2. Endocrines and Populations 280 



abnormal, deficient, or decreased in amount, and that the composition of 

 the ejaculate will in all probability not provide an optimal medium for 

 sperm (Burrows, 1949; Lutwak-Mann et al., 1949; Cavazos and Melampy, 

 1954). Therefore it is possible that male fertility is depressed appreciably 

 by increased population density, although the gross examination or rela- 

 tively minor changes in weight would provide no indication of such a reduc- 

 tion in male fertility. Male fertility is assessed in most studies on small 

 mammals by (1) the position of the testes, (2) whether or not the epididy- 

 mal tubules are clearly visible, or (3) possibly by epididymal smears to 

 determine the presence or absence of spermatozoa (Southwick, 195oa; 

 Louch, 1956; Strecker and Emlen, 1953; Crowcroft and Rowe, 1957). 

 Degenerative changes can be detected in testicular smears (Christian, 

 1950a, 1951) , but with much less assurance than properly prepared, stained, 

 and critically examined sections of the testes. Morphologic studies of the 

 testes, if properly done, can be extremely revealing, but the weights of the 

 testes and accessory organs, while providing a useful index of reproductive 

 function in general, do not tell whether or not the male is actually fertile 

 unless inhibition is severe. In the absence of good evidence to the contrary 

 it is generally assumed that the males in a population are fertile — an as- 

 sumption that may be misleading. Studies of male fertility in relation to 

 population density are needed that make use of sperm counts, determina- 

 tion of the motility and viability of the sperm, and possibly direct determi- 

 nation of fertility by mating with proven females. Admittedly such a pro- 

 gram would present problems, but they are by no means insurmountable. 

 Furthermore, once the techniques are worked out for obtaining ejaculates 

 from the males in populations of small mammals, it should be a valuable 

 and useful procedure for routinely assessing male fertility and reproductive 

 competence in freely growing populations of small mammals. But it is in- 

 correct to assume that male fertility is not affected by increased population 

 density, especially in view of the evidence indicating that the secretion of 

 testicular androgens is extremely sensitive to changes in population density 

 or to social rank. 



Southwick (1955a) observed a decrease in fertility in female mice from 

 populations which were slowly declining from peak levels. He also noted, as 

 have others (Southern, 1948; Calhoun, 1949), that in populations of high 

 density or with poor social organization (Calhoun, 1949, 1950) that "copu- 

 lation pressure" on females in estrus was high; that is, a number of males 

 gathered around her and attempted to copulate although they were fre- 

 quently pushed off by others, so that the mean copulation time per male 

 was reduced. "Copulation pressure" was used as a possible explanation 

 for the decline in fertility. This explanation overlooks several facts. The 

 populations were progressively declining from peak levels and did so for six 



