66 HELEN DEAN KING AND HENRY H. DONALDSON 



and they are usually kept together throughout life. Oc- 

 casionally, for some reason as yet unknown, one of the males 

 in a cage will grow much faster and become considerably 

 larger than its brothers. When this male is powerful enough 

 it dominates the 'home.' The other males are not permitted 

 to mate with the females, as a rule, and they are forced to 

 stay in a corner of the cage away from the nest. Sometimes 

 this dominant male becomes very vicious and develops 'killer' 

 instincts. He then either kills the other males or injures 

 them so severely that they have to be removed from the cage ; 

 females are rarely molested in any way. It would appear 

 probable, from this finding, that in a state of nature gray 

 rats are inclined to be polygamous, the most vigorous male 

 driving out or killing the other males and then occupying 

 the burrow with several females. Such a condition exists in 

 many other wild species, as is well known. 



Occasionally a female that has become larger than her 

 sisters will exhibit the 'killer' traits shown by males. One 

 such female, weighing over 400 grams, killed two sisters and 

 two males in the cage. She lived peacefully for some months 

 with the remaining male that was considerably larger than 

 herself, and took very good care of her numerous offspring 

 as long as they remained with her. 



. A most pronounced characteristic of gray rats, and one 

 that time and captivity have not changed to any extent as 

 yet, is their decided aversion to the presence of strange rats. 

 If the inhabitants of the cage are adults, they will at once 

 attack and soon kill the intruder, no matter how large it 

 may be. Even if the new arrival had formerly been housed 

 in the cage, and had been absent but a day or two, it promptly 

 meets with the same fate as that of a total stranger. When 

 a rearrangement of adult rats of mixed sexes is necessary, 

 the only way to insure each of the individuals more than a 

 fair chance of life is to etherize all of the rats, give them a 

 bath in a fairly strong solution of some disinfectant, such as 

 creolin or larkspur, and then place them in a recently steri- 

 lized cage containing a quantity of sterilized bedding. The 



