BBEEDING OF EATS. 15 



information on the life history of an animal so common as the brown 

 rat should be meager, nor that diverse opinions about its breeding 

 should be current. When wild rats are kept in confinement, they 

 rarely breed, and the conclusions as to their breeding have been 

 inferred from observations of domesticated white rats. 



BREEDING. 



Observations show that climate and food supply greatly affect the 

 rate of multiplication of rodents. The rat is no exception. It 

 increases most rapidly in a moderately warm climate and with an 

 abundant supply of food. Extremes of heat and cold retard its mul- 

 tiplication, decreasing both the number of litters produced in a year 

 and the number of young brought forth at a time. 



As already stated, the brown rat is more prolific than either of the 

 two other species mentioned. The female brown rat generally has 12 

 mammae — 3 pairs of pectoral and 3 pairs of inguinal, although these 

 numbers are by.no means constant. The black rat and the roof rat 

 have but 10 mamma? — 2 pairs of pectoral and 3 pairs of inguinal, 

 with but little tendency to vary. Records of actual observations 

 confirm the deductions to be drawn from the above facts. At Bombay, 

 India, during the recent investigations of the India Plague Commis- 

 sion, 12,000 rats were trapped and examined. The .average number 

 of embryos observed in pregnant brown rats was 8.1; the highest 

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

 number,, 9. a 



In temperate latitudes the average litter of the brown rat is prob- 

 ably considerably greater than the number above given. Instances 

 of very large litters observed in England have been recorded in the 

 Field (London). In two cases 22 and 23 young, respectively, were 

 found in one nest, and in two other instances 17 and 19 were found 

 in gravid females. A reliable observer residing in Washington, D. C, 

 found 19 young rats in a single nest. The writer, on March 25, 1908, 

 caught 6 rats in one trap, 3 of which were pregnant females, con- 

 taining 10, 11, and 13 embryos, respectively. While we have hardly 

 enough data on which to base definite conclusions for this latitude, 

 we may safely conclude that the average litter is not less than 10. 



The number of litters of young produced in a year by the brown 

 rat is not definitely known, and probably varies with local conditions. 

 Dehne's observations on white rats showed that in one instance an 

 interval of only 71 days intervened between two litters from the 

 same rat. A young female of the first litter gave birth to 6 young 

 when she was only 103 days old. 6 Frank T. Buckland in his Curi- 

 osities of Natural History, relates that a white rat which he kept in 



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

 b Brehm's Thierleben: Saugethiere, Vol. II, p. 353, 1877. 



