LONG-TAILED HARVEST MOUSE Reithrodontomys megalotis longicauda (Baird)

Field characters.—Size and general form of House Mouse; each upper incisor tooth with a single groove down its front surface; tail about equal to head and body, very scantily haired. Head and body 2-1/2 to 3-1/4 inches (61-83 mm), tail 2-1/2 to 3-1/8 inches (63-79 mm.), hind foot about 3/4 inch (16-18 mm.), ear about 1/2 inch (11-15 mm.); weight about 1/3 ounce (8.5-12.5 grams). Coloration above buffy and black mixed in fine pattern, blending along sides with dull white of under surface; ear clothed with very short tawny-colored hairs.

Occurrence.—Common resident in Lower and Upper Sonoran zones on west slope of Sierra Nevada. Recorded from Snelling and Lagrange eastward to Sweetwater Creek, to El Portal, and to Smith Creek, 6 miles east of Coulterville. Lives chiefly in grassland, occasionally on brush-covered slopes, if these be shaded and damp or near water.

The Long-tailed Harvest Mouse is a small dun-colored animal, of retiring habits, that to the casual eye is simply a field mouse. It dwells in grasslands, among weeds along fences and irrigation ditches, and in similar places. Unlike the meadow mice it leaves no reliable indication of its presence and so must be specially sought for, else it will escape observation entirely.

Externally, the harvest mouse has much the appearance of the House Mouse, the size of the head and body and the relative length of the tail being about the same in the two. In the harvest mouse, the pelage is longer and silkier, and the tail is less conspicuously scaly, than in the House Mouse. The ear is clothed with short tawny hairs. Perhaps the best character for surely distinguishing these two species pertains to the upper incisor teeth. In the harvest mouse the front of each tooth has a conspicuous groove running the full length, the effect of which is to suggest that the mouse has four rather than two upper incisors. The harvest mouse is much smaller than even the smallest of the local white-footed mice and so is not likely to be confused with any of that group at all. Lack of fur-lined cheek pouches distinguishes it from the pocket mice.

For practically all of the harvest mice caught in the Yosemite section we have records of the circumstances of capture and so are able to state satisfactorily the local haunts of the species. The animals inhabit a considerable variety of situations ranging from the immediate vicinity of water to dryish rocky and brushy hillsides. By far the greater number, however, were captured in rather damp grassy places. At Snelling, cat-tails, grass, wild oats, horehound, and blackberry were growing at points of capture. Marshy places, meadow, dry ravine bottoms, rolling lands, bottoms of small gulches, grain fields, and weed growths along fences were all places which the species frequented. At Smith Creek, east of Coulterville, and at El Portal, a few harvest mice were taken in the runways of meadow mice. And at the last-named place some harvest mice were obtained amid rocks on a steep greasewood-covered hillside which was several hundred feet higher than the grasslands bordering the river, and as far removed from any stream. In all instances our specimens were trapped on the ground. Harvest mice are said sometimes to use birds' nests above ground as foundations for their own nests, but we found no evidence on this point.

The total population of this species must be great. We have no means of stating it in relation to any given area occupied, and furthermore the density of the population varies greatly from place to place. But in favorable situations, especially amid grassy growths, our trap lines produced harvest mice as long as the lines remained in place. It was no uncommon thing to obtain along with other small rodents 5 or 6 individuals of this species in one night, from 40 to 60 traps set over a half-mile of favorable country.

At Snelling, in January and May, the males numbered 20 and the females 15, in the cases where sex was recorded. A preponderance of males in January, when breeding activity was commencing, might be expected; for males then range more widely than females and hence are likely to be caught more commonly in traps. The sexes in the Harvest Mouse are in reality probably about equal.

The breeding season for the Long-tailed Harvest Mouse is a long one. As just indicated, males began to show breeding activity during the first week of January. By May, young of nearly adult size were abroad in small numbers at Pleasant Valley, and females with embryos were common. The number of embryos ranged from 3 to 6, averaging close to 4. A female with large embryos was taken at Smith Creek on July 13, 1920. The absence of trapping records from August to October leaves doubt as to how late the breeding season continues, but it seems likely, from information gained elsewhere, that it continues until some time in the fall. At El Portal in December, where numbers were taken, no breeding individuals were noted.

A species which produces four young at a birth, and in which the young mature rapidly and probably breed late in the season in which they are born, has the potentiality of a rapid numerical increase within a single season. Despite this ability to increase its numbers, the Long-tailed Harvest Mouse has never been found to play any economic role, either harmful or otherwise, for it retires before cultivation and occupies marginal areas only.