The morphological characters that separate oreas from austerus, namely, larger size, richer color, and longer tail, are all features that might be associated with a more arboreal existence in dark, coniferous forests. Our observations show that oreas is, to a large extent, arboreal. Traps nailed to tree trunks six to eight feet from the ground and set for flying squirrels often caught oreas in the Skykomish River Valley. On one occasion I walked up a leaning tree trunk to set a trap, fifty feet from the ground on the trunk of another tree that was upright. An oreas was found in this trap the next morning. [Svihla] (1933: 13) relates how a specimen of oreas that escaped from a live trap took refuge in a tree when pursued. We have set numerous traps for flying squirrels in the area about Puget Sound. As far as memory and field notes serve, we have never taken austerus in these traps. Our observations on the habits of austerus are much more abundant than those on oreas, but for austerus no evidence of arboreal activities has been obtained.
There are, then, two subspecies that do not interbreed, each capable of existing in any ecologic niche that will support deer mice. Where the ranges of the two subspecies come together, they compete. The large size, richer color, longer tail, and perhaps arboreal habits of oreas give it an advantage in coniferous forests. The small size and dark color of austerus give it an advantage in other habitats, especially, perhaps, in winter, when the deciduous trees have shed their leaves.
The differences separating austerus from oreas would be expected to have developed under complete isolation. That oreas developed from austerus or austerus from oreas seems impossible. A glance at the distribution map (Fig. 107) shows that the range of austerus clearly separates the range of oreas into two segments. The range of austerus tapers out to the north, east and west. On the south austerus intergrades with rubidus from Oregon, a subspecies from which it is but slightly differentiated.
The geographic behavior of the four subspecies of deer mice that occupy extensive areas in Washington may be summarized as follows: P. m. gambelii occupies southeastern Washington and intergrades normally with oreas in the eastern Cascade Mountains and with artemisiae at the northern edge of the Columbian Plateau; artemisiae, occupying northeastern Washington, closely resembles populations of mice that are known to be intergrades between oreas and gambelii and itself intergrades with both oreas and gambelii; oreas occupies most of western Washington, intergrades with its neighbors artemisiae and gambelii to the east, but does not intergrade with austerus, its morphologically more similar neighbor in the west; austerus occupies a range in western Washington that is almost surrounded by the range of oreas, a subspecies with which it apparently does not interbreed.
The relations of these four subspecies appear to be the result of certain great changes in the range of deer mice in the Pacific Northwest that occurred during and since the last Pleistocene glaciation. We reconstruct these changes as follows. In the Puyallup interglacial cycle, just previous to the last (Vashon-Wisconsin) continental glaciation, clines, or gradual transitions existed in the ranges of the deer mice along the Pacific Coast. The small, dark, short-tailed mice of the coastal redwood forests of California and Oregon became gradually larger, redder and longer-tailed to the north, climaxing in a large, red, long-tailed form in the spruce forests of southern Alaska and northern British Columbia. A similar cline existed inland. Small, pale, short-tailed mice of the Great Basin became increasingly larger, darker, and longer-tailed to the north, reaching a maximum in the spruce forests of northern British Columbia.
The development and advance of the Vashon-Wisconsin ice sheet exterminated mice over much of British Columbia and the northern United States. Long-tailed northern mice were forced southward and contacted the southern, short-tailed forms. The preglacial clines were thus destroyed.
What might be the southern part of the western cline may be noted in the deer mice of western Oregon today. From the southern coast of Oregon the mice (P. m. rubidus) do become larger, redder and longer-tailed to the north. The climax of this cline is now reached in austerus, of the Puget Sound area of Washington. The cline is not continued farther northward because the range of oreas is encountered.
The advance of the Vashon-Wisconsin ice from the north probably forced species of mammals southward, ahead of it, including the long-tailed northern deer mice which generation by generation encountered progressively shorter-tailed forms of resident mice. Perhaps the unfamiliar, and certainly the extremely frigid, conditions in the fore of the glacier exterminated the short-tailed individuals but favored the long-tailed mice, since the latter originally were adapted to a boreal environment. The climax of the ice advance found the glaciers just within the political limits of the United States and found the long-tailed mice spread before the ice front.
In western Washington the Vashon glacier advanced as far south as the southern edge of the Puget Sound area. Long-tailed northern mice advancing before it reached the Columbia River. This glacially swollen stream served as a natural barrier and prevented their southward extension. At this time the northern mice had traversed more than half the length of the original western cline. The northern mice, originating in a boreal habitat a thousand miles away, were unable to interbreed with the southern mice and such individuals as might have crossed the Columbia River never became established in Oregon. During the existence of the glacier in western Washington, the long-tailed northern emigrants competed with the resident deer mice of western Washington to the total elimination of the resident mice. The retreat of the Vashon Glacier from Washington found the northern mice in complete control of that part of the state from the Pacific Ocean to the Cascade Mountains.
In eastern Washington the Wisconsin Glacier advanced south to the northern edge of the Columbian Plateau. Northern mice advancing before it probably did not survive on the treeless plateau but existed in forested areas of northern Idaho and driftless areas of northern Washington. No natural barriers separated the northern mice from the pale, short-tailed forms. The nonuniform topography perhaps allowed more mingling of the two types where climatic conditions permitted. Intergradation in some places as well as competition and elimination of one form or another in other places occurred. Following the withdrawal of the Vashon ice and the establishment of soil and forests on the deglaciated land, the long-tailed mice of western Washington (P. m. oreas) apparently spread northward, unhindered by competition, until they reached southern Alaska. The deep coniferous forests of western Washington presented conditions acceptable to oreas and it persisted there despite postglacial changes in climate.