122 



NORTH AMERICAN, FAUNA 66 



Young Norway rats superficially resemble the native American rice 

 rats {Oryzomys palasMs). They may always be distinguished from 

 this species, and from other cricetines, by the upper molar teeth, 

 which in the genus Rattus (and in the other introduced Murid genus 

 Mm) are provided with small rounded cusps (tubercles) arranged in 

 three longitudinal rows in contrast to two rows of longitudinal cusps 

 in most cricetines. 



Measurements. — An adult from the District of Columbia measures 

 as follows : Total length 470 ; tail 208 ; hind foot 43 ; ear 21 ; greatest 

 length of skull 52.6; zygomatic breadth 27.6; interorbital breadth 

 7.4 ; length of maxillary toothrow 7.3. 



Habitat and hahits. — This is essentially a water-loving and burrow- 

 ing animal. In the spring of 1963, large numbers of them were inhabit- 

 ing burrows in the banks bordering the Loch Raven Eeservoir north 

 of Baltimore. They would emerge from the burrows in broad daylight, 

 dive into the reservoir, and swim considerable distances to obtain 

 scraps of bread thrown into the water by visitors who were feeding 

 the numerous carp which swim in the area near the dam. In Wash- 

 ington, D.C., the population of Norway rats has recently risen to 

 alarming proportions. Their burrows may be seen around many of the 

 downtown government buildings and monuments, and at dusk they 

 come into the open and actively forage for food among the refuse 

 and rubble left by tourists during the day. 



The city of Baltimore has always had a Norway rat problem, but 

 these animals are not as numerous as was thought at one time. In a 

 careful study of Baltimore's rat population in 1949, Davis and Fales 

 (1950, p. 146) estimated there were approximately 43,000 animals, 

 with a range of from 26,000 to 68,000, of which about 15,000 were in 

 commercial areas. They had estimated that the population in 1947 

 was 165,000, so that there was a considerable decline in the period 

 1947 to 1949. With improved sanitation and methods of extermina- 

 tion developed during the intervening years it may be assumed that 

 the rat population of the city at present is no higher, and probably 

 lower, than in 1949. Nevertheless, rats are still a serious economic and 

 public health problem in Washington and Baltimore. 



The Norway rat is known to occur throughout the State both in 

 commercial buildings and habitations, and in some places in the wild, 

 particularly in the summer. 



The species is extremely adaptable, and about the only factor essen- 

 tial for its success is the presence of water ; it drinks freely, and is a 

 good swimmer and diver. It will eat virtually anything, and finds 

 sewers particularly attractive places to live because of the abundant 

 water supply and the offal usually found therein upon which it can 



