LIFE PROCESSES IN CAPTIVE GRAY RATS 59 



waltzing character (w), long known in mice (Yerkes, '07) and 

 reported in other species of rats by Bonhote ('12) and by 

 Hagedoorn and Hagedoorn ('22), appeared in the fourth 

 generation of mutant albinos (King, '36). Another new muta- 

 tion, which has been designated 'stub' (st), was found in the 

 eighth generation of cinnamon rats. Stub is a simple men- 

 delian recessive that has a very deleterious effect when in a 

 homozygous condition. It reduces the number of tail verte- 

 brae, frequently produces malformations in the posterior re- 

 gion of the body, markedly affects the viability of the young 

 and greatly retards body growth in individuals that survive. 

 A full account of this mutation will be published elsewhere. 



There is little in the breeding history of gray rats to sup- 

 port an assumption that genes for these various mutations 

 were present in the foundation stock and remained latent until 

 chance matings brought them to light. The hooded gene, as 

 shown above, was carried by one pair of wild rats (female 2 : 

 male 2), so this mutation requires no further comment. All 

 other feral rats that bred in captivity produced only normal 

 gray young, as far as known. The Fj generation comprised 

 twenty-one litters containing 139 young, of which 106 were 

 raised to maturity. These litters, as well as those born in 

 later generations, were carefully examined at or soon after 

 birth, in order to detect any structural anomalies or any varia- 

 tions in coat color that might be disclosed through changes 

 in eye pigmentation, such as albinism or ruby-eyed dilute. 

 Since black rats cannot be distinguished from grays until the 

 pelage develops, this mutation might have escaped detection 

 in early generations. 



Although most of the offspring of wild rats were reared by 

 albino foster mothers, all of them were as savage and as 

 difficult to care for as were their parents. Tests made indi- 

 cated that individuals that had been reared apart would not 

 always live together amicably when adult. Therefore, to avoid 

 the loss of valuable animals, it was deemed advisable to keep 

 members of the same litter together, whenever it was possible 

 to do so. All FI individuals were inbred, brother and sister, 



