202 U.S. NATIONAL MUSEUM BULLETIN 2 75 



Comparisons. Rattus rattus can be distinguished from Rattus 

 norvegicus by the following characters: Longer and more uniformly 

 colored tail; one pair of postaxillary, pectoral mammae, rather than 

 two pairs; smaller front feet; lyre-shaped temporal ridges, as opposed 

 to parallel temporal ridges; first molar lacking the small accessory 

 cusp, the latter being present in R. norvegicus. 



Remarks. Earlier workers in the systematics of rodents have 

 applied subspecific names to the three common color types of the 

 commensal or black rat (Rattus rattus). These color types are charac- 

 terized as: black or gray above and below; brown above and dark 

 gray below; brown above and white below; and correspond, respec- 

 tively, to the subspecies R. r. rattus (Linnaeus), R. r. alexandrinus 

 (GeofFroy), and R. r. frugivorus (Rafinesque). In the past, the com- 

 mensal rats of the larger coastal towns of Libya have been variously 

 assigned to one or more of these subspecies. More recently, ecological, 

 behavioral, and genetic studies indicate that these so-called subspecies 

 do not conform to the modern concept of subspecies and accordingly 

 should be regarded only as color phases or genetic variants within 

 the species. 



Caslick (1955), in an attempt to resolve some of the relationships of 

 these color types in Rattus rattus, conducted a series of selective 

 matings of wild-trapped black rats of mixed origin, and although, in 

 color, the Utter mates satisfied the requirements of subspecies and 

 "unclassifiable intergrades" were of rare occurrence, he concluded that 

 the current forms of R. rattus were best regarded as color phases within 

 the species. 



Setzer (1952) also recognized these same three color types in 

 commensal rats from the Nile Delta and noted that specimens from a 

 given locality segregated out as the three expected color types plus 

 intermediate forms. These intermediate forms, to which he attributed 

 a single gene character for melanism, were suggestive of intergrades, 

 but the presence of three distinct subspecies occupying the same en- 

 vironmental niche (or geographic area) was clearly untenable. Other 

 workers have frequently reported black rats of different color phases 

 sharing the same habitat, and the individuals in a given population 

 that did not conform to one of the so-called subspecies were considered 

 intergrades. 



Johnson (1946) commented on a Utter from the same nest that con- 

 tained representatives of both the white-belUed and black-beUied color 

 phases and stated that these color phases had no morphological 

 distinctions, except those of color, and that the ecological differences 

 mentioned by other workers were not evident in the population that 

 he studied. 



CasUck (1955) suggests that the color phases of the commensal rat, 

 recognized as the subspecies R. r. rattus, R. r. alexandrinus, and R. r. 



