288 /• /. Christian 



lished) . Severe fighting broke out in this population and the growth of the 

 population ceased completely until social order was reestablished 6 weeks 

 later. The cessation of growth resulted from a complete cessation of all 

 aspects of reproductive activity. These considerations, plus the fact that 

 the physiologic responses are equivalent in populations of maximum self- 

 limited size irrespective of the number of mice (Christian, 1956), make it 

 likely that the growth of populations of house mice, and possibly voles, is 

 limited by the total amount of social pressure rather than by the number 

 of animals. 



The reproductive competence of males has not been examined in detail 

 for any species from freely growing populations other than house mice ; and 

 no studies have adequately explored the problem. In most studies on popu- 

 lation density the criteria of male fertility are position of the testes, gross 

 size of the testes, visibility of the epididymal tubules, and occasionally the 

 presence or absence of sperm in the epididymes (Strecker and Emlen, 1953; 

 Brown, 1953 ; Southwick, 1955a; Crowcroft and Rowe, 1957) . These criteria 

 are actually poor indicators of relative fertility and only suggest whether 

 or not an animal is mature. Detailed morphologic studies provide more 

 evidence of fertility (Christian, 1956), but they do not provdde conclusive 

 evidence. No information on subtle changes in fertility is provided by any 

 of these criteria. The ability of males to fertilize females with respect to 

 population density has not, to my knowledge, been investigated. 



Many influences which depress reproductive activity cause striking 

 degenerative changes in the germinal cells in the testes (Selye, 1947). The 

 formation of giant cells from cells of the spermatogenic series is a common 

 indicator of such changes (Selye, 1950; Steinberger and Dixon, 1959). 

 Furthermore, influences which can produce severe degenerative changes 

 or inhibit spermatogenesis completely can produce more subtle changes if 

 the damaging stimulus is less severe (Steinberger and Dixon, 1959). It 

 seems likely that the conspicuous changes that occur in the testes of house 

 mice as a result of increasing population density [a delay in the onset of 

 spermatogenesis, reduction in the number of mature spermatoza, and 

 formation of giant cells (Christian, 1951, 1956)] can also be extended to 

 include less obvious abnormalities of the sperm, such as decreased motility 

 and viability. There also may be a decrease in the nmnber of sperm pro- 

 duced, and therefore in the number of sperm in the ejaculate. All these 

 factors presumably affect fertility (Chang and Pincus, 1951). To these 

 must be added the likelihood of an altered medium for the sperm which 

 would be suggested by the changes in the sex accessories associated with 

 testicular changes, as indicated by the decline in their weights (Burrows, 

 1949; Lutwak-Mann et al., 1949; Leathem, 1950; Mann, 1954) . Atrophy of 

 sex accessories carries the implication that their secretory products may be 



