I. LIFE PROCESSES 63 



is responsible for the majority of deaths among older rats, 

 is seemingly more common in males than in females. On the 

 other hand} cancer and tumors are far more prevalent in 

 females than in males. In the first ten generations of gray 

 rats eighteen females and only one male were killed because 

 of such growths. A third ailment, frequently found in rat 

 colonies, is the middle-ear infection which produces disequili- 

 bration and circular movements (Donaldson, '24). Only six 

 cases of this disease were found in the gray rats. The affected 

 individuals were killed as soon as they showed the character- 

 istic symptoms of the disease, as the infection is readily 

 transmitted to other individuals in the cage. 



From the findings recorded in this section it appears that 

 in the first ten generations of captive gray rats mortality 

 was very low at birth. It increased during the suckling 

 period, but fell during adolescence and early maturity. After 

 the animals reached the age of one year the death rate rose 

 considerably. As the generations advanced, the mortality at 

 twelve and at twenty months of age decreased in both sexes, 

 but among the older individuals " the decrease was more 

 marked in females than in males. The chief causes of death 

 were lung disease and malignant growths. 



BEHAVIOR 



Since little is known of the general behavior of gray rats 

 living in their natural habitat, it seems worth while to record 

 here the outstanding traits of the wild animals which were 

 brought into the colony to start the strain of captive Grays, 

 and to note the changes in these traits that occurred a* tin- 

 animals became adjusted to their new environment. 



When first captured, wild rats were much frightened and 

 so appeared very savage and vicious. They were constantly 

 in a state of high nervous tension due, apparently, to an in- 

 stinctive fear of man and to the restriction of their activity 

 that captivity imposed. If the cage was approached they 

 began running about, fighting and screaming in obvious 

 terror. When they had been exhausted )>y such efforts, they 



