LIFE PROCESSES IX CAPTIVE GRAY RATS 63 



continuation of their reproductive activities to a more ad- 

 vanced age (fig. 7). Possible explanation for this lengthening 

 of reproductive life are discussed briefly. 



Fertility of gray rats, as measured by litter production, 

 increased steadily, reaching its maximum in the nineteenth 

 generation where females produced an average of 10.18 litters 

 as contrasted with an average of 3.50 litters produced by fe- 

 males of the first generation. A slight decline in litter pro- 

 duction during later generations resulted from the unfavor- 

 able effects on reproduction of changes in the location of the 

 colony (table 8, fig. 9). 



Sterility and low fertility of females, ascribable to the 

 effects of captivity, had disappeared by the tenth generation. 

 Subsequently all females reared were fertile, except those in 

 which the reproductive organs became diseased. 



In litters of the second to the twenty-sixth generation the 

 range in litter size was from one to fifteen with the mean at 

 6.1 (table 9, fig. 10). Average litter size in any generation 

 did not differ materially from the mean for the entire series. 



The age of the mothers had a marked effect on litter pro- 

 duction, litter size and the variation in litter size. The number 

 of litters cast increased until the females were 7 months old 

 and then decreased steadily until the end of reproductive life 

 (table 10). Average litter size reached its maximum (6.55) 

 when mothers were 8 months old and then declined, reaching 

 its lowest point in litters cast at the end of the series (fig. 11). 

 Variation in litter size was slightly below the mean for litters 

 cast by very young females, and significantly lower in litters 

 produced when females were at the height of reproductive 

 activity. When fertility began to wane, variation in litter size 

 increased gradually and reached its maximum in litters cast 

 near the end of reproductive life (table 10). 



The sex ratio showed no definite trend as the generations 

 advanced, and for the entire series was 97.3 0.57 (table 11, 

 fig. 12). This ratio is significantly lower than that (105.2 

 2.00) found in a strain of albinos maintained under the same 

 conditions of environment and nutrition as the gray rats. A 



