26 HELEN DEAN KING 



were vigorous and seemingly free from disease. The changed 

 conditions of life to which these rats were subjected affected 

 reproductive processes adversely, causing sterility in some 

 females and greatly reducing fertility in others. Only six of 

 the wild females bred in captivity, and the litters they cast 

 were small. Under their new environment wild females, with 

 one exception, seemed incapable of suckling their offspring, 

 and their litters were either destroyed soon after birth or 

 neglected. In order to save the young it was necessary to 

 remove them from the nest soon after their birth and give 

 them to lactating albino females to rear. Restrictions to 

 breeding resulting from change of habitat began to disappear 

 in individuals of the second generation, and subsequently the 

 great majority of females reared were fertile and able to rear 

 their young. 



It has been stated (Eaton and Stirrett, '28) that wild rats 

 become sexually mature when they are about 3 months of age, 

 but this statement lacks confirmation. In light of the breed- 

 ing data obtained from trapped wild rats and from indi- 

 viduals in the early generations of captive grays, I am in- 

 clined to the opinion that gray females living under natural 

 conditions do not begin to breed, as a rule, until they are at 

 least 4 months of age. 



With the advance of the generations there was a marked 

 increase in the length of the reproductive period, as is shown 

 by the data in table 7. 



In later generations, as in the earlier ones, there was con- 

 siderable variation in the time at which individual females 

 began breeding. While some females cast their first litters 

 when less than 3 months of age, the majority of them did not 

 breed until they were at least 4 months old. The average age 

 of females at the onset of breeding decreased steadily as the 

 generations advanced, and was but 119 days at the twenty- 

 fifth generation (table 7). Thus, at the end of the period of 

 captivity covered by this report, the reproductive life of 

 females began 147 days earlier than did that of females in 

 the first generation whose average age at the inception of 



