CHAPTER 11 

 LIFE HISTORY AND DISTINGUISHING CHARACTERS 



1. Introduction. 2. Life history, a. Span of life. b. Gestation period, 

 c. Number of litters, d. Number in litter, e. Proportion of sexes, f. Open- 

 ing of eyes. g. Age of sexual maturity. 3. Comparison of Norway with Albino. 

 4. Similarities of Norway s and Albinos in western Europe to those of the United 

 States. 



1. Introduction. To obtain more complete information con- 

 cerning the rat it is important to note differences which may ap- 

 pear between the domesticated Albino and the wild Norway. 

 Since the wild Norway represents the parent stock it might seem 

 proper to use that form as the standard and to record the devia- 

 tions of the Albino from it. As a matter of fact however our in- 

 formation with regard to the Albino is so much the more complete 

 that the best results will follow from using it as the standard, 

 despite the fact that zoologically it is but a variety of the Norway. 



2. Life history of the Norway rat. As regards behavior, the 

 Norway rat is very responsive to sounds, gnaws its cage, bur- 

 rows when opportunity offers, is hard to handle and appears 

 fierce because usually in a state of terror, yet after some days in 

 a cage, it often becomes quite docile. 



Mus norvegicus when mature weighs 300-500 grams. (550 

 grams = 1 \ pounds avd., has been reported but is very unusual). 

 We have recorded one male with a body weight of 523 gms. The 

 color above ranges from light gray or orange to brown and dark 

 gray, usually with more or less white or light gray on the ventral 

 surfaces. Melanic sports occasionally occur (see p. 14, note 5). 

 Mus norvegicus is distinguished from Mus rattus, the house rat, 

 by the following superficial characters: larger size; blunter head; 

 smaller ears which are thicker and more covered with hair; tail 

 shorter than body; claws usually relatively dull. Its movements 

 are less rapid. Commonly the female Norway has twelve, some- 

 times fourteen nipples, while the house rat has very constantly 

 ten. 



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