SIERRA CANTANKEROUS MEADOW MOUSE Microtus mordax sierrae Kellogg

Field characters.—Body size more than twice that of House Mouse; tail slightly more than 1/2 head and body; pelage soft and dense. (See figs. 16b, 20a). Head and body 4-1/4 to 5 inches (108-128 mm.), tail 2 to 2-5/8 inches (50-66 mm.), hind foot about 4/5 inch (20-23 mm.), ear from crown 1/3 to 2/3 inch (13-17 mm.); weight about 1 to 1-3/4 ounces (30.3-48.0 grams). Coloration above dark brown with a grayish east; sides of body conspicuously grayish; under surface grayish white; tail distinctly darker above than below.

Occurrence.—Common resident, chiefly in Canadian and Hudsonian zones, on both slopes of Sierra Nevada. Recorded commonly from Merced Grove Big Trees and Chinquapin eastward to Warren Fork of Leevining Creek and Walker Lake; present on floor of Yosemite Valley in some numbers and taken once at El Portal. Lives chiefly along banks of swift-flowing mountain streams and in marshes but also on dry hillsides at some distance from water. Largely nocturnal.

Besides the path-cutting meadow mice (californicus and montanus) there is present in the Yosemite region a free-ranging species, the Cantankerous Meadow Mouse. It occurs in greatest numbers on the ground beneath the bushes which line the banks of mountain streams, but strangely enough is also found in some numbers on dry hillsides well away from water.

Fig. 19. Cross-section of the Sierra Nevada through the Yosemite region showing zonal and altitudinal ranges of Meadow Mice (genus Microtus).

The present species is a long-tailed meadow mouse, but is so only by comparison with others of its own tribe (figs. 16b, 20a). The tail of mordax is as a rule slightly over one-third the total length (one-half head and body), whereas in the California and Yosemite voles the length of tail is somewhat under the proportions given. In other features mordax closely resembles other meadow mice with its blunt nose, black bead-like eyes, small ear, and soft dense pelage.

The range of the Cantankerous Meadow Mouse includes the whole of the high Sierras. Nominally it embraces the Canadian and Hudsonian zones, the 'boreal' portion of the region; but the species locally extends well below the limit of the lower of these zones. Thus on the floor of Yosemite Valley, in the little swamp near the Happy Isles power house, and again in an area near Rocky Point, some of these mice were found; and on one occasion (November 21, 1914) an individual was captured at El Portal. It is an observed fact that along the course of a river or large creek a tongue of the next higher zone will often extend down into the zone below. This is due to the fact that the colder water and greater evaporation keeps down the temperature in the neighborhood of the stream. This, in the case of the Cantankerous Vole, would operate to permit the animal to reside comfortably at lower levels as illustrated by its occurrence in Yosemite Valley. The occurrence at El Portal may, of course, have been purely fortuitous, due to an individual having wandered or been carried down-stream from some Canadian Zone location on the slopes above. Altitudinally, this mouse was recorded as high as 10,700 feet in the head of Lyell Cañon, close to timber line. The lower limit of its regular range on the west slope is between 5000 and 6000 feet.

Fig. 20. (a) Sierra Cantankerous Meadow Mouse; Yosemite Valley, December 29, 1914. See p. 129. (b) Yosemite Meadow Mouse; same data. See p. 122. (c) Mountain Lemming Mouse; Ten Lakes, October 8, 1915. See p. 133. All photographed from freshly trapped specimens, about 5/8 natural size.

The Cantankerous Meadow Mouse does not regularly construct runways as do the California and Yosemite meadow mice. As a rule, it merely runs about here and there on the surface of the ground. In a few places, however, notably at Glen Aulin and Vogelsang Lake, we did notice ill-defined pathways on the ground beneath the thickets of bilberry and Labrador tea bordering the streams; and along these Cantankerous Meadow Mice were caught. Extensive use of the paths was indicated by the numerous small, elongated black droppings of this species. These natural avenues of travel are used also by other small mammals such as white-footed mice and chipmunks. One of these meadow mice was captured in a trap set on top of a heap of dead branches of aspen, about 2-1/2 feet above the ground. Foraging is carried on down close to the water's edge, as many individuals were trapped close beside streams; and occasionally one is seen swimming in the water.

Fig. 21. Willow and grass covered seepage slope in head of Lyell Cañon; altitude about 10,000 feet, Hudsonian Zone. Habitat of the Sierra Mountain Beaver or Aplodontia. In the willow thickets were Hudsonian White-crowned Sparrows. The grassy banks contained burrows and runways of the Yosemite Meadow Mouse. Photograph taken July 24, 1915.

This mouse is more restricted than its path-traveling relatives to nighttime foraging. Being a free-ranging animal it might be subject to capture by day-prowling, carnivorous birds or mammals in the same way as is Peromyscus. For that reason, probably, it is abroad but little during the day. Only on one occasion did we see an individual of this species alive. In Glen Aulin at about 9:30 A.M. on October 1, 1915, one was seen scampering over the leaf mold on the floor of a lodgepole pine forest.

The breeding season of this mouse, as revealed by our trapping records, embraces most of the summer season; we are unable to give its exact limits. A quarter-grown youngster collected at Merced Grove Big Trees on June 13 suggests commencement of breeding activity at some time in late April or early May. When we first came into the range of the species on June 10 many of the females contained embryos; this condition obtained throughout June and July. The latest records of breeding females are for August 30 at Vogelsang Lake and September 10 at Walker Lake. Continued trapping within the range of the species during October failed to reveal further breeding; hence the warmer six months of the year seem to encompass the breeding period. The numbers of embryos ranged from 3 to 7, the average for 21 cases being close to 5 in a litter. It may well be that females bear more than one litter a year, as is known to be the case with other meadow mice. A few females gave evidence of having bred before attaining the dimensions of a fully grown animal.