Rattus tanezumi, Temminck, 1844

Dinets, Vladimir & Asada, Keishu, 2021, Noble savages: human-independent Rattus rats in Japan, Journal of Natural History 54 (37 - 38), pp. 2391-2414 : 2404-2407

Rattus tanezumi

Rattus tanezumi is apparently common in evergreen subtropical forests ( Hämet-Ahti et al. 1974) that grow in the southernmost parts of the main islands (southern Shikoku and southernmost Kyushu). In central Kyushu and in the southwestern half of Honshu it was found only in exceptionally well-preserved patches of very old deciduous forests, while in the northeastern half of Honshu it wasn’t detected even in the most pristine forest habitats still existing. It is also common in evergreen subtropical forests on many small islands farther south, where it is not native. It is remarkably arboreal (even considering a certain bias towards exposed locations that is unavoidable when searching for animals using a thermal imager).

Localised distribution of Rt in natural habitats on the main islands of Japan likely results from the loss of high-quality habitat in the suboptimal northern part of the range, and small extent of warmer areas (characterised by subtropical evergreen forest) where it can live in lower-quality habitat. It might be speculated that following the widespread logging during the Edo Period ( Morris-Suzuki 1995) it was replaced in much of its former range by two species of large native mice ( Apodemus specious and A. argenteus ) which are capable of living at high densities in all forest types and are by far the most abundant forest rodents on the main islands of Japan ( Ohdachi et al. 2015; also VD pers. obs.). Alternatively, the present range of Rt in natural habitats might be confined to areas where non-coniferous forests survived during the last glacial maximum ( Tsukada 1985). It is also possible that all Rt present in Japan today originate from an ancient introduction rather than from the Pleistocene rats, which may have been driven to extinction by ice ages and/or the massive Akahoya Eruption, which covered much of southern Japan with volcanic ash in 4500 BC ( Maeno and Taniguchi 2005).

Small human-independent populations of Rt on the main islands of Japan might be under threat from feral cats and introduced Siberian weasels ( Mustela sibirica ), northern raccoons ( Procyon lotor ) and masked palm civets ( Paguma larvata ) ( Ohdachi et al. 2015). Hybridisation with introduced R. rattus has been recorded in some Japanese cities and small islands ( Chinen et al. 2005; Kambe et al. 2013) and might eventually threaten the genetic integrity of native populations. On some of the Ryukyu Islands, Rt is now the most common non-volant forest mammal, and an important food source for endangered Iriomote cat ( Prionailurus bengalensis iriomotensis ) ( Sakaguchi and Ono 1994). Its rarity in northern Okinawa might be a result of mongoose eradication programme that started in 2000 ( Yamada et al. 2015), as thousands of Rt per month were caught in mongoose traps and removed ( Kambe et al. 2012); however, a similar programme on Amami-oshima does not seem to have had such an effect.

The distribution of Rt in natural habitats in its presumed native range in mainland Asia is difficult to elucidate from the published literature. It is usually unclear which populations are human-independent because, as in Japan, it is often impossible to find forests with no human settlements nearby ( Lekagul and McNeely 1977). Another problem is the complicated and rapidly evolving taxonomy of R. rattus sensu lato; in many cases multiple taxa with different habitat preferences are mentioned but it is unclear what exactly they are. Some mainland Rt differ from Japanese animals phenotypically. In China they often have yellowish spots on the throat, and are considered to be two separate species – R. fulvipectus (Milne-Edwards, 1872) and R. yunnanensis (Anderson, 1879) – by many Chinese zoologists; these forms are treated as subspecies or synonyms in non-Chinese literature ( Ying et al. 2002; Musser and Carleton 2005), and have identical karyotypes (Mikhail Kosoy pers. comm.). Yellowish throat spots are common in some Indochinese populations of Rt as well ( Abramov et al. 2007). In mainland Asia and elsewhere Rt is mostly confined to human-modified habitats ( Zhan 1982; Zhang et al. 2000; Stuart et al. 2012). In Korea it occurs in forests on small Ulleung Island (VD pers. obs.), but not on larger Jeju Island or the mainland ( Jo et al. 2012, 2018). In China it is largely absent from forests as far south as Hong Kong ( Chung and Corlett 2006) and Taiwan ( Yu 1994). However, in southernmost China (Yunnan and Guanxi) and northern Indochina, where Rt is apparently native ( Guo et al. 2019), it is an uncommon but widespread component of forest fauna ( Zhang et al. 2000). At the northern edge of its native range in China it occurs at low densities in primary montane subtropical forests of central Yunnan, where the habitat is remarkably similar to the sites where we found it on Honshu ( Wu and Deng 1988; Wenbo Chen pers. comm.; VD pers. obs.). Rt also occurs at low densities in evergreen tropical forests of southern Yunnan, where animals in primary and secondary forest are phenotypically different ( Wu et al. 1996), and in deciduous tropical forests and karst forests in northern Thailand ( Chaisiri et al. 2010; Latinne et al. 2011). Note that we found it in karst forests in Okinawa ( Table 1). Rt is apparently absent from forests in mainland Vietnam ( Lunde and Truong Son 2001); no Rt were recorded during an extensive camera trapping survey of primary forests of Pu Mat National Park in north-central Vietnam (VD and Nicholas Wilkinson, in prep.). Rattus rattus sensu lato has been reported to be common in primary forests on islands off southern Vietnam ( Abramov et al. 2007), in karst and mangrove forests of southern Thailand ( Lekagul and McNeely 1977; Latinne et al. 2011), and in other natural (but mostly disturbed) habitats of southern Indochina ( Lekagul and McNeely 1977; Aplin et al. 2011); these populations are currently thought to be taxa different from R. tanezumi sensu stricto, based on mtDNA and sometimes phenotypical differences ( Van Peenen et al. 1970; Pagès et al. 2010; Aplin et al. 2011; Balakirev and Rozhnov 2012; Abramov et al. 2018), but nuclear DNA data contradict this ( Pagès et al. 2013). Some forms of R. rattus sensu lato, possibly Rt, are common in disturbed forests of Laos ( Aplin and Singleton 2003) and unlogged forests of Myanmar ( Thazin Wai et al. 2018). Unlike R. rattus sensu stricto, Rt is not known from natural habitats outside Asia; all records from other continents are from human settlements ( Aplin et al. 2011; Bastos et al. 2011; Lack et al. 2012).