LIFE PROCESSES IN CAPTIVE GRAY RATS 47 



MORTALITY 



No attempts were made to determine the potential duration 

 of life in gray rats, since it seemed more important to ascer- 

 tain the condition of various body organs before senility had 

 altered them. 



In all generations, the study of life processes was termi- 

 nated when rats reached the age of 20 months, and a stated 

 number of individuals given to Doctor Donaldson for a deter- 

 mination of organ weights. Exceptions were made in the 

 case of females that were unusually vigorous and had cast 

 litters when they were over 19 months of age. Such females 

 were housed with young males, and not killed until their re- 

 productive activity had ended in order that the series of litter 

 data might be complete. 



Mortality at birth was very low in all generations, and only 

 268 stillbirths were recorded among the 53,077 young born in 

 the first twenty-six generations. Mortality among the young 

 during the suckling period decreased considerably after the 

 tenth generation. By this time females were so well adjusted 

 to conditions of captivity that they rarely destroyed their 

 offspring returned to the nest after they had been removed 

 for examination, and nests were less frequently disturbed by 

 other inmates of the cage. Offspring were destroyed or ne- 

 glected only when the mother was in poor physical condition, 

 or when one or more of her nipples had been injured by the 

 young when suckling. 



Table 15 gives mortality data for the eleventh to the twenty- 

 fifth generation, with a summary of the records for the entire 

 series grouped to indicate the general trend in mortality rate 

 as the generations advanced. 



In later generations the mortality rate in both sexes was 

 low during the first year of life, the relatively high percentage 

 (11.9) in the twenty-third generation being due to the acci- 

 dental killing of four young females (table 15). Mortalitv 

 increased greatly after the rats were a year old, and only 

 79.9 per cent of males and 76.6 per cent of females lived to the 

 age of 20 months. 



