120 



NORTH AMERICAN FAUNA 66 



brownish dorsum, similar to that of frugivorous^ but with a gray 

 venter, and is most abundant in the Middle East and North Africa. 

 The third color phase, known as R. r. rattus^ has a black dorsum, with 

 a dark gray venter, as in alexandrinus^ and is mostly associated with 

 the cold temperate countries of northern Europe. In general, R. r. 

 frugivorous is a wild-living animal, whereas alexandrirms and rattus 

 are nearly always associated with man and his habitations. None of 

 these varieties, however, is exclusively limited to any of the geographic 

 or habitational areas mentioned above, and all three forms may occur 

 at any one locality or in any one habitat. Because of this it seems 

 advisable at present to consider them as color phases rather than as 

 distinct subspecies (Caslick, 1956, pp. 255-257) . All of the color phases 

 may be encountered in the United States. 



Rattus rattus is most easily confused with the Norway rat {Rattus 

 norvegicus) . It may be distinguished from that species by its smaller 

 size, more slender build, more elongated nose, larger longer ears, and 

 much longer and more slender tail (as long as, or longer than, the 

 combined length of the head and body). Cranially, the two species 

 differ in that the braincase of R. rattus is shortened and rounded, 

 whereas that of R. norvegicus is narrow and elongated, the well- devel- 

 oped temporal ridges extending parallel to each other for a consid- 

 erable distance on each side of the cranium. 



Measurements. — Two adults from Washington, D.C. (taken on a 

 river boat at the Seventh Street Wharf on 23 April 1923) measure 

 as follows : Total length 405, 423 ; tail 218, 238 ; hind foot 37, 39 ; great- 

 est length of skull 43.6, 43.2 ; zygomatic breadth 20.8, 20.9 ; interorbital 

 constriction 6.4, 6.7 ; length of maxillary toothrow 7.0, 6.9. 



Habitat and habits. — This rat is essentially an arboreal animal and 

 seldom inhabits burrows. Where it infests buildings and houses, it is 

 found usually in the walls, ceilings, or roof, but seldom in basements or 

 in sewers. It shuns water and seldom enters it voluntarily. This is, how- 

 ever, the common rat on ships, to which it gains access by climbing 

 the moorings. It is occasionally introduced with shipments of grain or 

 fruit. 



In diet, the black rat is omnivorous, consuming a wide variety of 

 grains, fruits, vegetables, and animal matter. 



The species is polyestrous all the year round. The duration of ges- 

 tation is about 21 days, and the average litter size is seven to nine. 



Black rats enter into close relations with man wherever they occur, 

 and for this reason they are often involved in the transmission of dis- 

 eases, principally the bubonic plague. 



Specimens examined. — District of Columhia: Five (three taken 

 on river boat at Seventh Street Wharf, and two taken in the Central 

 Market in a box of fruit from Egypt in February 1912) . 



