LIFE PROCESSES IN CAPTIVE GRAY RATS 49 



The summary of data in table 15 shows no general trend 

 in the mortality rate at 12 months in either sex. In males, 

 the lowest rate (2.8 per cent) comes for individuals in the last 

 group, while in females the rate for this group is high (4.2 

 per cent) only because of accidental deaths. In both sexes, 

 however, the mortality at 20 months tended to decrease as 

 the generations advanced. In males of the last group, mor- 

 tality was 18.7 per cent below that in the first group; in fe- 

 males it was 15.9 per cent less. Conditions of life in captivity 

 thus tended to prolong the average life span of the individuals, 

 possibly because they received more adequate nutrition and 

 were protected from various diseases prevalent among wild 

 rats living in their natural habitat. 



The main causes of death among older individuals were the 

 lung infection, commonly called 'pneumonia,' and tumors of 

 various kinds. Pneumonia rarely attacks rats until they are 

 a year old, and males seem more susceptible to it than females. 

 Autopsies made on rats 20 months old have shown very few 

 individuals that were entirely free of this disease. Tumors 

 occur mainly in females, and are usually found in the ovaries 

 or mammary glands. The latter growths, when small, can be 

 removed easily, and rarely return. Cancer has appeared but 

 rarely in this strain, as it has been found only in one male 

 and in three females. Gray rats are very resistant, appar- 

 ently, to the middle-ear infection commonly found in other 

 strains of rats. Only eight of 2737 individuals reared for 

 study developed this disease, and in two of these cases it could 

 be attributed to the fact that gray females were serving as 

 'foster mothers' to young from an albino strain in which many 

 individuals were affected. 



BEHAVIOR 



After 14 years of life in captivity the behavior of gray rats 

 had changed so greatly that many of the obnoxious traits ex- 

 hibited by the early descendants of feral animals had dis- 

 appeared. These changes in behavior occurred so slowly, 

 however, that there was no noticeable difference in the conduct 



