328 /• J- Christian 



(A the population. As social pressure increases, it acts as a progressively 

 stronger stimulus to physiologic adaptation via the mechanisms on the 

 right of the broken vertical line. These in turn act to decrease reproduction 

 and increase mortality, and therefore the rate of population growth declines 

 with increasing density and eventually ceases, as indicated by the"reproduc- 

 tion" and "mortality" arrows on the left. We have already seen that repro- 

 duction may cease altogether at maximmii densities and that mortality 

 may achieve precipitous proportions. An alteration in age composition of 

 the population is assumed to be inherent m these effects. The details of the 

 various mechanisms have been discussed earlier, and will not be discussed 

 further here. However, it is hoped that this diagram will help in visualizing 

 the dynamic mterrelationships between the endocrine responses in the 

 members of the population to changes in the population as a whole. It will 

 be observed that a food deficiency has been shown as decreasing directly 

 the production of gonadotropins, but, as we have discussed earlier, it may 

 serve to increase competition and therefore effective density under some 

 circumstances. Theoretically it seems unlikely that a food deficiency would 

 reach proportions in natural populations which would enable it to exert its 

 effects dhectly on the pituitary without competitive factors first becoming 

 operative. This diagrammatic hypothesis appears to fit the facts available 

 for a Imiited number of studies on a few species of rodent, and undoubtedly 

 it will be necessary to modify it as information from more critical studies 

 become available. It is of primary importance to obtain results from careful 

 studies on ungulates, similar to those already conducted on rodents, before 

 regulation of population growth of ungulates by sociopsychologic-physio- 

 logic feedback mechanisms can be considered to be more than hypothetical. 

 However, at present it is very probable that these mechanisms are basically 

 important in regulating the growth of populations of rodents and lago- 

 morphs. 



An attempt has been made ui this chapter to point out the areas where 

 critical data are lackuig and some of the problems and pitfalls which may 

 be encountered in investigating these physiologic phenomena in relation to 

 population density. It is hoped that it will serve as a useful guide for further 

 research by investigators who wish to explore the mechanics of population 

 dynamics. 



References 



Ackerman, N. B., and Arons, W. L. (1958). The effect of epinephrine and norepinephrine 

 on the acute thyroid release of thyroid hormones. Endocrinology 62, 723-737. 



Agate, F. J., Jr. (1952). The possible role of the adrenal cortex in fluid shifts in the ham- 

 ster following renal damage. Ann. N. Y. Acad. Sci. 55, 404-411. 



Albert, A., Tenney, A., and Ford, E. (1952). The effect of cortisone and corticotropin on 

 the biologic decay of thyroidal radioiodine. Endocrinology 50, 324-326. 



