Montana Field Guide

Great Basin Pocket Mouse - Perognathus parvus

The Great Basin pocket mouse is the largest member of the genus Perognathus. Tail length is 110 to 120% of head and body length, and distinctly bicolored. The hind legs are elongate, but not to the extent observed in bipedal heteromyids such as kangaroo rats. They have external, fur-lined cheek pouches, hence the name pocket mouse. The dorsal pelage is pinkish-buff or ocherous-buff overlain with black hairs; the belly is white to buffy. Adults from Oregon attain the following body measurements: total length 138 to 205 millimeters; tail length 53 to 115 millimeters; hind foot 19 to 27 millimeters; and weight 9.5 to 29.5 grams (Verts and Carraway 1998). On the skull, the auditory bullae are not greatly inflated but meet or nearly meet anteriorly, the upper incisors are grooved, the nasal septum is perforated (connecting right and left infraorbital canals), and the molars are hypsodont (high-crowned and fully covered in enamel). There are 20 teeth in the skull (dental formula: I 1/1, C 0/0, P 1/1, M 3/3).

The combination of grooved incisors, hypsodont molars, perforate nasal septum, fur-lined external cheek pouches, and very long tail distinguish the Great Basin pocket mouse from other non-heteromyid rodents. For other heteromyid species in Montana, Ord's kangaroo rat (Dipodomys ordii) differs by having greatly elongated hind legs, a hind foot length more than 30 millimeters, and the mastoid (rear) region of the skull greatly expanded. The hispid pocket mouse (Chaetodipus hispidus) has mastoids that do not project beyond the occipital plane, auditory bullae that are clearly separated, rough pelage, and naked soles on the hind feet. The olive-backed pocket mouse (Perognathus fasciatus) has an unlobed antitragus (fleshy projection) in the ear, an occipitonasal length (anterior tip of nasal bone to posterior tip of occipital bone at base of skull) less than 24 millimeters, a uniformly-colored tail, and a hind foot length less than 20 millimeters. The range of the Great Basin pocket mouse does not overlap in Montana with the above species (Foresman 2001a, 2001b).

Montana Range

The Great Basin pocket mouse is non-migratory. Little other information exists for Montana. Home ranges in British Columbia, Washington, and Oregon are 267 to 4005 square meters, and vary with year, sex, age, population density, and resource availability. The distances home range centers shift between successive years are 2 to 43 meters for males and 1 to 10 meters for females; all evidence indicates the species is relatively sedentary (Verts and Kirkland 1988).

Occupied habitats in Montana are arid and sometimes sparsely vegetated. They include grassland-shrubland with less than 40% cover, stabilized sandhills, and landscapes with sandy soils, more than 28% sagebrush cover, and 0.3 to 2.0 meters shrub height (Hoffmann et al. 1969, Frissell 1978, Hendricks and Roedel 2001, 2002, P. Hendricks unpublished data).

Data from other portions of its range suggest a variety of western arid and semiarid habitats are occupied, including pine woodland, juniper-sagebrush scablands, sandy short-grass steppes, and shrubland covered with sagebrush, bitterbrush, greasewood, and rabbitbrush; heavily forested habitats are avoided. They are captured more often than expected (based on availability) at sites with more than 40% ground cover. On plots where fire killed the shrub cover, the species was one-third as abundant as on adjacent unburned plots. They usually are found in habitats with light-textured, deep soils, and sometimes in shrublands among rocks. Presence is positively correlated with percent sand and negatively with percent clay. Adults sleep and rear young in underground burrows (Verts and Kirkland 1988, Verts and Carraway 1998).

Food habits have not been studied in Great Basin pocket mice in Montana. Information from other areas of the species' range indicate that this species is primarily a seedeater of grasses, legumes, borrages, composites, nettles, and mustards, and in spring also feeds on insects (20 to 25% of diet and contents in cheek pouches) and some green vegetation. Seeds are carried back to burrows in cheek pouches and stored in underground chambers (Verts and Kirkland 1988, Verts and Carraway 1998). They sometimes forage in grain fields but significant damage has not been noted, although they may consume newly planted or sprouted seed, and may caches seeds of one type of grain in a field sown with a different type. Their diet in Montana has not been studied or reported.

Very little is known about this pocket mouse in Montana. Information from other parts of its range suggests the Great Basin pocket mouse is not considered a social animal; individuals occupy separate nests. During winter it enters torpor and is not surface-active, but it may also enter torpor for various periods at any season. Male emergence from winter torpor is in late March or April, depending on location, and females emerge about a month later (Verts and Kirkland 1988). All Montana captures have occurred between mid-June and mid-August (Hoffmann et al. 1969, Frissell 1978, Hendricks and Roedel 2002). It is nocturnal or crepuscular when active away from its burrow.

Frequently it is the most abundant small mammal in the Great Basin, especially in the northern portion of its range where it may represent up to 90% of the small mammals captured. At the community level it is the prime energy mover, and contributes 4 times more to energy exchange than the deer mouse and 11 times that of the northern grasshopper mouse. Population density may reach 80+ per hectare in years with abundant precipitation, and density is positively correlated with October to April precipitation. Predators include rattlesnakes, Burrowing and Short-eared owls, coyotes, weasels, skunks, badgers, foxes, northern grasshopper mice, and deer mice (Verts and Kirkland 1988, Verts and Carraway 1998).

No information on the reproductive biology of Great Basin pocket mice for Montana is available. Information from other parts of its range suggests Great Basin pocket mice are reproductively active during spring and summer. Estrous in Washington is during April and May. Pregnant females in Idaho are present from late May to early August. Gestation probably lasts about 21 to 28 days, with females producing 0 to 3 litters per year, depending on winter-spring precipitation. The number of fetuses per female ranges from 2 to 8 (average about 5). Lactation may extend into September or sometimes October. Young are weaned in about 3 weeks (Verts and Kirkland 1988, Verts and Carraway 1998).

No special management activities are currently recognized as necessary for maintaining viable populations of this species in Montana. Land management designed to maintain a mosaic of sagebrush cover, size, and age classes will benefit this species, especially if it promotes the growth of grasses and forbs within sagebrush stands; large-scale sagebrush removal should be avoided. Livestock probably competes with pocket mice for grasses and reduce shrub and grass cover (Hendricks and Roedel 2001).