190 LIFE HISTORY 



a.) 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. 



6.) 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. 22. 



c.) 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. 



d.) 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 and he himself in 1902 examined one female bearing 16. 

 Donaldson (MS, '09) also noted in a rat taken in Paris, 16 

 fetuses. 



The India Plague Commission reports ('08) that the average 

 number of fetuses found in females was 8.1 from a total of 12,000 

 Norway rats. 



According to Lantz ('09) the maximum size of litters recorded 

 in England (Field) are 17, 19, 22 and 23; in India however 14. 



The maximum numbers just given as recorded in England are 

 not trustworthy as they represent merely the number of young 

 found in a single nest. Since two different litters are sometimes 

 reared in the same nest the inference from the number in the nest 

 to the number in the litter is not convincing. Lantz ('09) as- 

 sumes the average litter (in north temperate latitude) to be about 

 10. This is what Miller ('11) (vide infra) and Crampe ('84) 

 (vide supra) found. 



Miller ('11) observed in a group of eight litters 7-12 young in a 

 litter, with an average of 10.5. 



e.) Proportion of the sexes. Lantz ('09) and others state that 

 the males are in excess. Donaldson ('12) found the same in 

 trapped series taken in Paris and London. In a small series 



