2. Endocrines and Populations 323 



2. There may be increased mortality due to parasitism or infectious 

 disease due to decreased host resistance (presumably from suppression of 

 inflammatory and immune defense mechanisms by increased adreoncortical 

 activity) . 



3. There may be partial or complete inhibition of reproductive function, 

 especially in the younger animals and with prolonged effects on the next 

 generations of progeny, apparently initiated by deficient lactation on the 

 part of mothers subjected to mcreased density, but also owing to the fact 

 that the young probably are most severely affected by increased density. 



It is assumed that in the absence of mass mortality normal or moderately 

 increased mortality rates can produce striking declines in density when 

 reproduction is partially or wholly suppressed. The bulk of evidence in the 

 literature indicates that this is more usually the case than mass mortality. 

 Furthermore, food shortages, epidemics, or predation may be the immediate 

 cause of the mortality, although it is equally clear that the mortality still 

 occurs when these factors are absent (Elton, 1942; Rausch, 1950; Chitty, 

 1954; Godfrey, 1955; Pitelka, 1957a, b). It is also possible that some pre- 

 cipitating factors may act through the physiologic adaptive mechanisms by 

 increasing intraspecific strife, as was shown to be the case in experimental 

 populations of voles (Frank, 1953). 



4. Summary: Natural Populations 



There is considerable evidence to indicate that there is a density-de- 

 pendent increase in the activity of physiologic adaptive mechanisms in 

 natural populations of Norway rats, voles {Microtus, Clethrionomys, Dicro- 

 stonijx, Lemmus, Ondatra), hares (Leptis), rabbits {Onjctolagus) , white- 

 footed mice (Peromijscus) , and probably other species. A relationship be- 

 tween population density and adrenocortical activity has been demon- 

 strated for Norway rats, meadow voles {Microtus niontanus and M. 

 pennsylvanicus) , white-footed mice (Peromyscus leucopus), and sika deer 

 (Cervus nippon). An inverse relationship between density and reproductive 

 function exists for Norway rats, red-backed voles (Clethrionomys) , meadow 

 voles {Microtus agrestis, M. montanus, M. californicus, M. pennsylvanicus), 

 muskrats {Ondatra), cotton rats {Sigmodon), and hares (Lepus). However, 

 in few of these studied were reproductive function and adrenocortical func- 

 tion correlated in the same population. Furthermore adequate assessment 

 of other envu-onmental factors has not been made. In many investigations 

 there were no adequate means of determining age, so that a true indication 

 of the degree of inhibition of reproductive function was not possible, espe- 

 cially with a simultaneous suppression of growth. Determinations of 

 age should be possible in a variety of species if a concerted effort is made 

 to find the proper criteria. There is evidence that, as in the laboratory, 



