Population Density and Reproductive Efficiency1

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Abstract
I have discussed how various behavioral-endocrine feedback systems act to increase mortality (and emigration) and decrease natality (and immigration) and thereby regulate and limit growth of many mammalian See PDF for Figure populations. Figure 18 is a graphic summary of these interrelationships. It is not in correct form for systems analysis, although knowledge of the mechanisms controlling growth of populations and the physiology of a number of species is sufficient to warrant examining them from this point of view. The role of the autonomic nervous system and limbic system in the evocation of these responses has not been discussed here, although its importance is clearly recognized (e. g., Welch and Welch, 1969). I also have suggested that we have sufficient information to explain the cyclic fluctuations of microtine populations. Most of the behavioral, endocrine, and demographic changes can be accounted for adequately by data at hand, although variations and at times the action of other factors would be See PDF for Table See PDF for Figures expected. Unquestionably two major variables in populations of different species of small mammals are intraspecific competition and the sensitivity of the reproductive inhibitory mechanisms. However, there may be quantitative variations in the relative importance of different mechanisms in a species at different times. The two extremes among those small mammals for which information is available appear to be microtines on the one had with potentially very high rates of reproduction and with reproductive cut-off mechanisms that appear to be less sensitive in terms of density than those in other species, for example as in Peromyscus. Therefore, it appears that increased pituitary-adrenocortical activity, in response to aggressive behavior, is greater in microtines than in Peromyscus, although adrenocortical secretory activity has not been studied quantitatively with respect to changes in density in sufficient detail and in enough species for firm conclusions. Nevertheless Peromyscus seems to have evolved a more sensitive reproductive inhibitory mechanism than microtines. House mice and Norway rats appear to be between these two extremes. One might expect that as studies are made of other species that a continuous spectrum of graded balance between responding systems will be found. In any event it seems clear that endocrine—behavioral negative feedback mechanisms operate to increase mortality and decrease reproduction as density increases and can regulate and limit population growth if some other outside limiting factor does not do so first. Even in instances where other limiting factors stop population growth, the internal feedbacks should operate in some proportion to the density achieved. A final comment is in order with reference to population density. Density and size are frequently used synonymously. However, size should not be understood to imply and specific relationship to space. Nevertheless, it is clear that space per se must be important, as Myers el al. (1971) have pointed out.

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