]84 Order RODENTIA 



of the brown variety, R. r. alexandrinus (E. Geoffroy-Saint- 

 Hilaire), inhabited some warehouses at St. Louis, Missouri, in 

 1945 but it was soon exterminated. 



RATTUS NORVEGICUS (Berkenhout) 

 Norway Rat Barn Rat House Rat Sewer Rat 



Description. — The Norway rat, fig. 100, generally is grayish 

 brown on the upper parts and gray on the under parts. The 

 fur is coarse. The gray or brown tail is slightly but noticeably 

 lighter below than above, is scaly, nearly devoid of hair, and 

 slightly shorter than the head and body. 



Length measurements: head and body 7i4-9^ inches (187- 

 250 mm.); tail SYz-IYa inches (138-185 mm.); over-all 13-17 

 inches (325-435 mm.); hind foot IYz'Wa inches (38-44 mm.). 

 Weight: average about ^ pound, but there is much individual 

 variation. 



The skull is 41.5-45.0 mm. (nearly 1^ inches) long; the ros- 

 trum is long; ridges may be present on the lateral margins of 

 the braincase; the cusps on the cheek teeth of the upper jaw 

 are in three longitudinal rows, fig. 74c. Dental formula: I 1/1, 

 C 0/0, Pm 0/0, M 3/3. 



Life History. — The Norway rat, an Old World species acci- 

 dentally introduced into the New World, is at home not only in 

 dwellings, warehouses, stores, and sewers of American cities 

 but around garbage dumps, along ditchbanks, under straw 

 piles, and in cornfields, feed lots, and barns. Usually it chooses 

 a protective shelter from which it can burrow into the ground, 

 fig. 8. It is a sociable animal, and usually several of its kind 

 live together. 



This rat reproduces at a rapid rate. Litters are made up of 



2 to 15 young and average about 7. The young are weaned at 



3 weeks and are capable of breeding when about 3 months old. 

 The gestation period is 21 to 23 days. Litters may be produced 

 almost continuously throughout the year if adequate food is 

 available. Theoretically, one pair of Norway rats and their 

 progeny can produce more than 3,000 rats in a single year. 



In some years, Norway rats become excessively abundant. In 

 1903, hordes of rats were found migrating over several counties 

 in western Illinois. In 1939, rats were again very abundant in 

 this state. 



