Treated as a distinct species (Abe, 1973a, b, 1984) until Aimi (1980) included rex in M. rufocanus, an allocation followed by Corbet (1978c) and Musser and Carleton (1993). Kaneko and Sato (1993), however, presented morphological traits that distinguish the two as species on Rishiri Isl (also Kaneko et al., 1998) and demonstrated their sympatry and habitat affinities. Subsequent evaluation of ribosomal and mitochondrial DNA sequences clearly sustains the specific integrity of M. rex (Suzuki et al., 1999b; Wakana et al., 1996). Imaizumi’s (1971) description of M. rex, differential comparisons with M. rufocanus, and records of habitat segregation (M. rufocanus in open grassy fields, M. rex in coniferous forest) are lucid and thorough.
Imaizumi (1972) described montanus (Hidaka Mtns, Hokkaido) as the other species in his M. rex group. A wealth of morphological (Abe, 1973a, b, 1984; Aimi, 1980), chromosomal (Kashiwabara and Onoyama, 1988; Tsuchiya, 1981), allozymic (Yoshida et al., 1989), and DNA sequence data (Suzuki et al., 1999b; Wakana et al., 1996) represents montanus as another population of M. rex on Hokkaido. Reports of M. rex on Shikotan and Shibotsu Isls in the S Kurils and on Sakhalin Isl (Kostenko and Allenova, 1978) require verification (Kaneko et al., 1998:25).
Imaizumi (1971, 1972) regarded M. rex to be closely related to M. rufocanus but more primitive in external, cranial, and dental traits. His view is broadly consistent with phylogenetic interpretations of mitochondrial and ribosomal DNA sequences and biogeographic reconstructions of species diversification (Wakana et al., 1996). At present, M. rex, M. rufocanus and M. rutilus occur on Hokkaido (Kaneko, 1994; Kaneko et al., 1998; Wakana et al., 1996).