2. Endocrines and Populations 297 



limited either in source or amount (Christian, 1959c). These results are 

 consistent with those from experiments with freely growing confined 

 populations of house mice or voles in which competition for food in every 

 case was rare or absent (Strecker and Emlen, 1953; Louch, 1956; South- 

 wick, 1955a, b; Christian, 1956). Strecker and Emlen (1953) provided two 

 confined populations with a limited amount of food but the growth of only 

 one of these was limited by the limited food supply. Growth of this popula- 

 tion was rapid and ceased abruptly when the limit set by the food supply 

 was reached. Growth ceased because reproduction stopped abruptly 

 with involution of the reproductive organs, especially of the females, all of 

 which were reproductively quiescent at autopsy, although the animals 

 otherwise appeared to be in excellent condition and usually excessively fat. 

 Several points of interest emerge from these experiments. One is that 

 population growth ceased very abruptly with a truncated growth curve, 

 indicating that a limited food supply does not act as a damping factor, but 

 exerts its effects at one point on the growth curve. Therefore, it is unlikely 

 that a typically sigmoid growth curve would result from a food shortage. 

 The second point of interest is that reproduction was so sharply limited by 

 the limited food supply. These results agree with those from populations of 

 fixed size and indicate that the food limitations completely inhibited re- 

 production, probably by suppressing the secretion of gonadotropins, with- 

 out producing increased activity of the pituitary-adrenocortical adaptive 

 system. The last point is that the population immediately adjusted to the 

 supply and the general condition of the mice was unaltered by the limitation 

 in food. The inhibition of reproductive activity in all the females contrasts 

 sharply with the results of other experiments conducted on self-limited 

 freely growing populations of mice or voles supplied with an excess of food. 

 In the latter populations there were always some reproductively active 

 females, although the actual proportion in the population varied with each 

 population. Data on adrenal activity were not collected in these experi- 

 ments on the efTects of food limitation on freely growing populations. It is 

 probable that there would have been density-dependent increases in adreno- 

 cortical activity, but the effects of food limitation were independent of any 

 density-dependent effects. In another series of experiments, the food supply 

 was limited similarly, but egress from the population was allowed (Strecker, 

 1954) . In these populations there was a low constant rate of egress until 

 the food limitation was reached and then a high rate of egress began and 

 continued until the experiment was terminated three months later. The 

 egress apparently involved all segments of the population except the 

 youngest. Reproduction continued at a good rate in this population in 

 sharp contrast to the confined population without egress. The implications 

 of these experiments are clear: f1) food limitation can limit population 



