- 

 sures increase accordingly. The dotted lines above and below the arrow for 

 population increase serve to indicate that the actual population size may 

 vary considerably from population to population for a given degree of social 

 pressure, which we see hypothetically as the fundamental growth-regulating 

 and growth-limiting factor in all mammalian populations and the factor to 

 which physiologic adaptive mechanisms respond. However, that this factor 

 is always present and always operative does not necessarily imply that it is 

 always the factor which limits population growth. Environmental factors 

 probably operate through this mechanism by increasing or decreasing social 

 pressures, although conclusive evidence for this statement presently is 

 lacking. There is good e^'idence, howe\^er, that the social factors will limit 

 population growth despite an abundance of all environmental requirements 

 and that the degree of social density is related to the behavioral composition 



