4 CIRCULAR 4 2 3. U. S. DEPARTMENT OF AGRICULTURE 



pair. The number of litters under average conditions, however, 

 probably varies from three to six a year. The period of gestation is 

 considered to be 21 to 25 days. Accurate data on the white rat, 

 which is an albino form of the common house rat, show that the 

 period varies from 21 days and 15 hours to 22 days and 16 hours. The 

 nests, built of scraps of paper, rags, grasses, or any other soft mate- 

 rial, are placed in underground burrows or under floors, wood piles, 

 or any other structures or accumulations that afford shelter near an 

 available food supply. 



The rat's tremendous reproductive potential offers a great tempta- 

 tion to writers to dilate at length on the increase possible in the rat 

 population. Using the above averages, but ignoring the death rate, 

 the progeny from one pair of rats could exceed 350,000,000 in 3 years. 

 This figure is of value, however, only as it indicates the ability of the 

 rat to recuperate quickly from any unusual depletion in its numbers 

 and as it shows how persistent man's resistance must be to this 

 pest if it is to be kept under control. 



The common house rat, or brown rat, is naturally a burrowing ro- 

 dent. At times it may be found in open fields and particularly along 

 ditch banks and water fronts, but by far the greater number live in 

 burrows under or adjacent to buildings or within man-made struc- 

 tures. Although not physically adapted for extensive burrowing, the 

 rat is communistic in its mode of living, and cooperation in extending 

 underground runways sometimes results in an extensive maze of 

 tunnels interspersed with nest chambers, though the burrows rarely 

 extend downward more than 18 inches. As many as 281 rats have 

 been taken from a single system of burrows under one small chicken 

 house. 



The brown rat is an expert climber, although in this respect it is 

 not equal to the black rat, which lives largely in the upper parts of 

 buildings and in trees. Although frequently found in the upper 

 stories of buildings, the brown rat goes there for the most part only 

 upon foraging expeditions and retires during the day to the lower 

 floors and basements or to its burrows under the floors. It is also 

 an excellent swimmer and does not hesitate to take to the water in 

 cases of necessity. It is extremely quick but is not able to run so 

 fast as some of our native rodents of the same size. Tests have 

 shown that it can jump vertically to a height of 25 inches. 



It is difficult to keep rats out of a building in which food supplies 

 are stored unless the building is constructed of masonry or metal, 

 as rats readily gnaw their way through wooden partitions. They 

 are practically omnivorous and will take almost any food that either 

 man or domestic animals eat, which explains the great ease with 

 which they adapt themselves to almost any environment. 



ABUNDANCE 



Although it is obviously impossible to count rat noses, rough 

 estimates of the number of rats in the United States are made 

 possible by the many available statistics and local observations on 

 their abundance. The number varies locally from year to year, 

 though there is now no known cycle of rat abundance. Under favor- 

 able conditions, however, rats increase amazingly, reaching plague- 

 like proportions in some sections, followed by a few years of relative 



