324 /. /. Christian 



there is suppression of lactation, as well as of other reproductive functions, 

 with increased density which may account for the prolonged effects of high 

 density on the young. However, the effects of high density may also be 

 exerted on the young animals directly. There is also evidence that resistance 

 to disease and parasites is decreased, especially m the young, at high popula- 

 tion densities. In extreme cases, there may be mass mortalities of popula- 

 tions due to hypoglycemic shock, presumably following exhaustion of some 

 part of the entire physiologic mechanism or of the reserves of readily 

 available necessary metabolites or both. 



None of the data from natural populations is conclusive evidence that 

 density-dependent responses of physiologic adaptive mechanisms are active 

 in the regulation and control of the growth of mammalian populations. 

 However, when the available evidence is viewed in the light of the results 

 obtained from carefully controlled studies on laboratory populations, there 

 is sufficient reason to conclude that these mechanisms are operative in 

 natural mammalian populations, and more precisely in populations of 

 rodents and lagomorphs. It is equally apparent that sociopsychologic pres- 

 sures are the stimuli to the various physiologic responses involved. There- 

 fore, there is a wide range in responses in the individuals of a population, 

 the subordinate, and therefore younger, animals being the most severely 

 involved. Finally, it appears that these sociopsychologic-physiologic mecha- 

 nisms are basic controlling factors in populations of mammals and other 

 factors are secondarily important in most cases. 



There is a great need for a coordinated study on a small rodent for which 

 there are criteria for determming age in which adrenocortical function, re- 

 productive function, resistance to disease, mortality, social factors, and 

 enviroilmental factors, can be adequately studied for a complete cycle of 

 the population. Until the results of such a study are available, conclusions 

 regarding density-dependent physiologic mechanisms must lean heavily on 

 extrapolations from data gathered from experimental populations in the 

 laboratory. There are in addition to the general problem many specific 

 problems that need investigation. Many of these have been mentioned m 

 the course of the discussion, but a few have not. A major problem is what 

 factors determine where the reproductive process will be affected in a 

 particular population. For example, why is it intra-uterine mortality in one 

 case, total inhibition of reproductive function in another, and depressed 

 lactation in a third? The explanations are largely conjectural at present, 

 but that these differences exist cannot be questioned, yet all represent a 

 common mechanism and effect: inhibition of reproduction. The answer to 

 another question may provide the answers to those already asked : how do 

 social pressures affect the individual members of a population? 



