BIG-EARED WHITE-FOOTED MICE. Peromyscus truei (Shufeldt)⁹

Field characters.—Size about twice that of House Mouse, and somewhat greater than Common White-footed Mouse; tail about equal to head and body; ears large (20 mm. or over). (See fig. 10d) Pelage long and dense; tail well haired, with a slight 'pencil' at tip. General coloration above dark brown, sharply set off from pure white of under surface; feet white.

Occurrence.—Resident on west slope of Sierra Nevada, chiefly in Upper Sonoran Zone. Recorded from Pleasant Valley eastward to 6 miles east of Coulterville and to El Portal (subspecies gilberti). Also east of Sierra Nevada on Williams Butte (subspecies truei).⁹ Lives about rocks and brush. Nocturnal.

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⁹Two slightly differing subspecies of this group occur on the opposite slopes of the Sierra Nevada. Their ranges do not touch at any point in the Yosemite region, but they do come together at other localities to the south, and intergradation takes place there. Hence the two are considered as subspecies of one species.

GILBERT WHITE-FOOTED MOUSE, Peromyscus truei gilberti (Allen). The form common through much of California west of the Sierra Nevada. It was found locally from Pleasant Valley eastward to Smith Creek (6 miles east of Coulterville) and to El Portal.

TRUE WHITE-FOOTED MOUSE, Peromyscus truei truei (Shufeldt). A slightly paler, more silky haired subspecies, which occupies a wide range of territory east of the Sierra Nevada. It was recorded by us on the south slope of Williams Butte in small numbers.

Measurements.—Gilberti: head and body 3-1/2 to 4-1/3 inches (88-110 mm.), tail 3-3/4 to 4-1/3 inches (94-111 mm.), hind foot 7/8 to 1 inch (23-25 mm.), ear from crown 4/5 to 1 inch (20-26 mm.), weight 5/6 to 1-2/5 ounces (23.5-41 grams). Truei: head and body 3-2/3 to 4 inches (91-100 mm.), tail 3-2/3 to 4 inches (92-100 mm.), hind foot about 7/8 inch (22.5-24 mm.), ear from crown 4/5 to 1 inch (20-24 mm.), weight 7/10 to 1-1/6 ounces (20.6-33.0 grams).

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White-footed mice of the big-eared or truei group are present on both slopes of the Sierra Nevada but never in as large numbers or so widely distributed as the common species (maniculatus). The range of this species on the west slope lies chiefly in the Upper Sonoran Zone; on the east side it was found in the belt of piñon pines. The Gilbert Mice of the west side were found on the ground in brushy places or under pines and other trees near the chaparral, and on one occasion about a deserted building. The True Mice of the east slope were mostly taken in or near rocks, though this species is not necessarily a rock dweller.

At Blacks Creek, west of Coulterville, on the nights of May 10 and 11, 1919, a line of 37 traps, set through a mixed stand of brush plants on a shaly hillside, produced 7 and 8 Gilbert Mice. They seemed to be the only mice of the white-footed group (Peromyscus) present there. They were evidently finding daytime shelter in heaps of shale at the mouths of old prospect holes, in weathered outcrops of the same rock, and in tangles of dead brush.

At El Portal and Pleasant Valley the numbers of this species were less than those of the Gambel and Boyle white-footed mice though greater than those of the Parasitic Mouse. No conspicuous differences in habitat were evident between these several species and it yet remains to work out their ecology. A postulate in animal distribution is that no two species can permanently occupy exactly the same niche in nature, and the evidence in many cases is convincing. But with the several species of white-footed mice there is still much to be explained in this connection.

One of the Gilbert Mice obtained at Blacks Creek was caught only by the tail and as it seemed uninjured it was carried to a pool in the creek bed to test its swimming abilities. The instant it touched the water the mouse began to swim, using all four feet, and soon gained the bank. It walked slowly up the rocks, but when the observer made an attempt to follow, the mouse put on speed, ran quickly up the hill, and was lost to view in the brush.

The breeding season of this mouse is not known with any certainty. Between May 25 and June 3, 1915, 5 females, each containing 3 or 4 embryos, were trapped. Blue-pelaged juvenals were obtained at about the same time, and suckling females were captured in July. These meager data point to a breeding season three months in extent, with the probability that it is of somewhat longer duration.