294 /• /• Christian 



Brown, 1953; Clarke, 1955; Southwick, 1955a, b; Christian, 1956; Louch, 

 1956). Litter survival apparently declines approximately linearly as the 

 logarithms of the population increases (Christian, 1959b). However, litter 

 mortality is not always a factor in diminishing the rate of population 

 growth. Crowcroft and Rowe (1957) found very little mortality of young 

 mice in their populations and furthermore were able to attribute all the 

 observed mortality to interference on their own part in making routine 

 censuses. The decline in productivity in their populations was due almost 

 entirely to prenatal loss or complete inhibition of all reproductive activity. 

 It has been suggested (cf. above) that diminished fertility probably was 

 more of a factor than was realized in several other investigations; neverthe- 

 less litter mortality has been an unportant factor in many populations. 

 Decreased litter survival may be due to diminished lactation (Christian, 

 1956; Christian and LeMunyan, 1958) or to behavioral factors (Brown, 

 1953; Strecker and Emlen, 1953; Southwick, 1955b). 



Partial inhibition of lactation with increased density was described 

 earlier. The results suggested that lactation, controlled by complex endo- 

 crine mechanisms, was partially inhibited owing to inhibition of gonado- 

 tropin secretion and a resultant diminished production of estrogens and 

 progestins. Lactation therefore reflected population density in a similar 

 fashion to other reproductive functions. It was also pointed out that the 

 progeny nursed by mothers subjected to increased density were profoundly 

 affected by the inhibition of lactation. Experiments with freely growing 

 populations of house mice suggest that deficient lactation may be an im- 

 portant factor in decreasing litter survival (Christian, 1956). These young 

 were stunted and were weaned early. At autopsy their stomachs contained 

 little or no milk. Calhoun (1949, 1950) suggested that the socially sub- 

 ordinate rats in freely growing confined populations were incapable of 

 raising then* young, even though they were protected from disturbance. 

 He indicated that physiologic and psychologic disturbance in the sub- 

 ordinate females affected the young through either poor fetal nutrition or a 

 breakdown of maternal instincts. 



Dimmished lactation may have been an appreciable factor in reducing 

 the survival of young in the above experiments, and diminished lactation 

 is a logical consequence of a generalized inhibition of reproductive function 

 presumably due to diminished gonadotropin secretion. 



Decreased litter survival was attributed entirely to social or behavioral 

 factors in a number of experiments with freely growing populations of 

 house mice or voles. Brown (1953), Southwick (1955b), and Louch (1956) 

 related litter survival to the type of nest constructed and maintained. 

 Brown and Louch each noted that litter survival was good so long as the 

 female mice or voles were able to maintain covered or bowl types of nests, 



