YOSEMITE MEADOW MOUSE. Microtus montanus yosemite Grinnell

Field characters.—Body size about three times that of House Mouse; tail short (fig. 16a), less than 1/2 head and body; pelage long and lax; ear nearly buried in fur. (See fig. 20b). Head and body 4-2/5 to 5-2/5 inches (112-138 mm.), tail 1-3/8 to 2-1/8 inches (35-54 mm.), hind foot about 4/5 inch (20-22 mm.), ear from crown 2/5 to 2/3 inch (10-17 mm.); weight 1-1/3 to 2-5/8 ounces (38.0-74.8 grams). Coloration dark brown above, sometimes with a reddish tinge on back; under surface dark gray. Workings: Pathways or runways 1 to 1-1/2 inches wide, cut along surface of turf and connecting with small round holes in earth, which are always open.

Occurrence.—Common resident chiefly in Canadian and Hudsonian zones on both slopes of Sierra Nevada. Recorded from Gentrys (on Big Oak Flat Road) and from Mono Meadow (south of Glacier Point) eastward across the mountains to Mono Lake Post Office and to near Williams Butte; present in numbers on floor of Yosemite Valley. Lives in meadows and grasslands, usually at no great distance from water. Active to some extent during daytime, but otherwise nocturnal.

The level or sloping grasslands of the Yosemite region are inhabited by many of the small furry-coated animals known popularly as field mice and to students of natural history as meadow mice or voles. Two distinct groups are represented, a long-tailed, free-ranging type, the Cantankerous Meadow Mouse, and several short-tailed species, the Tule, Mariposa, and Yosemite meadow mice, which cut pathways in the grassland. Of the latter group the first two inhabit respectively the San Joaquin Valley and the western foothills. The last, the subject of the present chapter, is distributed over most of the high Sierras from the neighborhood of Yosemite Valley eastward to the plateau country on the western side of Mono Lake. (See fig. 19.)

The local meadow mice are all much alike in general outward appearance and so the Yosemite Meadow Mouse may be taken as an example for detailed treatment. (See fig. 20b). The nose is blunt, the eyes prominent and bead-like though not so large in proportion to the head as those of white-footed mice. The ear is rather short and therefore nearly or quite buried in the copious fur. The body is rather chunky, the tail short and slender, and both front and hind pairs of feet are inconspicuous. The pelage of these mice is distinctive, being rather long, dense, and soft, and of fluffy appearance. This type of pelage is found in various other animals which like the meadow mice live about water where the pelage must perform the dual function of keeping the animals both dry and warm.

Fig. 16. Tails of (a) Yosemite Meadow Mouse and (b) Sierra Cantankerous Meadow Mouse. Natural size.

The Yosemite Meadow Mouse like the other path-cutting species has a short tail, less than half, even but a third the length of the head and body; also the tail is of nearly uniform color. These features will usually serve to distinguish any of the path-cutting species from the Cantankerous Meadow Mouse. But within the first-named group there is no good external character to separate the species. The distinctive features are found in the skulls and hence can be determined only from museum specimens. The Yosemite Meadow Mouse is generally more blackish, but old individuals have as much reddish coloration on the upper surface as does the, foothill and valley species (californicus). In the field, distribution is the best clue for separating the short-tailed species. No specimen of yosemite has been found below 3800 feet (the floor of Yosemite Valley), while mariposae of the foothills penetrates the mountains no farther than Cascades (3600 feet).

As a group the meadow mice are grass feeders, and when green vegetation is available they will take little or nothing else. (See fig. 21.) Sometimes when the animals are unusually plentiful they gnaw the bark of trees. Since the main item of diet is grass, their whole scheme of existence is modeled for obtaining this sort of food.

In order to be close to their source of food supply and also to be able to escape if danger threatens while they are foraging, these mice carry on their whole existence right in the meadows. They dig small shallow burrows or tunnels down through the sod to a depth of several inches and place their nests in enlarged parts of these tunnels. When they venture out to forage, they follow runways which they cut through the vegetation; along these they can run readily without interference and are to some extent shielded from observation. These surface runways constitute the most obvious indication of the presence of meadow mice and are the things which a naturalist always searches for when he wishes to locate these rodents. Sometimes in making a runway the grass is merely trampled down, but usually the mice carefully cut away the stems so that the floor of the run is at or even slightly below the level of the ground. In the latter case, to the eye of a person, the runway shows as of earth color in contrast with the surrounding green vegetation. The floor of the run is kept free of obstructions by the mice, but often the growing grass arches over the top forming a canopy under which the animals may travel for long distances without much danger of being observed by a foraging hawk or owl overhead. The system of surface runways is often extensive, with many intersections; as many as five and even six different paths may radiate from a single hole. The direction and extent is probably dictated by the relative abundance and location of forage material. The usual width is from 1 to 1-1/2 inches. The average dimensions are slightly greater in the runways of the Mariposa than in those of the Yosemite Meadow Mouse; the former mouse is on the average a slightly larger animal. In late summer on Tuolumne Meadows many small narrow runways were noted which were thought to have been made by the young of the year after they had taken up an independent existence apart from their parents.

At the end of September (in 1915), on Tuolumne Meadows, the meadow mice were exceedingly active. Extensive surface runways were seen, as well as many holes leading to underground burrows. In some places there were 4 to 6 holes per square yard of surface. There was much cut grass in the runways and in one place, under an overhanging bank, a mass aggregating about two handfuls was noted. Droppings were scattered along all the runways, and occupation of the runs for a considerable period of time was indicated.

These mice are abroad to a considerable extent during the daytime, although they are perhaps most active in the early hours of the night. The protection afforded by the runways, particularly those arched over by grass, probably promotes daytime activity. Several of the animals were seen running about by members of our party. One individual noted in Sentinel Meadow, Yosemite Valley, on October 8, 1914, darted along 20 feet or more of tunnel with amazing speed considering the turns which had to be negotiated. The sensitive 'whiskers' may be of considerable service in traversing the runs; the animal literally "feels its way." Its relatively smaller eye, as compared with that of a free-ranging animal like the white-footed mouse, may be indicative of the meadow mouse's lessened dependence upon acute sight.

Meadow mice are active during winter as well as summer even in localities above the snow line. In Yosemite Valley in late December runways were found through the snow at the margin of the Merced River, and openings to the surface were found at short distances along the runs. In other parts of the region winter nests were seen which had been built on top of the ground where, of course, they had been fully protected by the blanket of snow. Considerable activity on the part of the mice is to be observed during the fall months, evidently in preparation for the long Sierran winter. This was noted on Tuolumne Meadows in late September of 1915, and in Yosemite Valley in early October of 1914. In the latter instance excavation of underground burrows was being diligently prosecuted.

Our data relative to the breeding of the Yosemite Meadow Mouse are plentiful, chiefly because we spent much time and did a great deal of trapping within the range of this species. The evidence in the form of females with embryos or recently born young points to a breeding season extending at least from March or April until October. Thus, on May 4, 1916, a female meadow mouse taken near Williams Butte contained 6 embryos. Another captured October 22, 1915, at Gentrys, held 5 small embryos. Between these inclusive dates 30 females with embryos were obtained. The number of embryos ranged from 4 to 9, with an average of between 6 and 7 (6.3). With the long breeding season indicated it is probable that each female rears more than one and perhaps several broods. The species thus has the ability to increase rapidly when conditions are favorable. A still further factor greatly enhancing the potentiality for increase is that the females may breed at an extremely early age. On May 22, 1916, a female containing 6 embryos was captured; this animal was still in the juvenal pelage and hardly half-grown, as it weighed only 24.5 grams, scarcely half the weight of an adult. Two other females taken May 21 and May 4, weighed 30.0 and 32.5 grams respectively, and these also contained embryos. These latter animals had molted out of the short blackish juvenal pelage but were otherwise immature. It seems safe to assume that the youngest of these mice was not much over two months of age when it began to breed, and that both had been born during February or March.

In Yosemite Valley during the fall months more, than half the population of meadow mice was found to consist of obviously young animals—at least this was so indicated by our trapping records. Of 43 individuals obtained October 8 to 10, 1914, 23 were juveniles.

Over the entire northern hemisphere meadow-inhabiting mice are to be found wherever the ecologic niche of this group is represented, but under very diverse climatic conditions. The Lemmings of the Arctic regions and the Meadow Mice of the Great Basin are extreme examples. From time to time great fluctuations in the populations of these mice are to be noted. Sometimes the animals are so scarce as to be found only by diligent search; in other years they are extremely plentiful. A case of the first sort is related of the Mariposa Meadow Mice at Bean Creek east of Coulterville in 1915; on Tuolumne Meadows the same year the Yosemite Mice were so numerous that 5 or 6 holes per square yard were counted in places. Under certain favoring conditions, regarding the nature of which we still have much to learn, meadow mice may increase enormously, even far beyond the population indicated for Tuolumne Meadows. Then a 'mouse plague' results. It was a race closely related to the Yosemite Meadow Mouse which over-ran Humboldt Valley, Nevada, in 1907 and 1908 and caused great damage to agricultural interests there by destroying practically all the surface vegetation and even the roots of the alfalfa.