Rattus rattus species group. The authority is usually cited as 1845, but was published in 1844 (Holthuis and Sakai, 1970). The name tanezumi is the oldest for the 2n = 42 group of Asian houserats that is distinguished from the 2n = 38/40 R. rattus not only by chromosomal characters but also morphological and biochemical traits (see account of R. rattus). The probable indigenous range is generally north and east of peninsular India (judged by Musser’s study of material in AMNH, BMNH, FMNH, MZB, RMNH, USNM, and geographic distribution of karyotypes with 2n = 42; Duncan and Van Peenen, 1971; Gadi and Sharma, 1983; Markvong et al., 1973; Niethammer, 1975; Niethammer and Martens, 1975; Ray-Chaudhuri and Pathak, 1970; Raman and Sharma, 1974, 1977; Gill and Gupta, 1989; Tripathy et al., 1985; Yong, 1969; Yosida et al., 1971). In the southern portion of the Western Ghats (along SW peninsula in Karnataka and Maharashtra States) at Sagar, three individuals trapped in forest exhibited 2n = 42, and three from a nearby "domestic site" had 2n = 38. The former diploid number was attributed to R. r. wroughtoni and the latter to R. r. rufescens (Lakhotia et al., 1973; Pradhan et al., 1991), but holotypes and paratypes of wroughtoni represent R. rattus (which includes rufescens as a synonym), and the 2n = 42 sample is R. satarae (see that account). Dark-bellied populations identified as R. rattus rufescens are found occurring together with white-bellied rats in India, which prompted Tiwari et al. (1972) to recognize rufescens as a separate species, but it is simply a phenotypic morph of R. rattus found in the same populations containing the white-bellied morph (see account of R. rattus). Distributional and ecological relationships between R. rattus and R. tanezumi along the N portion of the Indian subcontinent where indigenous ranges are either parapatric or overlap still requires resolution (see discussion in Corbet and Hill, 1992).

More than one species may be present in our concept of R. tanezumi, but careful taxonomic revision using morphological, chromosomal and molecular characters will have to test such an hypothesis. Verneau et al. (1997), for example, split Vietnamese R. flavipectus from R. tanezumi because of a difference in Fundamental Numbers of karyotypes. Wang (2003) recognized yunnanensis as a separate species, but no data substantiates that treatment (see G. M. Allen, 1940, and Osgood, 1932). He also listed brunneusculus as a species with sladeni, molliculus, canna, tistae, and hainanicus as subspecies. We associate the first four taxa with R. tanezumi and hainanicus with R. andamanensis (see that account). An extensive taxonomic survey of these populations, based on a combination of DNA sequencing and morphological studies, is currently underway by K. Aplin, H. Suzuki and their various collaborators (Aplin et al., 2003b; K. Aplin, in litt., 2004). Their preliminary analysis, using the mitochondrial cytochrome b gene, suggests that the 2n = 42 Asian group includes two distinct taxa, with a level of divergence equal to or greater that characterizes the 2n = 38 cluster. Aplin et al. (2003b:489) summarized the fundamental division as between ". . . an endemic Southeast Asian taxon (recorded from Vietnam, Cambodia and southern Laos) and a northern and South Asian taxon (recorded thus far from Japan, Hong Kong, northern Vietnam, northern Laos and Bangladesh). These taxa are probably regionally sympatric in parts of Vietnam and Laos." Their work also suggests that island populations off the Sunda Shelf contain a complex admixture of both major haplotype groups, suggestive of prehistoric introductions from multiple areas with subsequent hybridization (K. Aplin, in litt., 2004). Aplin et all. (2003c:489) suggested that ". . . it would be premature to speculate on the appropriate names for each of the two Asian taxa," and caution that ". . .the apparent extension of the north Asian taxon (which probably includes Japanese tanezumi) through to the Indian subcontinent raises the possibility that tanezumi may not be the earliest available name for this group."

Females in most populations of R. tanezumi have five pairs of mammae (one pectoral, one postaxillary, one abdominal, and two inguinal), but in some populations the postaxillary pair is twinned with the teats close together (less than 1 cm apart, usually 5 mm). This configuration is different from the six pairs of teats characterizing certain species of Rattus in the R. rattus species group (e. g., R. andamanensis) in which there are two distinct pairs of postaxillary teats, usually 1 cm or more apart. We have not seen such variation in R. rattus.

Phallic morphology of Chinese flavipectus described by Yang and Fang (1988) in context of assessing phylogenetic relationships among Chinese murines. Standard karyotype of Philippine samples described by Rickart and Musser (1993). Taxonomic, chromosomal, and ecological aspects of Chinese populations (under flavipectus) described by Zhang et al. (2000). Md Nor (1996) documented its distribution and ecology on the small islands off the northern tip of Sabah. The species does not occur on Christmas Isl or Madagascar (R. rattus was introduced on those islands), or in the Andaman and Nicobar Isls except for Greater Nicobar Isl (see above); R. rattus from Barren Isl is the only recorded introduction of a commensal Rattus in the Andaman Isls (see account of R. andamanensis).