<国立科博専報>日高の陸棲哺乳類 : とくに固有のヤチネズミ類とその起源について Land Mammals of the Hidaka Mountains, Hokkaido, Japan, with Special Reference to the Origin of an Endemic Species of the Genus Clethrionomys
According to the investigation executed by the party from the National Science Museum, Tokyo, in the summers of 1970 and 1971,the small mammal fauna of the Hidaka Mountains, Hokkaido, is fairly different from most of the plains but rather similar to that of Rishiri Island, a small island off the northern extremity of the main island, in the combination of important species. They resemble each other not only in the presence of a vole of the Clethrionomys rex group and a tiny shrew nearly related to Sorex gracillimus THOMAS, 1907,but also in the absence of Sorex shinto saevus THOMAS, 1907,one of the two common shrews in the plains of the main island of Hokkaido. Voles of the rex group, containing Clethrionomys rex IMAIZUMI, 1971,of Rishiri Island and C. montanus sp. nov. of the Hidaka Mountains, are evidently more primitive in several external, cranial and dental characters than those of the rufocanus group, which contains C. bedfordiae (THOMAS, 1905) of the main island of Hokkaido and C. sikotanensis (TOKUDA, 1935) of Rishiri I. and Shikotan I. of the Habomai Islands. Moreover, within the respective groups, the forms of small islands are less advanced than those of the main island as shown in Fig. 1. These nearly related forms belonging to the same clade with compete each other when they are in a sympatric condition of distribution, because they are probably poorly differentiated in the ecological niche. In such cases more advanced forms tend to destroy the primitive ones, as clearly seen in three lineages of Japanese moles, Mogera minor, M. wogura and M. kobeae (IMAIZUMI, 1964). A lineage, here used by the author, is a taxon applied not only to good species but also ill-defined hemispecies or even some of subspecies based on the biological species concept, and differs from other lineages by the relative time of emergence or of animated expansion of the distribution. Such competition may be more violent in a combination of nearest two lineages than that of rather remote ones, as the former is less different in physical characters and also in ecological niches than the latter. Evident correlations between the degree of specialization and the relative time of emergence of taxa have been confirmed among the lineages of the glareolus group, of the Japanese moles, etc. If this is acceptable as a general rule, the earliest of the four lineages of the voles that arrived in the Hokkaido district might be Clethrionomys rex, now confined to Rishiri Island (Fig.1). At first probably C. rex and then C. sikotanensis expanded their distribution nearly all over the district. Later, both of the populations were exterminated from the main island of Hokkaido through violent competitions with newly arrived and more advanced lineages, C. montanus of the former group and C. bedfordiae of the latter one. On the other hand, not so violent but rather loose competitions have been carried on between these conquerors, and montanus was gradually exterminated from the plains and finally almost confined to the alpine and subalpine zones of the Hidaka Mountains. Very similar process of emergences and competitions might be carried out between Sorex shinto saevus and a tiny shrew similar to Sorex gracillimus. The former, which has now nearly exterminated tha latter from the plains of the main island, probably failed to invade into Rishiri Island because of its later arrival. Thus, the similarities between the small mammal faunas of Rishiri Island and the high mountain zone of Hidaka Mountains can easily be understood as a result of successive appearances and expansions of slightly advanced lineages in the same clade and violent competitions between them which can be hindered by some topographical factors.
国立科学博物館専報 5, 131-149, 1972