23 



The number of times rats breed in a year is not definitely known, 

 and probably varies considerably with local conditions. Kolazy 

 makes the almost incredible statement that two female white rats, 

 kept by him in confinement and well fed, within thirteen months 

 gave birth to 26 litters of young, numbering 180 in all. One of them 

 produced young regularly at intervals of 25 days. a 



The writer recently kept two young female brown rats with a male 

 in a large open cage for several months. One of the females gave 

 birth to young on April 15; the other on April 17. The number in 

 these litters was not observed, as some were devoured soon after 

 birth, and all within three days, presumably by the male rat. On 

 May 23 both females gave birth to young, 24 in number, all in one 

 nest. 



The known facts concerning the breeding of the brown rat may be 

 briefly summarized as follows: The animals breed from three to five 

 times a year, each time bringing forth from 6 to 19 young. After a 

 gestation period of twenty-one days, the females give birth to their 

 young in nests built in underground burrows or under floors, stacks, 

 lumber, woodpiles, or other shelter. The young are blind and 

 naked when born, but grow rapidly, and young females are capable 

 of breeding when less than three months old. 



Early spring and summer are the periods of greatest reproductive 

 activity among rats. Young, however, are to be found every month 

 of the year. 



The above statements apply in the main also to the black and the 

 roof rat, but the number of young in a litter is somewhat smaller. 

 The newly born young of the black rat have not the bright pink 

 color of those of the brown and the roof rat, but are bluish, especially 

 on the upper parts. Black-and-white spotted rats are at first bluish- 

 and-red spotted, the red areas representing the white of the adults. 



ABUNDANCE OF RATS. 



From the foregoing account of the breeding habits of rats, the 

 great difficulty of ridding cities or large areas of the animals may be 

 readily understood. Ordinarily, they breed more rapidly than they 

 are destroyed. Although few are seen in daytime, at night they 

 fairly sw^arm along river fronts and wharves, as well as in sewers, 

 stables, warehouses, markets, and other places where food is abun- 

 dant. Their real numbers may sometimes be discovered when any 

 such harbor is demolished. 



An ordinary farm sometimes supports an astounding number of 

 rats. In 1901, an estate of 2,000 acres near Chichester, England, was 

 badly infested with the pests. They were systematically destroyed 



Verh. Zool. Bot. Gesel. Wien., pp. 731-734, 1871. 



