ALLEN JUMPING MOUSE. Zapus pacificus alleni Elliot

Field characters.—Body size somewhat larger than that of House Mouse; tail very long, one-third longer than head and body; tail almost bare of hairs, and scaly (pl. 26a). Front surface of upper incisor teeth grooved. Head and body 3-3/8 to 4 inches (86-102 mm.), tail 4-3/4 to 5-1/2 inches (120-140 mm.), hind foot 1-1/8 to 1-1/3 inches (28-33 mm.), ear from crown 1/2 to 2/3 inch (12-16 mm.); weight about2/3 to 7/8 ounce (18-24.5 grams). Coloration above bright reddish yellow with a dark tract along middle of back; whole under surface pure white; tail and feet dusky.

Occurrence.—Common resident in Canadian and Hudsonian zones on both slopes of Sierra Nevada. Recorded from Merced Grove Big Trees and Chinquapin eastward to Mono Lake Post Office and Walker Lake. Present in Yosemite Valley about foot of Yosemite Falls. Lives in wet meadows and cañon bottoms close to water. Nocturnal.

Besides the meadow mice and shrews in the high mountain meadows there is present, amid the same surroundings, another mammal not familiar to many people, namely, the Jumping Mouse. In general form of body and mode of progression this animal recalls the kangaroo rats and pocket mice found at lower levels on either side of the Sierra Nevada, but its habitat predilections are quite different, for it lives in damp meadows and along the banks of streams.

The body of the Allen Jumping Mouse is perhaps a fourth larger than that of a House Mouse; it is similar in size to a Gambel White-footed Mouse. It is of slender form, the ears are comparatively small, and the pelage is long haired and rather harsh. The forelegs and feet are relatively short and small; the hind legs and particularly the hind feet are proportionately very much longer in the jumping mouse than in the white-footed mice. The tail is the most striking feature; it is fully one-third again the length of head and body (pl. 26a). These departures from the normal mouse form are all adaptations to the particular and unusual mode of progression used by this mouse. Instead of running on the surface with all four feet, it bounds along, using the hind pair of feet alone for propulsion and the tail as a counterbalance and support.

At Chinquapin an Allen Jumping Mouse was captured alive, the tail only having been slightly injured by the trap. The animal was released on a sunlit slope and an attempt was made to photograph it, but to no avail. It was off on the instant, bounding downhill two to three feet at a jump. These leaps were the results of catapultic extension of the two hind legs simultaneously. The front feet apparently took no part in the leaping. All the movements were so rapid that it was impossible to observe in detail the methods of its locomotion. Upon reaching a small pool the mouse took to the water readily, and swam steadily and rapidly.

Near Porcupine Flat a jumping mouse was startled from its nest at 7:30 A.M. on June 25, 1915. One of us, in walking through a small grassy place beside a stream, chanced to touch the nest where the animal had been resting and it thereupon darted out into the open. In three leaps it covered about 8 feet and then stopped, humped up in the shadow at the butt of a willow stalk but not under cover. There it remained motionless for some minutes, with its eyes closed and its long tail curled around to one side of its body. When in motion the reddish yellow color of the animal quickly attracted the naturalist's eye; and when the mouse came to rest, partly in sunlight and partly in shadow, its coloration was anything but protective against the gray stem of the willow and the brown leaf-littered ground. Later an unsuccessful effort was made to drive the mouse out into the open, but it alertly avoided this, and darted off in a zigzag course among the willow roots, always by quick hops, barely touching the ground at each bound. Finally it disappeared in a hole beneath a clump of willows.

The nest out of which the mouse at Porcupine Flat was flushed was a spherical affair about 5 inches (130 mm.) in diameter, snugly ensconced in a depression in the ground and surrounded and overtopped by dead and new grasses. There was a short curved outlet run at one side about one inch (25 mm.) wide and 5-1/4 inches (130 mm.) long which led directly into the grass and then disappeared. The nest proper consisted externally of long flexible blades of grass of the previous year's growth; these were arranged concentrically around the outside. Within was a soft lining which consisted of finely shredded, last-year's grass blades together with a few green ones. It was all perfectly dry though the surrounding meadow was, as usual at this season, quite damp. This nest may have been merely 'living quarters' for an adult, and not intended for the reception of young.

A local colony of Allen Jumping Mice was found on the floor of Yosemite Valley near the foot of Yosemite Falls. To our own senses the air is notably cold in that particular part of the Valley. A cold breeze comes down from the falls much of the time, and the ice-cold water dropping directly from snow fields 3000 feet above until well along in summer also affects the temperature of the place. There is, in addition, a thick canopy of shade from the enclosing dense stand of yellow pines, incense cedars, white firs, and black oaks. In this 'boreal' spot jumping mice, characteristic of the Canadian and Hudsonian zones above the Valley rim, were present in numbers during June of 1915. The ground was covered with a mat of dead pine needles and deciduous leaves, and there were scattered plants of thimble berry, azalea, creek dogwood, and fern. Possibly colonies of this rodent occur elsewhere in similar, cool situations on the Valley floor, but we found no others.

No data were obtained as to breeding, save that indications pointed to the summer months as the breeding period. Elsewhere it has been found that the Allen Jumping Mouse has rather large litters, 6 perhaps being an average. Immature individuals about two-thirds grown were captured at Merced Lake August 25 to 29, 1915, and near Williams Butte September 15, the same year.