Red-backed Voles of Japan — JAMESON 
603 
GEOGRAPHIC ORIGIN OF RED-BACKED 
VOLES IN JAPAN 
In the Pleistocene and earlier the present ar- 
chipelago of Japan was connected to the main- 
land in the south, between Kyushu and the 
Korean Peninsula, and in the north, between 
Hokkaido, Sakhalin, and adjacent Siberia. At 
approximately the same time, changes resulted 
in the separation of Kyushu and Korea, and 
also the formation of the Tsugaru Strait be- 
tween Hokkaido and Honshu. Consequently, the 
islands of Kyushu, Shikoku, and Honshu no 
longer received immigrants from the continent, 
and they were isolated from the effects of faunal 
movements on the mainland. Today, the fauna 
of the old islands of Kyushu, Shikoku, and Hon- 
shu is, in many respects, quite different from 
that of the immediately adjacent mainland, and 
has its closest affinities to the southwest in 
China. This faunal difference has been men- 
tioned as early as Thomas’ 1905 paper. On the 
other hand, Hokkaido and Sakhalin remained 
part of the mainland and were subjected to 
the faunal changes that affected southeastern 
Siberia and Korea. There are no land mammals 
in Hokkaido that are more than subspecifically 
distinct from those of nearby Siberia and Korea. 
The important and extensive movements of 
mammals in this general area during the Pleis- 
tocene (see Simpson, 1947: 643) changed the 
fauna of Hokkaido, replacing with more recent 
forms the original fauna that now remains in 
the older Japanese islands. 
Eothenomys smithi probably entered Japan on 
the old route between Korea and Kyushu via a 
connection which remains today as the islands 
of Tsushima and Iki. E. smithi moved on from 
Kyushu to Shikoku and is now the only micro- 
tine rodent known from the latter island. This 
vole moved also to Honshu and now occupies 
a large part of that island, at least to southern 
Nagano-ken. E. smithi, or its progenitor, at one 
time must have occurred in Korea although the 
genus is not there now. E. kageus may have 
come from the north via Sakhalin and Hok- 
kaido, although its occurrence is now confined 
to the northern half of Honshu south to where 
it abuts the geographic range of E. smithi. The 
changes which resulted in the extinction or the 
emigration of Eothenomys from Korea and Si- 
beria apparently affected Hokkaido as well. Ac- 
tually, it seems likely that E. smithi and E. 
kageus are offshoots from a single ancestor 
which differentiated slightly along the coast 
from Korea to Siberia so that different subspe- 
cies entered Kyushu and Hokkaido. This could 
account for the great similarity between these 
two forms that now occupy adjacent but not 
overlapping geographic areas. 
Similarly, the species of Clethrionomys came 
to Japan by two routes. C. imaizumii, which is 
now known only from the low elevations of 
Wakayama-ken, may have its relatives among 
one of the many kinds of red-backed voles 
described from China. This is conjecture, but 
this vole is now associated with a semitropical 
flora of broad-leaved hardwoods. This species, 
moreover, is morphologically quite distinct from 
C. under soni, C. niigatae, and C. rufocanus bed- 
fordiae, which are found in forests of fir ( Abies 
spp.) and spruce ( Picea spp. ) . C. niigatae and 
C. andersoni are almost certainly arrivals via 
Hokkaido. Although niigatae and andersoni are 
close relatives of C. rufocanus bedfordiae, they 
may have- been derived from an earlier stock. 
Hokkaido and Honshu were separated since be- 
fore the Pleistocene, and niigatae and andersoni 
were immune to the immigrants of C. rufocanus 
bedfordiae. Not only are the three species close 
morphologically, but they share three species of 
fleas which are parasites of C. rufocanus in Hok- 
kaido: Catallagia striata, Megabothris sokolovi, 
and Rhadinopsylla alphabetica. One ( Catallagia 
striata ) occurs also in Siberia and another 
(. Megabothris sokolovi) in the Kuriles. 
The occurrence of C. rutilus needs little ex- 
planation. It is obviously a rather recent ar- 
rival in Hokkaido. If it ever extended to Hon- 
shu, it did not persist, and it may well have 
entered Japan after the separation of Hokkaido 
from Honshu. 
SUMMARY 
There are seven species of red-backed voles in 
Japan. Clethrionomys ( Clethrionomys ) rutilus 
occurs in Hokkaido. The species andersoni, nii- 
gatae, and imaizumii (from Honshu) and rufo- 
canus (from Hokkaido) are considered to rep- 
resent Aschizomys, which is placed as a sub- 
genus of Clethrionomys. Eothenomys smithi in- 
