Rattus norvegicus (Berkenhout, 1769)

Dinets, Vladimir & Asada, Keishu, 2021, Noble savages: human-independent Rattus rats in Japan, Journal of Natural History 54 (37 - 38), pp. 2391-2414 : 2393

Rattus norvegicus

Brown (or Norway) rat Rattus norvegicus (Berkenhout 1769) , hereafter Rn, first described from Europe, originates from Southeast Asia ( Zeng et al. 2018) or, more likely, the Far East ( Puckett and Munshi-South 2019); in any case it is native to northern China, Mongolia, Korea and Russian Far East, as evidenced by high genetic diversity in those areas ( Puckett et al. 2016; Puckett and Munshi-South 2019), and has been introduced worldwide ( Amori and Cristaldi 1999; Burgin 2017a). East Asian Rn are smaller, browner (with lighter pelage in winter), and shorter-tailed than those in the Western Palearctic, and have been described (from Transbaikalia) as R. caraco (Pallas, 1778) , now usually considered a subspecies; introduced Rn on other continents are either R. n. norvegicus or a mixture of the two forms ( Kuzyakin 1951); the latter also appears to be the case in Tokyo ( Ohno et al. 1994).

Rn is a native species in the main islands of Japan: there are Middle Pleistocene fossils (pre-dating the human arrival) from Honshu, as well as Late Pleistocene–Holocene fossils from Honshu and Kyushu ( Kawamura 1989; Iwasa 2015b). Elsewhere the earliest fossils are from the Late Pleistocene of Korea ( Yeong-Seok 2015) and the Late Pleistocene of central China ( Zheng 1993). Today Rn occurs on all four main islands of Japan and on Tsushima ( Iwasa 2015b). It is also present on almost all other islands of Japan (with the notable exception of Amami-oshima), but those populations are purely human-associated and presumably introduced ( Iwasa 2015b); the extensive fossil record from the central and southern Ryukyu Islands shows that Rn arrived there much later than humans ( Otsuka and Takahashi 2000). It largely replaced R. rattus in European cities during the Industrial Revolution ( Amori and Cristaldi 1999), and in North American cities in the eighteenth century ( Lack et al. 2013), but it is unknown whether a similar replacement of Rt by Rn has ever taken place in the cities of the main Japanese islands; in parts of China and certain other regions, the opposite might be happening ( Zhang et al. 2000).

Some field zoologists have always recognised Rn, Japan’s only somewhat aquatic native rodent, as an integral element of the country’s native fauna ( Abe et al. 1971), but the Japanese zoological literature often treats it as an invasive species confined to human settlements and agricultural habitats (see e.g. Iwasa 2015b), despite the fact that it has been recorded during surveys in remote mountain forests and large natural wetlands on Hokkaido ( Ota 1968; Abe et al. 1971; Maekawa et al. 2002).