30 BIOLOGY 



Institute colony a considerable number of rats of both sexes 

 surpass these weights to-day. Moreover, exceptional rats of 

 large size are now appearing. A male of 615 grams has been 

 reared by Osborne and Mendel (MS.) and Ibsen ('21) reports 

 one of 745 grams. 



In the Institute colony several males over 600 grams have 

 been raised. These results indicate a progressive reponse of 

 the Albino to domestication and reflect also the influence of 

 better food and care through a series of generations. Wild 

 Norways of such body weights have not been recorded. 



Minimum weights. Individuals under weight at birth tend to 

 remain under weight during later growth (Dunn, '08). Under 

 usual conditions "runts" are rare among rats, but in these the 

 weight of the brain and of the spinal cord is low (King, '16). 



10. Behavior. Under natural conditions. When wild Nor- 

 ways are confined in a wooden cage they almost always gnaw 

 the boards of which it is made. This the domesticated Albinos 

 do not do. When put in a tank of water the Albino rat swims. 

 Often when unable to climb out over the side, it will dive and 

 search for a way out below. This must be a reaction, for many 

 generations unexercised. 



Greenman and Duhring ('23) have specially described the 

 clicking of the teeth — a sign of content — and other behavior of 

 the gentle rats— when these are taken in the hand. They have 

 also described the nest building of the pregnant female and the 

 behavior of the associated male when the pair are kept in the 

 same cage after the birth of the litter. There is no justification 

 for the common statement that the male kills the young. When 

 the young are killed it is the female which is usually responsible. 

 Such rats are designated "killer" females — Stotsenburg ('23) — - 

 and these individuals often have rather definite periods for 

 killing. The cause of this habit has not been determined, but 

 the ovaries of these animals are of normal size (Stotsenburg, 

 '23) so that the explanation must be sought elsewhere. 



The killing is usually accomplished by a bite severing the 

 vessels in the neck. In well tamed rats the killing of the suck- 

 ling young occurs only rarely. 



