POCKET MICE. Genus Perognathus Maximilian¹²

Field characters.—Size small, body size usually about that of House Mouse; tail long, about equal to head and body (pl. 26b, c); forelegs and feet short and small; hind feet long and relatively large (see footnote for detailed measurements); a fur-lined pouch in each cheek, opening at side of mouth. Coloration yellowish brown above, white on under surface. Workings: Small holes about 3/4 to 1-1/4 inches in diameter, usually in sandy soil about bases of bushes; occupied holes plugged with earth during daytime.

Occurrence.—Resident at lower altitudes on both sides of Sierra Nevada, from Snelling east to El Portal and again around Mono Lake, east of the mountains.¹² Lives chiefly in areas of sand or other easily worked soil. Nocturnal.

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¹²Three distinct species of Pocket Mice occur in the Yosemite region; in fact two distinct systematic groups are represented. But they are all treated together here, due in part to our scanty knowledge of their habits and in part to the slight attention that is likely to be given to such elusive animals by most visitors to the region. It will be noted that the ranges of these three species do not overlap, so that specimens found in the field can be referred with confidence to the proper species on the basis of locality alone.

CALIFORNIA POCKET MOUSE, Perognathus californicus californicus Merriam, a large-sized spiny-haired species (pl. 26b) which occurs widely through the Upper Sonoran Zone in central California, was found from Pleasant Valley eastward to Smith Creek (6 miles east of Coulterville) and to El Portal where it lives on dry chaparral-covered slopes. The largest and darkest colored species in the region. Head and body 3-1/5 to 3-1/2 inches (81-90 mm.), tail 4 to 5 inches (103-125 mm.), hind foot about 1 inch (24-27 mm.), ear from crown 1/3 to 1/2 inch (8-14 mm.), weight 2/3 to 1 ounce (19.9-30.0 grams). Pelage coarse with many long grooved spine-like over-hairs on side and rump; tail with a 'pencil' or tuft at tip; soles of hind feet naked. Upper surface reddish buff, darkened by numerous black hair tippings; under surface white.

SAN JOAQUIN POCKET MOUSE, Perognathus inornatus inornatus Merriam, a small-sized, soft haired species (pl. 26c) of the San Joaquin Valley, was recorded at Snelling where it lives in sparse grass on the dry mesa. Head and body 2-1/2 to 2-7/8 inches (65-74 mm.), tail 2-1/2 to 3-1/8 inches (66-79 mm.), hind foot 2/3 to 4/5 inch (17-20 mm.), ear from crown 1/3 inch (8-9 mm.), weight about 1/3 ounce (10.2 grams). Tail not tufted. Coloration pale sandy buff above, with numerous black hair endings; under surface pure white.

GREAT BASIN POCKET MOUSE, Perognathus parvus olivaceus Merriam, a medium-sized, soft haired species distributed through much of the Great Basin country east of the Sierra Nevada, was found abundantly in the neighborhood of Mono Lake, being recorded from Silver Lake and near Walker Lake north and east to Mono Lake Post Office and Mono Mills. It inhabits dry sandy situations and makes its burrows under sagebrush. Head and body 2-3/4 to 3-5/8 inches (69-92 mm.), tail 3-1/8 to 4 inches (80-100 mm.), hind foot 7/8 to 1 inch (22-26 mm.), ear from crown about 1/4 inch (6-7 mm.), weight 2/5 to 7/8 ounce (12.5-25.3 grams). Tail not tufted. Coloration above plain buff, with many black hair endings; under surface pure white.

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The pocket mice constitute but one of several groups of small nocturnally active animals which pass unnoticed, even in places where they are abundant, unless special search is made for them. The naturalist when hunting for pocket mice looks at the loose sandy or fine soil about the bases of desert, valley, or foothill bushes, and if he finds little burrows plugged with earth he sets his traps there with the expectation of capturing some of the animals when they come abroad at night. They may not, however, take the bait (usually rolled oats or cornmeal is used) and will thus refuse to disclose their specific identity. The pocket mice are diminutive relatives of the kangaroo rats, their mode of life and niche or place in nature being much the same.

The three species of pocket mice in the Yosemite region belong to distinct groups, and were we as fully informed upon the details of their life histories as we are for example upon those of the chipmunks, these three pocket mice would doubtless merit separate consideration. But at the present time we know little more than their structural characters, their ranges, and the sort of immediate surroundings which each inhabits. The San Joaquin and Great Basin pocket mice live in open situations, recalling in this respect the Merced and Pale-faced kangaroo rats, while the California Pocket Mouse lives in places beneath the foothill chaparral (Adenostoma) which are somewhat gravelly or rocky. The latter species parallels in choice of habitat the Heermann Kangaroo Rat. In certain parts of the country 2 or even 3 species of pocket mice are to be found in a single locality, each occupying a separate type of habitat or niche; but in the Yosemite region the ranges of the 3 species are, distinct geographically as well as ecologically.

Pocket mice are exclusively nocturnal. They spend the day below ground in their short simple burrows, coming out as soon as darkness falls to forage on the surface of the ground. Their mode of progression is like that of a kangaroo; they bound along on the enlarged and proportionately long hind feet, using the tail as a stabilizer and counterbalance. The forefeet come into particular service when the animals feed. Then they function as hands and are used with great dexterity to hold food materials and to thrust these into the fur-lined pouches or pockets on either side of the face. When the cheek pouches are filled with seeds or other food the animals make for their burrows and store the food there for use at times when it is too cold or rainy out-of-doors for them to venture forth.

A specimen of the Great Basin Pocket Mouse was captured alive at the Farrington ranch on June 21, 1916, and retained in captivity for a time. It was kept in a can, well wrapped with cloth. One morning the mouse was found cold and stiff, seemingly dead; but when the sun had warmed the air it revived completely.

One afternoon this mouse was taken to a large clear sandy area and set loose in order that its habits might be observed. It seemed quite averse to facing the sun and would always turn its back to the strong light. In attempting to dig a burrow the mouse used its front feet to shove out the loosened sand. Its actions in this respect resembled somewhat those of a pocket gopher. When not disturbed the mouse moved along the sand slowly like a cat when stalking a bird, but when alarmed the animal bounded over the sand in three-foot leaps using only its hind legs, at such a rate that the observer could scarcely keep up. When offered some rolled oats the mouse, using its forefeet, stuffed the material into its cheek pouches but ate none.

Scanty data were obtained relative to the breeding of the local pocket mice. At Snelling on May 26, 1915, two female San Joaquin Pocket Mice were captured which contained 2 and 6 embryos respectively. A nearly full grown young-of-the-year in the bluish-tinged soft pelage of immaturity was taken at the same station three days later. On the California Pocket Mouse we have only three notes: Two females with mammae conspicuous were taken on May 21 and 27, 1915, at Pleasant Valley, and a nearly grown juvenile was collected at Smith Creek (6 miles east of Coulterville) on July 28, 1920. For the Great Basin Pocket Mouse, although numerous specimens were obtained, the data are likewise scanty. A female containing 3 embryos was captured at Mono Lake Post Office on July 1, 1916, and an immature animal was trapped on Dry Creek, June 12, 1916. At Walker Lake, September 9 to 13, 1915, and near Williams Butte, September 17 to 22, 1915, numerous smooth-pelaged gray-tinged but nearly or quite full-sized young-of-the-year were procured.