LIFE PROCESSES IN CAPTIVE GRAY RATS 19 



two groups of about 3 per cent. At 300 days this difference 

 has more than doubled. The weight excess of the larger fe- 

 males was 11.9 per cent at 400 days and had become 25.6 per 

 cent at the last weighing. 



Autopsies made on a number of these large and small indi- 

 viduals disclosed no conditions that would account for the 

 difference in their size. In the large individuals the body had 

 increased as a whole and there was no evidence of differential 

 growth in any of the organs. 



As the number of these exceptional individuals increased 

 as the generations advanced, it seems probable that the condi- 

 tion is inherited, and that these rats contained some genetic 

 factor, or factors, that acted mainly during adult life to sup- 

 plement the action of growth activating agencies. Genetic 

 factors are known to play an important role in inciting and 

 regulating growth processes, and their action is probably 

 physiological, since there is no evidence, as yet, of the exist- 

 ence of specific size genes in mammals. Green ( '31, '33) found 

 that genes producing brown and dilute color in mice also 

 increase the body size of the individuals. In confirming 

 Green's finding, Castle, Gates and Reed ('36) have given evi- 

 dence indicating that these genes influence body size "by 

 virtue of their own physiological action. ' ' 



Some of these large individuals became exceedingly savage 

 and often severely injured or killed other inmates of the cage. 

 Males with this 'killer' trait did not molest females of any 

 age, but attacked adult males smaller than themselves and 

 young males approaching maturity. Vicious females usually 

 harmed only w r eaker individuals of their own sex. The 'killer' 

 trait in these rats was doubtless a heritage from their wild 

 ancestry, associated with the polygamous condition common 

 to many animals living in their natural habitat. 



Individuals much larger than their litter mates of the same 

 sex have appeared occasionally in all strains of rats that have 

 been under investigation in the colony during the past 20 

 years. 'Killers' have been found, however, only in captive 

 grays and in mutant strains, particularly in mutant albinos. 



