296 /. ./. Christian 



plays a more or less constant, probably unimportant, role in causing litter 

 mortality at all levels of population density, whereas inhibition of lactation 

 and changes in maternal behavior are density dependent and probably 

 reflect physiologic (endocrine) responses to changes in population density. 

 Southwick (19o5b) noted that aggressiveness, as indicated by fights per 

 mouse per unit of time, was reciprocally related to litter survival at high 

 densities, but that this relationship did not necessarily hold at lower 

 densities. On the other hand, there was a correlation between fighting and 

 density in only one of three populations of voles (Louch, 1956). The be- 

 havioral changes observed in relation to litter survival may be outward 

 manifestations or "symptoms" of more profound physiologic changes at 

 higher population densities. 



A final consideration of importance is that fecundity and litter survival 

 are sufficiently sensitive to changes in the population to be responsive to a 

 variety of different circumstances affecting density. Litter survival was 

 markedly increased in a population of house mice in which there was an 

 appreciable mortality of adults (Christian, 19o9b). The growth form and 

 eventual size of the population were comparable to those of populations 

 without appreciable adult mortality. The loss of adults in this population 

 was compensated for by increased litter survival. Similar relationships were 

 observed in three populations of voles (Louch, 1956). There was a signifi- 

 cant correlation between fertility and population density in one population 

 of voles in which there was no correlation between density and adult 

 mortality. In a second population there was a good correlation between 

 density and adult mortality, but none between density and fertility. The 

 third population was intermediate between these two. Therefore, there 

 appears to be sufficient flexibility in the physiologic adaptive responses to 

 compensate for losses of adults from populations of voles or mice. This 

 compensation apparently can occur at any stage of the reproductive pro- 

 cess from ovulation to weaning. 



d. Effects of food supply. It is often assumed without question that food 

 shortage is responsible for limiting population growth. However, we have 

 seen that the growth of a population may be entirely self-limited without a 

 shortage of food. The role of a limited food supply in the regulation of 

 population growths has been investigated (Strecker and Emlen, 1953; 

 Strecker, 1954). Clarke (1955), Christian, (1955b, 1956, 1959b), South- 

 wick (1955a, b), and Louch (1956) have emphasized the fact that in 

 their experiments food was always abundant and usually scattered so that 

 any animal could obtain food irrespective of other animals present. Further- 

 more, experiments with populations of fixed size indicated that inanition 

 was not a stimulus to increased adrenocortical activity in house mice, nor 

 did it constitute a factor for increasing competition when it (food) was 



