HOUSE MOUSE. Mus musculus Linnaeus

Field characters.—Size small; tail about equal to head and body (pl. 25c); tail nearly naked, scaly; eye small. Head and body 3 to 4 inches (75-106 mm.), tail 3 to 3-5/8 inches (74-92 mm.), hind foot 2/3 to 4/5 inch (17.5-20 mm.), ear from crown 1/2 to 3/5 inch (11-14 mm.); weight about 1/2 ounce (12.7-18.8 grams). Coloration above dark grayish or yellowish brown; under surface uniformly dusky brown, buff or whitish in different individuals; feet dusky.

Occurrence.—Not native; came with the white man; now common in and around practically every town or settlement on west slope of Yosemite region, from Snelling and Lagrange eastward to Yosemite Valley. Lives about dwellings, barns, and store houses, and also to a limited extent in grassy places away from buildings.

The same House Mouse which is found in our cities is to be found in the western part of the Yosemite region. This alien interloper, so much more aggressive than most of the local rodents, is in firm possession of territory in and about the towns and settlements from the San Joaquin Valley eastward through the foothills and even into Yosemite Valley. It was not detected about Mono Lake, though it does occur farther to the southeast, at Laws, Inyo County.

The general appearance of the House Mouse is familiar to so many people that description is scarcely necessary. It is the standard (pl. 25c) by which other small animals are judged when said to be 'mouse-like.' The tail comprises about half the total length and is scaly in appearance, there being only a very few short hairs and no 'pencil' or tuft at the end. The general coloration is the same over the entire upper surface of the body, a mixed yellowish brown or grayish brown 'ticked' with black hair endings. The under surface of the House Mouse is ordinarily but little paler than the back; many individuals here in California, however, and especially in the Yosemite region are buff or even white beneath; the latter color, when present, is never so pure as on the White-footed Mouse, and is not so sharply demarked along the sides. The feet are usually dusky-colored, sometimes pale, but never white. The eye of the House Mouse is small, about half the size of the eye of the Common White-footed Mouse.

A striking similarity in external appearance is found between the House Mouse and the Harvest Mouse, the measurements, proportions of body and tail, and even the coat color being much alike, particularly with light-bellied specimens of Mus. The appearance of the upper incisor teeth at once separates the two, however. In the Harvest Mouse each of these teeth is marked by a vertical groove; in the House Mouse, the surface of the tooth is perfectly smooth.

The House Mouse is now well established in the Yosemite region and doubtless has been for a great many years. It was probably quite an early arrival, as the foothill districts bordering the Yosemite were among the first areas settled by white people in California; and this mouse, in America, has closely followed the white settler. Living about houses and barns, it often makes its nest amid household effects, or in bags of grain or bales of hay. When these are carried to a new locality the mice often go also, as stowaways; their spread in this manner is thus passive so far as the mice themselves are concerned. When the goods or other articles are set down in a new location the mice, being in new territory, speedily increase and take possession of their surroundings; and, sooner or later, because of their more aggressive nature, they compel the native small rodents of the neighborhood to give way and finally altogether displace them.

But the House Mouse at the lower and middle altitudes is not only about man's habitations. At Snelling and as far into the foothills as El Portal this mouse was found living apart from buildings, in fields and grassy ravines. At the former station specimens were trapped near bluffs fully a mile away from the town. These individuals were living in a really wild state; and this was true in winter (January) as well as in the spring and summer. Their numbers were fully as great as those of the Gambel White-footed Mouse which was present amid the same general surroundings.

Besides being an aggressive and adaptable species, this mouse is also prolific. It breeds practically throughout the year, has rather large broods, and these may follow one another at relatively short intervals. Adults taken at Snelling in January showed signs of breeding activity; while young, not fully grown, were captured about the barns in Yosemite Valley at the end of December. The broods elsewhere are known to average between 5 and 6.