2. Endocrines and Populations 299 



to be interpreted to indicate that acute starvation cannot ensue in local 

 populations or that a limited food supply is not at times important. The 

 relationships between food, competition, reproduction, and the growth of 

 populations require much more investigation, especially for different 

 species. Until additional critical information is available, any conclusions 

 regarding food and population growth must remain tentative. 



4. Growth 



In a recent experiment with a freely growing population of house mice, 

 it was shown that there was appreciably inhibited growth in all but the 

 mice originally introduced and those from the first litters (Christian, 

 1961) . In this experiment all mice were toe-clipped so that their ages were 

 known. In general the results from the reproductive organs and adrenal 

 glands confirmed those of earlier experiments and were even more pro- 

 nounced because the results could be equated with age rather than body- 

 weight categories. 



5. Summary of Results from Freely Growing Experimental Popu- 

 lations 



We have seen that, for the most part, freely growing populations of 

 house mice and voles are self-limited, primarily as a result of a density- 

 dependent activity of physiologic adaptive mechanisms. The activity of 

 these mechanisms is reflected in both sexes by a progressive inhibition of 

 reproduction and stimulation of the pituitary-adrenocortical activity with 

 increasing population size. The results agree with those obtained from 

 populations of fixed size. Diminished reproductive function in the female 

 may be apparent at any one or all phases of reproductive activity : inhibi- 

 tion of estrus, increased intra-uterine mortality, reduced nvmibers of im- 

 planted ova, increased postparturient mortality due to suppression of 

 lactation, and possibly by diminished maternal behavior. Inhibition of male 

 reproductive ability is shown by decreases in the weights of the sex acces- 

 sories and depressed spermatogenesis, although the problem of male fertility 

 needs further investigation. The decreases in reproductive function in both 

 sexes apparently stem from decreases in the secretion of pituitary gonado- 

 tropins with a secondary decline in the production of estrogens, progestins, 

 and androgens. Evidence for and against the role of "increased copulation 

 pressure" in diminishing the fertility of female mice and voles was dis- 

 cussed. Without much more conclusive evidence, it is likely that "copula- 

 tion pressure" is an unimportant factor and that fertility is depressed by 

 increased density operating through sociopsychologic and physiologic path- 

 ways. Increased adrenocortical activity was shown by increased adrenal 



