LIFE PROCESSES IN CAPTIVE GRAY RATS 



43 



Litters containing more than twelve young are included in 

 table 13 merely to complete the records. They are too few 

 to be of value statistically, and the ratios are not in accord 

 with those for other litter groups. 



The great majority of litters in this series contained from 

 four to eight young, and the sex ratios for these groups vary 

 little from each other or from the ratio for the series as a 

 whole. In litters of one and in litters containing eleven or 

 twelve young the sex ratios are considerably lower than those 



TABLE 13 



Litter size and the sex ratio in the second to the twenty-sixth generation of 



captive gray rats 



for the other litter groups, but the differences are not im- 

 portant statistically. Data in table 13 do not indicate any 

 definite relation between litter size and the sex ratio in gray 

 rats, and so are in agreement with the findings for many 

 other mammals : albino rat (King and Stotsenburg, '15 ; mouse 

 (Parkes, '26 b) ; deer mouse (Sumner, McDaniel and Huestis, 

 '22) ; guinea pig (Ibsen, '22; Schott and Lambert, '30; Haines, 

 '31) and the pig (Parker and Bullard, '13; Machens, '15; 

 Parkes, '23). 



