Racial characters.—Similar to columbiensis but underparts darker and tail paler. Larger and paler than oregonensis.
Measurements.—Three females from the Cascades average: total length 327; length of tail 145; hind foot 40.7; ear 25.
Distribution.—The Cascade Mountains, according to [Taylor] and [Shaw] (1929: 18), as revised here, east to Entiat River and south to Carson.
Perognathus parvus (Peale)
Great Basin pocket mouse
Description.—Great Basin pocket mice are slightly larger than house mice. The ears are tiny and round; the tail is relatively long, slightly longer than the head and body; the forefeet are small but the hind feet are large and powerful, more than one-fourth as long as the head and body. They have relatively large, fur-lined, external cheek pouches. The color of the upper parts varies with the subspecies from blackish olive-gray to buffy olive-gray; a line of clear buff or buffy-olive extends along the lower part of side, separating the darker color of the upper parts from the white underparts; the tail is short-haired, blackish above, olive beneath.
Pocket mice of the genus Perognathus range from Mexico northward to British Columbia. Only one species, parvus, occurs in Washington where it is represented by three subspecies. Pocket mice are most common in the Upper Sonoran Life-zone in sandy areas dotted with desert shrubs. They are found occasionally in dry, grassy places in the Arid Transition Life-zone. They may be locally abundant in rocky areas and are often trapped high on talus slides, many yards from the nearest soil. [Gray] (1943: 191-193) estimates their numbers on the sagebrush areas of the Yakima Valley at 32 per acre. They are completely nocturnal. Their ordinary movements are rather slow and specimens studied by the aid of a searchlight usually crept quietly into the protection of the nearest desert shrub. The trail of a pocket mouse in soft sand may be recognized by the distinctive mark left by the dragging tail.
Burrows of pocket mice usually are at the bases of shrubs where tough roots furnish protection. They are kept closed during the day by means of a plug of fresh earth or sand. Openings may often be recognized by a fan-shaped pile of fresh sand before the hole. Burrows excavated were usually less than four feet in length and branched from two to four times. No nests were found in the burrows but one contained a few fresh stalks of desert annuals. The air in the burrows seemed warm and humid.
The pouches of pocket mice from Washington often contain the fresh, green tips of desert plants, grass seeds, seeds of plants other than grasses, and plant leaves. Because of their occasional great abundance, pocket mice may be a menace to agriculture. Fortunately much of their habitat is unsuited to farming.
In Washington pocket mice breed in March and April. From 4 to 8 embryos were found in pregnant females.