2. Endocrines and Populations 291 



cortical activity in mice results in an increase in the secretion of weakly 

 androgenic steroids, and that these can depress the secretion of gonado- 

 tropms. 



h. Female. Reproductive competence of female mammals can be assessed 

 grossly more easily than that of males diu-ing studies of growing popula- 

 tions simply because the number of young produced can be counted. Some 

 comments have been made about the relationships of female fertility and 

 reproduction to population density in animals from freely growing popula- 

 tions. Most of the pertinent evidence indicates that fertility and reproduc- 

 tion in female mammals is depressed by increased density. Two kinds of 

 information has been gathered on female productivity in relation to popula- 

 tion density. In one case the number of young born at intervals throughout 

 the history of the population has been determined, and in the other female 

 fertility is assessed by external examination of the vagina, which is closed 

 by a thin membrane during anestrus in many species, or by examination of 

 the animals at autopsy. Information of both kinds is desirable, although 

 neither is sufficient by itself. 



It has been shown in studies with house mice and voles in freely growing 

 confined populations from which adequate data are available that the birth 

 rate for mature or potentially mature females in the population declines 

 steadily with increasing population size (Clarke, 1955; Christian, 1956, 

 1959b, 1961; Crowcroft, and Rowe, 1957). The decline in the number of 

 young born per female appears to decline approximately linearly as the logio 

 of the population increases (Christian, 1959b), 



The declines in productivity apparently resulted from suppression of all 

 phases of reproductive functions. Crowcroft and Rowe (1957) showed 

 there was complete suppression of estrus, as indicated by closure of the 

 vaginal orifice, in an increasingly larger proportion of females as the size of 

 a population increased. It was shown that closure of the vaginal orifice was 

 a reliable index of fertility in female house mice in these experiments by 

 the very high correlation between the condition of the vaginal orifice and 

 the development of the reproductive organs. In these populations the 

 majority of the young were produced by the introduced females initially or 

 by females from the first few litters. These results correspond to those from 

 populations of limited size in which it was indicated that the reproductive 

 competence of female mice was related to their social status. Similar results 

 were also observed in confined populations of Norway rats (Calhoun, 1949, 

 1950) . Reproductive function was completely suppressed in the majority 

 of female born into these populations. 



These results are comparable to those from experiments with freely 

 growing populations of house mice (Christian, 1956), thus indicating that 

 reproductive function in female mice was depressed for all body-weight 



