2. Endocrines and Populations 321 



possibilities has been explored and in retrospect the idea of attributing 

 hypoglycemic shock solely to adrenocortical exhaustion seems naive. That 

 the hypoglycemic shock syndrome does occur cannot be denied, but its 

 physiological causes remain to be determined. Such indirect evidence as is 

 available indicates that it is a very complex affair. Mice and voles have 

 been observed to die of hypoglycemic shock shortly after being placed 

 placed together in groups (Frank, 1953; Christian, 1955a), but if moribund 

 animals are removed from their cagemates, they recover (Frank, 1953; J. J. 

 Christian, unpublished) . The rapidity with which this occurs suggests some- 

 thing other than cortical exhaustion, perhaps medullary exhaustion or 

 exhaustion of readily available glycogen reserves. In addition, there is 

 evidence that adrenal cortical exhaustion may not occur per se, but may 

 be an apparent result of exhaustion elsewhere in the adaptive system (Ro- 

 senfeld, 1958). 



At the present time it appears that a failiu-e in reproductive function 

 may account for a decline in a population much more frequently than shock. 

 It has been observed in several species that the young born during peaks 

 of density appear to be much more susceptible to various mortality factors, 

 and their deaths largely account for precipitous population declines (Green 

 and Evans, 1940a, b, c; Chitty, 1952, 1954; Godfrey, 1955). This dispro- 

 portionate mortality of young animals may result from three different 

 factors that have already been discussed: (1) they are the subordinate 

 animals in a population and therefore, as we have seen, are much more 

 severely exposed to sociopsychologic pressures than older and more domi- 

 nant animals; (2) they appear to be more sensitive to these pressures and 

 respond accordingly; and (3) they are adversely affected by deficient 

 lactation on the part of their mothers, and then- progeny in turn are 

 affected. The combination of these factors would be expected to have pro- 

 found effects on the young, and we have already seen that their reproduc- 

 tive function is severely depressed. Excessive stimulation of the adaptive 

 responses in these animals would also be expected to result in exhaustion 

 and shock. Frank (1953) has shown that socially inferior voles are the 

 ones primarily affected and that succumb to shock. 



In addition to these effects of social pressure and increased activity of 

 physiologic adaptive responses, one would anticipate a marked decline in 

 resistance to infection, parasitism, or other harsh environment experiences. 

 It is not surprising that epidemics are frequently observed following peak 

 densities and a general decrease in host resistance. It is particularly signifi- 

 cant that a variety of agents may be found to produce death in a given 

 population, apparently as a result of decreased resistance (Chitty, 1954) . 



It is not the intent of this account to dwell on the causes of mortality in 

 natural populations except insofar as a decreased resistance may result from 



