THE HOUSE RAT 3 



inland, driving out the smaller black rat as it progressed, until now 

 it infests every State in the Union. It followed closely on the heels 

 of the early settlers, except in the high and dry mountain sections 

 of the West, where its progress has been relatively slow. Though 

 entrenched in some of the larger towns in Colorado and New Mexico 

 by 1890, it did not reach Wyoming until about 1919, and Montana 

 until 1923. 



As the house rat is largely dependent upon both the food and 

 shelter provided by man, its spread into new territory has been 

 nearly coincident with that of people, except at higher altitudes and 

 in extremely dry areas. The brown rat thrives best in the Temper- 

 ate Zone, where it has almost completely replaced the black rat, but 

 in the Southern States, particularly in Florida and in the southern 

 parts of the States bordering on the Gulf of Mexico, the black rat 

 has held its own and predominates in some places. 



DESCRIPTION 



House rats* are too common and well known in the United States 

 to need detailed description. The brown rat may be distinguished 

 from the black rat by its relatively larger size, more robust build, 

 shorter, thicker ears, and shorter tail, which, when bent forward, 

 does not reach the tip of its nose, whereas that of the black rat ex- 

 tends considerably beyond. The rather soft fur of the brown rat 

 is usually a grayish brown, fading to a dirty silver gray or pale 

 yellowish white on the belly. Individuals may vary in color from 

 an almost pure gray to a blackish or reddish brown, and partial 

 albinos are not rare. 



The black rat in the United States may be either a pure or a 

 mixed descendant of two geographic races, or subspecies. One of 

 these (Rattas rattus rattus), described from specimens taken in 

 northern Europe, is distinguished only by color from the other 

 (R. r. alexandrinus) , which developed a lighter coloration through 

 long residence in the warm climate of northern Africa. The former 

 is a uniform dusky black with a white or cream-colored belly, 

 whereas the latter is a gray with only slight tinges of brown and a 

 pale-colored belly. 



The average adult weight of the common house rat is about three- 

 quarters of a pound. Individuals weighing 1 pound may be con- 

 sidered unusually large. Occasionally a much larger rat is reported. 

 The heaviest specimen of which the Bureau of Biological Survey has 

 record weighed 25 ounces. The average length of adult brown rats 

 is 16 to 18 inches, including the tail, which is 7 to 7% inches long. 



BREEDING AND OTHER HABITS 



The number of young in the litter of the brown rat varies all the 

 way from 6 to 22, the average being 9 or, in the North Temperate 

 Zone, probably 10. The number of litters produced in a year is 

 reported to vary from 3 to 12. The young are blind and naked 

 at birth, but grow rapidly and breed when only 3 to 4 months old. 

 The life span of a rat is probably between 3 and 5 years. Abundant 

 evidence demonstrates that the house rat breeds every month in the 

 year, and there is one record of 7 litters in 7 months from a single 



