Peromyscus maniculatus.—Six subspecies of Peromyscus maniculatus occur in the state of Washington. The geographic range of one of these (rubidus) lies mainly in the states of California and Oregon and includes, so far as is known, a single small island in the Columbia River that is politically within the state of Washington. Another (hollisteri) is restricted to certain islands in northern Puget Sound and obviously has become subspecifically differentiated in postglacial time. The remaining four subspecies, namely oreas, austerus, artemisiae and gambelii, have extensive geographic ranges. These subspecies are not confined to their geographic ranges by geographic barriers. Deer mice occur in the deep forests and the open desert, on high mountains and in low valleys, and are almost everywhere the commonest species of mammal present.
The study of several populations of deer mice from any general area usually shows small but constant differences between them. [Dice] (1939: 21) studied stocks of deer mice from nine localities in southeastern Washington and found significant differences between several of them. A statistical study of mice from the San Juan islands shows that the population of almost every island is different in one or more respects from the mice of any other island. Geographically separated populations of "wild caught" mice of the subspecies austerus, of the mainland, were statistically compared and significant differences were found between these populations, too. Small, differentiated populations are to be found in many parts of the state, and each subspecies appears to be an assemblage of such tiny genetic units.
These genetic units probably are the microgeographic races of Debzhansky (1937). They have been intensively studied by [Sumner] (1917 A, B) and [Dice]. An especially important paper by the latter author (1940) summarizes much of the available information on speciation in Peromyscus and clearly discusses the microgeographic races of Peromyscus.
The numerous microgeographic races of Peromyscus maniculatus in Washington present diverse combinations of characters which could result from the random fixation and elimination of genes ([Wright]. 1932: 360-362). Such a hypothesis, however, requires at least partial isolation of the populations involved. The mechanism of such isolation, in such populations of deer mice as we have studied, is not readily apparent. Some microgeographic races are not separated by noticeable geographic or ecologic barriers and the distance between their ranges is not too great to be traversed by a deer mouse. The tendency to remain on a home range may have the same effect as isolation would be supposed to have.
The work of [Murie] and [Murie] (1931: 200-209, 1932: 79) is enlightening in this respect. These authors found that mice residing in a locality tended to remain in that locality; individuals trapped and marked were retaken in the same locality a year later. Individuals released some distance from the point of capture remained where released or returned to the point of capture. Transported individuals did not spread at random. The home instinct was developed in young as well as in old mice. Two mice in the gray pelage, four to eight weeks old, returned to their home ranges from distances one and two miles away. The authors fix the home range of an individual Peromyscus m. artemisiae in Teton County, Wyoming, at approximately one hundred yards in diameter.
This home-range instinct is essentially a lack of incentive for individual mice to emigrate to new localities where mice of the same species are already established. This may partly account for the microgeographic races of deer mice in Washington.
[Dice] (1939: 21) pointed out that, except in color, the differences in nine stocks of mice from southeastern Washington could not be correlated with environmental factors. We have found this to be true of microgeographic races throughout the state of Washington.
Of the four subspecies of deer mice that occupy extensive geographic ranges in Washington, one, oreas, is a long-tailed form that seems not to intergrade with austerus, a neighbor in western Washington that has a tail of moderate length. These two and gambelii, a short-tailed form with which oreas intergrades, are easily distinguished. In eastern Washington two short-tailed subspecies, gambelii and artemisiae, are currently recognized. The taxonomic relationships of these two subspecies are complex. The subspecies gambelii has an extensive geographic range in Oregon and California. These mice, with short tails, occur in the Wallula Water Gap of southeastern Washington and on the Columbian Plateau. To the west the desert conditions of the Columbian Plateau fade into the Transition Life-zone forests of the eastern Cascade Mountains. The pale, short-tailed desert mice (gambelii) gradually change to the dark, long-tailed subspecies, oreas, that occupies the Cascade Mountains.
North of the Columbian Plateau, in northeastern Washington, the deer mice are darker and relatively longer-tailed than on the Columbian Plateau. Some populations are distinctly reddish, almost as reddish as oreas. Although assigned to artemisiae, they are almost identical with populations of deer mice from the eastern Cascade Mountains, known to be intergrades between oreas and gambelii. This fact, and the presence of surprisingly oreas-like characters in some microgeographic races in extreme northeastern Washington, may indicate that the race called artemisiae is a group of intergrades between gambelii and an oreas-like mouse that has become extinct.
Intergradation between gambelii and artemisiae is normal and takes place gradually where the ranges of the two subspecies meet. The same is true of intergradation between oreas and gambelii in the eastern Cascade Mountains. West of the Cascade Mountains the range of oreas meets the range of austerus. These two subspecies appear not to interbreed in nature, since no intergrades were taken at any of the numerous localities in western Washington where the mice were trapped. Pure populations of the two subspecies exist within a few miles of each other. In the valley of the Skykomish River, in the western Cascades Mountains, from the town of Skykomish, King County, to the lowlands to the west, only austerus was found. In the coniferous forests of the mountains above Skykomish, only oreas was taken. Several pairs, each an oreas and an austerus of the other sex, were kept from four to six months, and one pair was kept for a year, but they did not reproduce. The oreas were from the upper Skykomish Valley and the austerus were from Seattle, King County. Along the border of the ranges of the two subspecies in the Skykomish River Valley, a definite habitat preference was noted. The coniferous forests were occupied by oreas and brush or deciduous forests by austerus. Within the range of austerus and within the range of oreas only one subspecies is found whether the habitat be coniferous forest or thickets of alder and willow, but where the ranges of the two subspecies meet austerus occurs only in the thickets of aspen and willow and oreas occurs only in the coniferous forest.