Fig. 109. Distribution of the northern lemming mouse in Washington. A. Synaptomys borealis wrangeli. B. Possible range of Synaptomys borealis artemisiae.
Description.—Lemming mice may be recognized by their short, thick bodies, slightly larger than the bodies of the common house mouse (Mus); short tails, which are less than 20 per cent of their total length; small, inconspicuous ears; and grooved upper incisors.
Mice of the genus Synaptomys range over much of boreal North America. Two species and eleven races are recognized by [Howell] (1927B: 9). Synaptomys borealis ranges westward across Canada from Labrador to the Pacific Coast and from Alaska south to Washington. The lemming mice are terrestrial and inhabit runways similar to those of meadow mice (Microtus). They are alpine in distribution. [Shaw] (1930: 7-10) found them among typical annual herbs in a cold, boggy mountain valley. Other than this, nothing is known of the species in Washington. Indeed little is known of the life history of any member of the genus.
The lemming mouse possesses four pairs of mammae, two pectoral and two inguinal. It is the most primitive of the microtines. It seems to be of boreal origin but is not known from the Old World. It was evidently forced southward by the Pleistocene glaciers into Washington and is now retreating northward in the wake of the ice. The only specimens recorded from Washington are from the northern border of the state.
Phenacomys intermedius [Merriam]
Heather vole
Description.—Heather voles are heavy-bodied, short-legged mice, closely similar in general appearance to other microtines that occur in Washington. Their dull, brownish upper parts, lacking a distinct reddish dorsal stripe, separate them from the red-backed mice (Clethrionomys) and the lack of grooves on their upper incisors separates them from Synaptomys. Their relatively short tail (30 per cent or less of their total length) separates them from most meadow mice (Microtus). There are no external characters which serve to separate them from all species of Microtus and the teeth must be examined certainly to identify Phenacomys. In Microtus the angles between the cusps of the inner and outer sides of the lower molars are of approximately equal depth, but in Phenacomys the angles on the inner side of the lower molars are at least twice as deep as those of the outer side of the jaw. In addition the molars of Phenacomys are rooted while those of Microtus are not.
The heather voles and their relatives are primitive microtines that range through boreal North America including the higher parts of the Rocky Mountains and the cool area along the Pacific Coast. Several species are included in the genus: an arctic form (ungava), a lowland Pacific Coastal species (albipes), two tree-inhabiting species (longicaudus and silvicola) and a mountain species (intermedius). Thus far only intermedius has been found in Washington but further search may reveal that one or more of the arboreal species and possibly albipes are also present.
Heather voles are alpine animals, seldom occurring below the Hudsonian Life-zone. They are rare as compared with most microtines, and are seldom taken in traps, even by the experienced collector of small mammals. Evidence of their presence is most often found in the dense meadows of heather (Cassiope and Phyllodoce) high on the mountains. Here their feces, runways, and old winter nests are not uncommon, and heather voles may be far more abundant than trapping records indicate. They are not restricted to the vicinity of heather, however, for one of my specimens was taken in a marshy willow thicket near Stevens Pass in King County, one in a patch of skunk-cabbage in a grassy glacial cirque near Tomyhoi Lake in Whatcom County, and one at the edge of a snowbank on a steep hillside at Deer Park, Clallam County. In each of these places, however, there were heather meadows within a thousand yards. [Shaw] (1924A: 12-15) also found the heather mouse on "comparatively dry hill slopes" and reports that it "rather characteristically occurs in such log-tangled areas as former fire zones."