336 LIFE HISTORY 



rats, similar to waltzing mice in behavior, but as viable as their 

 normal litter-mates and able to hear. 



Mus norvegicus when mature weighs 300-500 grams. Larger 

 animals sometimes appear and our records show one male (cage 

 bred and continually petted) which attained a maximum of 750 

 grams. The males in any series tend to be some 20 per cent 

 heavier than the litter mate females. 



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. 11, 

 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, sometimes fourteen nipples, while the house rat has 

 very constantly ten. 



As will be evident, the data which follow were largely obtained 

 from caged or partially domesticated animals. For the most 

 part however, they probably represent what would be found in 

 the strictly wild state. 



Span of life. The span of life of the Norway rat is not known. 

 It seems probable that it is between three and four years, though 

 here and there individuals may live somewhat longer. Norway 

 rats may breed at all seasons, but best in the spring. 



Period of gestation. 21 days Lantz ('09); 23.5-25.5 days 

 Miller ('11). The latter periods are possibly due to the effect 

 of nursing on gestation, see p. 21. 



Number of litters. Miller ('11) reports seven litters in seven 

 months from a single pair, and estimates that, in general, five 

 to six litters may be easily reared by a single pair in a year. 



Number of young in a litter. Climate and station appear 

 as general modifying influences. Larger litters are reported 

 from northern Europe than from India (Lantz, '09) . 



Crampe ('84) obtained an average of 10.4 in fourteen litters. 



Zuschlag ('03) states that among the rats examined at Copen- 

 hagen in 1899, fetuses to the number of 14 were found four times 



