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 : 2408

Rattus norvegicus

Rattus norvegicus appears to be a rare species of pristine upland forests (mostly alder and birch groves along streams), natural wetlands, and possibly seashores on Hokkaido, natural wetlands on other main islands, and seashores on Tsushima. Such habitat preferences correspond to its distribution in other parts of its native range (see below).

It is puzzling that Rn occurs in forests of Hokkaido but not in the mountains of Honshu where the climate is similar; possible explanations include the availability of invertebrate prey or the duration of snow cover, which is important for Rn in the forests of Russian Far East ( Kuzyakin 1951). It is also possible that Rn of Hokkaido are not related to those known from Pleistocene fossils from Honshu and Kyushu, but have colonised Hokkaido from mainland Asia via Sakhalin Island, like most other native mammals of Hokkaido ( Kuroda 1939). Unfortunately, the Pleistocene fossil record of small mammals from Hokkaido is very fragmentary ( Kawamura 1991). Similarly, Rn of Tsushima might be relatively recent natural colonisers from Korea, like some other mammals of the island ( Ohdachi et al. 2015).

Their dependence on wetlands and pristine forests make human-independent populations of Rn vulnerable to extinction in Japan, where virtually all primary forests were logged during the Edo Period ( Morris-Suzuki 1995), and very few large natural wetlands remain, particularly outside Hokkaido ( Washitani 2007). Daisetsuzan National Park (the largest protected natural area in Japan) and Kushiro-Shitsugen National Park (the largest remaining wetland in the country), where Rn was detected by previous surveys ( Ota 1968; Abe et al. 1971) as well as in our study, appear to be the main strongholds of human-independent Rn in Japan. Populations in Hokkaido wetlands might be under threat from introduced American mink ( Neovison vison ), while on other main islands they now have to compete with introduced coypu ( Myocastor coipus ) ( Ohdachi et al. 2015). Declines in human-independent populations of Rn have been noted in other parts of its native range: it is now virtually extinct in Transbaikalia where it was locally common in the twentieth century ( Karaseva et al. 1990), and on some of the southern Kuril Islands where it used to be abundant ( Grigor’ev 2008). Replacement of native subspecies caraco by invading ssp. norvegicus from the west has been noted in Siberia ( Kuzyakin 1951), and might be happening in Japan, where differences in mtDNA between brown rats in ports/large cities and the backcountry have been reported ( Ohno et al. 1994).

In its presumed native range outside Japan, Rn occurs in upland forests (mostly alder groves along streams), riparian forests, tallgrass floodplain meadows, and seashore habitats on southern Kuril Islands, where virtually all bodies of water once had Rn living along their shores ( Surkov 1986; Dinets and Rotshild 1998); in mixed forests (particularly with alder) and tallgrass meadows near streams on Sakhalin Island ( Surkov 1986); in floodplain and streamside forests (sometimes in pristine areas very far from human habitat) in Ussuriland ( Maak 1861; Karaseva et al. 1990; Anna Gritsuk pers. comm.; Anastasia Kadetova pers. comm.); along forest streams with large salmon runs in the lower Amur River basin (Khamaganov 1965); in riparian forests and sedge-reed marshes around oxbow lakes in Transbaikalia ( Radde 1862; Kuzyakin 1951; Karaseva et al. 1990); in reedbeds with forested islands in Manchuria (VD pers. obs.), and in coastal wetlands in Korea (VD pers. obs.). Note that the oldest Ussuriland record is from the mid-nineteenth century when there were no cities or agriculture there ( Maak 1861); there are also mid-nineteenth century records from Sakhalin, where human presence at the time was largely limited to small villages of native hunter-gatherers ( Yegerov 1946). In its non-native range in western Eurasia and North America Rn also readily colonises reedbeds, riverbanks and other wetlands, and sometimes deciduous forests and seashores ( Kuzyakin 1951; Karaseva et al. 1990; Amori and Cristaldi 1999; Dinets 2015; KryŠtufek et al. 2020).