COMMON WHITE-FOOTED MICE. Peromyscus maniculatus (Wagner)⁸

Field characters.—Size slightly greater than that of House Mouse; ear larger and tail shorter (pl. 25b and text fig. 10a); tail distinctly less than length of head and body, not scaly in appearance. (For measurements, see footnote 8.) General color above yellowish brown (blue-gray in young); below pure white, sharply set off from color of upper surface; tail bicolor, that is, white, with a dark stripe above; feet pure white.

Occurrence.—Abundant resident throughout the entire Yosemite section from the San Joaquin plains at Snelling eastward without interruption across the mountains to Mono Mills; range upward to at least 10,800 feet.⁸ Inhabit every sort of cover from stream margins to the dryest slopes and most barren rock slides. Nocturnal.

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⁸Two subspecies of these mice occur in the Yosemite section, one on the west slope, the other on the east side of the mountains, the two intergrading over the crest of the Sierras.

GAMBEL WHITE-FOOTED MOUSE, Peromyscus maniculatus gambeli (Baird). The form which is distributed throughout most of California west of the Sierran crest. It is found from Snelling and Lagranga eastward in more or less typical form to the vicinity of Tuolumne Meadows, and intergrades insensibly over the Sierran crest with the Sonora White-footed Mouse.

SONORA WHITE-FOOTED MOUSE, Peromyscus maniculatus sonoriensis (LeConte). A paler, less dusky, and slightly larger subspecies which inhabits the Great Basin and desert country to the east of the Sierran crest. It was recorded from Walker Lake east ward to Mono Mills.

Measurements.—Gambeli: head and body 3 to 3-3/4 inches (75-95 mm.), tail 2—2-3/4 inches (52-72 mm.), hind foot about 3/4 inch (18-20 mm.), ear from crown 3/5 inch (13-16 mm.), weight about 1/2 ounce (12.8-21.1 grams); sonoriensis: head and body 3-1/2 to 4-1/8 inches (83-106 mm.), tail 2-1/8—3 inches (55-75 mm.), hind foot about 4/5 inch (19-21 mm.), ear from crown 5/8 inch (15-17 mm.), weight 2/3 ounce (19.9-28.5 grams).

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The Common White-footed Mice are without any doubt the most abundant mammals in the Yosemite section. Indeed, it is not unlikely that the total population of this one species nearly or quite equals that of all the other mammals in the region together. Its numbers do vary somewhat according to place and season, but it is always present, and in some places it may be said to fairly swarm. In whatever locality we placed our traps this kind of mouse was sure to be caught. In places these mice simply have to be 'trapped out' before representatives of other species can be obtained. Yet White-footed Mice are practically never seen by daylight, for they are as strictly nocturnal as are bats.

Except for the fact that it does most of its foraging on or close to the ground there is scarcely any limitation to the range of this mouse. It frequents the very edge of running water, thickets and grass clumps on the banks of streams, the runs of meadow mice in damp grasslands, the sides of dry gullies, mixed growths of brush plants on the hill slopes, old buildings, logs and boulders in the forest, and heaps of slide rock on the mountain sides. On one occasion some mice of this species were found living in burrows on altogether open ground, a place where only kangaroo rats were expected to occur.

Fig. 10. Showing differences in ear between the four species of White-footed Mice found in the Yosemite section. (a) Gambel (Common) White-footed Mouse; (b) Boyle White-footed Mouse; (c) Parasitic White-footed Mouse; (d) Gilbert White-footed Mouse.

For nesting places and daytime retreats White-footed Mice make use of any available cover, such as is afforded by crevices or holes in rocks, hollows in trees or in logs, or holes in the ground. Often they use burrows made by other rodents, while in some cases it seems likely that they do a certain amount of excavating themselves.

Despite its great numbers this mouse does not leave any very obvious indications of its presence. Its small black droppings are the only regular and definite evidences to be found. Nothing distinctive pertaining to its nest, or route of travel, or choice of food, is left as a clue, as is the case with most other rodents. It seems to be the most adaptable of all the small mammals, fitting into types of habitat unused by any of the more specialized mammals and venturing into the special territory of these which may not be fully occupied. With this flexible nature it might be expected that the White-footed Mouse could and would become a pest about human habitations, but there has been no development in this direction. The species does hold the last line of defense for the wild species, living, as it does, about cabins and barns in newly settled territory; but it quickly retreats upon the arrival of that more aggressive alien, the House Mouse.

The Common White-footed Mouse is somewhat larger, differently proportioned, and differently colored than the well-known House Mouse. (Compare pl. 25b and c). The average weight of the Gambel Mouse is 0.62 ounce (17.5 grams), and of the Sonora Mouse, 0.86 ounce (24.5 grams), while that of the House Mouse is 0.58 ounce (16.4 grams). The tail of the Common White-footed Mouse is less in length than its head and body; in the House Mouse it is about equal. The ear of the Common White-footed Mouse averages larger, 5/8 inch (16 mm.), compared with about 1/2 inch (13.5 mm.) in the House Mouse. The White-footed Mouse is conspicuously white on its under surface, this white extending to the under side of the tail and including the entire feet. The House Mouse, on the other hand, is dingy gray underneath, with no sharp line of demarcation along the sides; the tail is monochrome, not bicolor; and the feet are dusky. The tail of the White-footed is well haired (though the hairs are very short), not nearly bare and scaly, as is that of the House Mouse.

It should be stated here that there are no less than four species of white-footed mice in the Yosemite section, and in certain places on the west slope of the mountains all four are to be found in close proximity to one another. (See fig. 11.) All four of the species bear a general resemblance to each other and two of them (the Boyle and True) are enough alike to make it difficult to identify individuals. The Common White-footed Mouse (Peromyscus maniculatus with subspecies), the subject of the present chapter, is the smallest of the four (see fig. 10 and pl. 25). Its tail is shorter than the head and body, 3 inches or less (75 mm.), and is distinctly bicolor, that is, pure white with a dark stripe along the top. The hind foot is shortest, measuring 3/4 to 4/5 of an inch (18-21 mm.); its ear is smallest, measuring 1/2 to 2/3 of an inch (13-17 mm.).

Fig. 11. Cross-section of the Sierra Nevada through the Yosemite region showing zonal and altitudinal ranges of White-footed Mice (genus Peromyscus).

The Boyle White-footed Mouse (Peromyscus boylii) is the next in point of size, and can be recognized further by the combination of medium sized hind foot (21 to 23 mm.) and medium sized ear (from crown, 17 to 20 mm.). In this and the following two species the tail is distinctly longer than that of the Common White-footed Mouse, equaling or exceeding the combined length of the animal's head and body.

The True and Gilbert white-footed mice (which are subspecies of the one species, Peromscus truei) have much larger ears, measuring 20 to 26 mm.; and the hind feet average longer, measuring 22 to 25 mm. Also the pelage is longer (hair on rump 11 mm., instead of 7 or 8 mm. as on the Boyle Mouse). The Gambel and Sonora Mice are short-haired, while the Parasitic is long-haired.

The fourth species in the series, the Parasitic White-footed Mouse (Peromyscus californicus), is decidedly larger than any of the other three; its hind foot is longer, measuring 25 to 28 mm., but its ear is no larger than that of the True and Gilbert, since it measures 21 to 23 mm. above crown of head. (See accounts of each of the species for detailed measurements.) The relative size of each of the four species of white-footed mice may be judged from the following weights, which are averages obtained from selected adult specimens: Peromyscus maniculatus, 21.0 grams; P. boylii, 26.5; P. truei, 29.5; P. californicus, 45.0.

The white-footed mice are practically all under cover through the daylight hours. Occasionally a few are trapped during the day, especially when traps are set in shaded places; but they are by no means as active then as are the meadow mice. Their 'day' comes at night. As soon as the dusk has claimed all but the nearest of objects, these mice begin to venture abroad. Most of their running about is done during the earlier hours of the night, but some are still abroad when the Wood Pewees utter their first calls shortly before the break of day.

The camper who goes early into his sleeping bag and there listens for the night sounds is likely to hear little rustlings among the leaves, indicating that the white-foots are abroad. One evening in mid-May at Hazel Green, one of us happened to put his sleeping bag close to the base of a large tree beneath which there was an accumulation of leafy debris. Soon after dark a Common White-footed Mouse began exploring the neighborhood. For some time it stayed within a radius of 6 or 8 feet, rustling among the leaves and occasionally making larger shifts of position. These were accomplished by swift runs; the rapid patter of small feet would be followed by several seconds of quiet while the mouse took account of its new surroundings. About this time the moon came up and the mouse could be seen clearly in the bright light. Whenever the observer moved, the mouse would scamper into some hiding place; but its fright was of very short duration and it would soon reappear.

That the Common White-footed Mouse does on occasion range higher than the ground is indicated by the fact that several individuals were trapped on pantry shelves up to six feet above the floor in a house in Yosemite Valley, and another individual was caught eighty feet above the ground on top of a prostrate tree in a windfall at Tuolumne Meadows. Practically all our traps were set on the ground, so we are unable to state the extent to which these mice may climb. The animals sometimes venture well out from shelter; individuals were taken on open ground as much as 20 feet away from cover of any sort. Most of those trapped, however, were obtained close to or under logs, rocks, or brush, where the majority of our traps were set.

At Snelling an adult and a juvenile mouse were caught together in the same trap, this incident suggesting that young individuals may forage for a time in company with their parents. The species is not colonial, in any definite sense of the term; although it occurs locally in considerable numbers, the adult individuals are, as a rule, intolerant of one another s presence.

It is not known with certainty that the Common White-footed Mice hibernate. There is even good evidence to the contrary. In the winter months their tracks are often to be seen in Yosemite Valley, on the surface of the snow. Individuals were trapped in December in dead grass and leaves in sheltered places. Here, it seemed likely, they had been running about among the bases of grass stems beneath the snow mantle.

The breeding season is of long duration and each female very probably bears more than one litter a year. Females with embryos were taken from May 13 until October 24, and evidence, in the form of blue-pelaged juvenals or sexually active males, suggested that, in the lower altitudes at least, the species was breeding practically throughout the year. The number of young to a litter ranges from 3 to 7, averaging 5. Of 38 sets of embryos examined, in two cases there were 3, in eight cases 4, in thirteen 5, in thirteen others 6, and in two cases there were 7. The young come quickly to maturity and some of them undoubtedly breed during the same season in which they are born. Thus within one favorable season, when all of the offspring would be able to find sufficient food of a suitable nature, the numbers of these mice might increase very greatly.

A maximum concentration of Sonora White-footed Mice was encountered at Mono Mills in 1916. A line of 30 traps about one-half mile in length was set on the ground in the sagebrush among Jeffrey pines near the mill. On the night of June 6, 10 Peromyscus m. sonorensis and 6 other rodents were caught in this one trap-line. During the day of the seventh 8 chipmunks and 2 Golden-mantled Ground Squirrels were obtained. The night of June 7, 21 Peromyscus and one pocket mouse were trapped; the night of the eighth, 20 Peromyscus and two pocket mice; and the night of the tenth, 15 Peromyscus and 1 Kangaroo Rat. Then the line was taken up. The collector's own footprints made as he visited his traps at night fall to bait and re-set them would in places by morning be obliterated by the multitude of tiny tracks made during the night. Many of the mice in the traps were partially eaten, probably by others of their own species. Food in general seemed scarce.

The suggestion presents itself that an unusually large population had resulted from exceptionally favorable conditions, including abundant food during a preceding period; and because of the discontinuance of these favoring conditions, the mice were on the verge of starvation just at the time the member of our party started trapping. The potential powers for the expansion of mouse population, as based upon the figures for rate of breeding given above, are enormous, possibly twenty-fold in a single year. A sequence of favoring conditions may on occasion bring about the full realization of this potentiality; but eventually there will be a return to normal numbers.

Many of the Sonora White-footed Mice trapped at Williams Butte in the fall of 1915 were sorely afflicted with huge rabbit-fly bots on the back or flank. An immature male, trapped September 22, 1915, had one of these maggots imbedded beneath the skin on one flank and opening toward the ankle. The mouse weighed 14 grams, and the fly larva 1.3 grams—nearly one-tenth the weight of the host!

In Yosemite Valley when the melting snows at higher levels cause a rise of water in the Merced River, the Valley meadows are flooded and the non-aquatic animals which live there are forced, at least temporarily, to seek higher ground. The white-footed mice then move up-slope, invading, en route, the gardens and even the houses of the people living in the Valley. One householder told us that on one particular night, during such an invasion, there were fully 20 of these mice running about the rooms in her house. After a few days the white-foots leave the neighborhood of the houses and seek their more natural retreats.