Systematics and Zoogeography of Sorex in the Bering Strait Area

Abstract

Shrew populations on the islands in the Bering Strait region (Sorex pribilofensis, S. jacksoni, S. hydrodromus) were regarded as related to Sorex arcticus of the Alaskan mainland. Our data indicate that S. pribilofensis and S. jacksoni are members of the S. (Otiosorex) cinereus group, which occurs both in Alaska and eastern Siberia. Sorex hydrodromus apparently does not exist on Unalaska Island, and is a synonym of S. pribilofensis. The Holarctic evolution of the S. cinereus and S. arcticus-S. araneus groups, and their occurrence on the Bering land connection in the late Pleistocene, is discussed. Changes in sea-level in the Bering Strait area during the Cenozoic, and particularly the Pleistocene, from time to time produced a broad land connection between northeastern Siberia and Alaska (Fig. 1), whose importance to mammal distribution has been appreciated for some time (Gilmore, 1943, 1946; Simpson, 1947). The extent of this land connection directly varied with the amount of water withdrawn from the world ocean by glaciers, and was greater in the Riss (Illinoian) glaciation than in the last glacial episode, the Wurm (Wisconsin), which followed it (Hopkins, 1959; Saidova, 1961). The Riss (Illinoian) land connection existed about 100,000 years ago; the Wurm (Wisconsin) dates from 70,000 to 10,000 years ago (Pewe, et al., 1965). The connection was broken, and Bering Strait most recently formed, to the west of St. Lawrence Island about 12,000 years ago (Creager and McManus, 1965). The land connection was continuous with adjacent unglaciated areas in eastern Siberia and the Alaska-Yukon, but was isolated from other unglaciated areas of North America by continental and Cordilleran ice-sheets (Fig. 1), and semi-isolated from the unglaciated areas of Siberia by extensive montane glaciers on the eastern Siberian mountain ranges (Moreau, 1955; Rand, 1954). This ice-free refugium has been called Beringia (Hulten, 1.937) or Amerasia (Gilmore, 1943). The environment of the land-bridge portion of Beringia during the last two glacial periods was one of tundra (Colinvaux, 1964); hence only species adapted to, or tolerant of, tundra or to other cold open habitats such as loess steppe, could occupy the land connection. Thus, only such species might be expected to occur on both the Eurasian and North American sides of Bering Strait. LATE-WISCONSIN AND POST-PLEISTOCENE