22 



HABITS OF RATS. 

 BREEDING HABITS. 



Both climate and food supply affect the rate of multiplication of 

 most rodents. The rat probably increases more rapidly in a tem- 

 perate and equable climate than in one of great variability. Ex- 

 tremes of heat and cold retard multiplication, decreasing both the 

 number of litters in a year and the number of young at a time. In 

 northern latitudes, apparently, more or less interruption of breeding 

 occurs in the winter months. 



Where the country is well settled the food supply of rats is not 

 likely to be deficient; and when the animals have access to stores 

 of grain, the young mature very quickly and probably reproduce 

 earlier than when grain is absent. 



The brown rat is more prolific than either the roof rat or the 

 black rat. The female brown rat has usually 12 mammae — 3 pairs 

 of pectoral and 3 pairs of inguinal — although these numbers are not 

 constant, one or more teats frequently being undeveloped. The 

 black rat and the roof rat have only 10 mammae — 2 pairs of pectoral 

 and 3 pairs of inguinal — with but little tendency to vary. Records 

 of actual observations on the number of young confirm the deduc- 

 tions that might be drawn from the above facts. At Bombay, 

 India, during the recent investigations made by the India Plague 

 Commission, 12,000 rats were trapped and examined. The average 

 number of embryos found in pregnant brown rats was 8.1; the 

 highest number, 14. The average for the black rat was 5.2; the 

 largest number, 9.° 



In temperate latitudes the average number of young produced by 

 the brown rat is undoubtedly greater. Instances of very large 

 litters observed in England are recorded in The Field (London). In 

 two instances 22 and 23 young, respectively, were found in a single 

 nest, though no evidence is offered that these were the progeny of a 

 single female; but in two other cases 17 and 19 embryos were found 

 in gravid females. A dealer in feedstuffs in Washington, D. C, 

 relates that he found 19 young rats in a single nest in his store. 

 Within the past few months the writer has examined four pregnant 

 brown rats taken in traps. The numbers of embryos they contained 

 were 10, 11, 11, and 13, respectively. While we have not enough 

 data for definite conclusions, we may safely state that the average 

 litter for this latitude is not less than 10. 



Frank T. Buckland, in Curiosities of Natural History, relates that a 

 white rat which he kept in captivity gave birth to 11 young when 

 only eight weeks old. As gestation in rats occupies three weeks, 

 this animal must have bred when only five weeks old. 



a Etiology and Epidemiology of Plague, p. 9, Calcutta, 1908. 



