2. Endocrines and Populations 293 



function in female mice and voles is reciprocally related to population 

 density. The degree of inhibition apparently may vary from none to com- 

 plete in the females of a given population, within limits depending on social 

 relationships of the particular individuals. 



We have already criticized the concept of "copulation pressure" which 

 has been used to explain the decline in female fertility seen in populations 

 of high density. As an explanation it is inadequate because it does not 

 coincide with the facts, even in those cases in which it has been used as an 

 explanation (Southwick, 1955b). Furthermore, no correlation could be 

 shown between "copulation pressure" and fertility in other studies (Louch, 

 1956) . In the light of those studies in which female reproductive function 

 was more precisely assessed, it appears that reproductive function was at 

 least partially inhibited in those populations in which "copulation pressure" 

 was used to explain the decline in fertility, and that the means used to assay 

 female fertility were not sufficiently sensitive. "Copulation pressure" con- 

 ceivably could have an effect on female reproductive performance, but 

 until the problem is reexamined more critically it must remain an unaccept- 

 able concept, especially in view of the preponderance of evidence indicating 

 that reproductive function is inhibited, either individually or on a popula- 

 tion-wide basis, as part of the adaptive responses to increased density. A 

 broad view of this evidence suggests considerable variability from popula- 

 tion to population in the degree of reproductive inhibition with respect to 

 the numbers of females involved as well as to the magnitude of inhibition 

 in particular individuals. These variations are evidently related to social 

 factors, and this aspect of the problem needs examination in greater detail 

 than heretofore. 



Prenatal mortality of the fetuses was an appreciable factor in reducing 

 the number of births in dense populations, presumably largely owing to a 

 suppression of gonadotropins, but also possibly to the effects of increased 

 adrenal corticoids and androgens. There seems to be some kind of "dual 

 response" to density in depressing female productivity. Judging from the 

 results described in this and preceding sections, it appears that the least 

 "dose-response" effect occurs when fertility is partially diminished, as by 

 decreased numbers of ova and increased losses between ovulation and birth, 

 without a total suppression on the ability to bear young. A greater effect 

 apparently is total inhibition of estrus and reproduction in potentially 

 mature animals. Delayed attainment of maturity probably reflects partial 

 inhibition. The possible mechanisms involved in producing intra-uterine 

 mortality have been discussed in an earlier section. 



c. Litter survival. A marked decline in litter survival with increasing 

 population density was observed in all but a few experiments with freely 

 growing populations of house mice or voles (Rtrecker and Emlen, 1953; 



