LIFE PROCESSES IN CAPTIVE GRAY RATS 29 



The better physical condition of the rats after they had been 

 in captivity for several years, and the less frequent occurrence 

 of tumors and infections of the genital tract, probably account 

 in great measure for the extension of reproductive life. There 

 is also a possibility that heredity may be a factor involved in 

 defining the limits of reproductive activity, since in some 

 cases relatively young females, seemingly in good physical 

 condition, cast only two or three litters; in other cases all 

 females of a given litter produced approximately the same 

 number of litters and stopped breeding at about the same age. 



Changes in the average length of the reproductive period 

 in females of the first twenty-five generations are shown by 

 the graph in figure 8. 



Avw*g tongtn in atyt 



1 2 3 4 5 6 7 8 B 10 II 12 13 14 15 18 17 18 19 20 21 22 23 24 K 



Fig. 8 Average length of the reproductive period in different generations. 



Although there are irregularities at the beginning of the 

 graph in figure 8, its general trend is upward. There are 

 noticeable depressions at the ninth and twentieth generations, 

 ascribable to the fact that in these generations breeding was 

 checked because a number of females had ovarian cysts or 

 developed pneumonia at an early age. The difference in the 

 level of the graph at its beginning and the level at its end 

 indicates that, at the twenty-fifth generation, the average 

 length of the reproductive period (440 days) was more than 

 twice that (204 days) in the first generation. This extension 

 of reproductive life and also the increased fertility of captive 

 gray rats, which will be shown in the following section, accord 



