Timing of Major Life Events
In late fall and winter, Great Basin pocket mice remain in their burrows in a state of torpor. They emerge from their burrows and mate in early spring. Males emerge slightly before females. In south-central Washington, Great Basin pocket mice emerged from March to April. Prebreeding enlargement of ovaries and testes begins in winter in the complete darkness of the burrow. Following emergence from the burrow, the lengthening photoperiod of spring apparently triggers final enlargement and development of gonads for breeding. Access to succulent green vegetation in spring may enhance reproductive success of females. Captive female Great Basin pocket mice from eastern Washington fed lettuce and seeds had significantly larger ovaries than control females fed only seeds. Great Basin pocket mice remain reproductively active through summer. Females produce one or two litters per year. Most first litters are delivered in May and second litters in August. Reports of average litter size have ranged from 3.9 in south-central Washington to 5.6 in Nevada. First-litter subadults first leave the natal burrow in early summer; subadults from the second litter first emerge in fall. In a 2-year study in south-central Washington, first-litter subadults first emerged in June, and second-litter subadults first emerged in October (1974) and November (1975).
As it signals the beginning of the breeding season, photoperiod may often signal its end. In the laboratory, an artificial short day-long night summer photoperiod caused gonadal shrinkage in Great Basin pocket mice. A favorable diet apparently overrides this effect, however, extending the breeding season. In nature, Great Basin pocket mice remain reproductively active through fall in years of favorable plant production. Juveniles typically breed in their second year, but first-litter individuals may first reach breeding condition before winter when plant productivity is high.
Great Basin pocket mice occupy open, arid terrain. They seek friable soil of a variety of textures for burrowing.
Home ranges of 7,060 to 9,630 square feet (656–895 sq m) have been reported for Great Basin pocket mice in British Columbia. Males may have larger home ranges than females. Average home ranges reported from south-central Washington are 23,030 square feet (2,140 sq m) and 33,640 square feet (3,125 sq m) for adult males and 15,564 square feet (1,446 sq m) for adult females. In big sagebrush habitat on the Malheur National Wildlife Refuge, Oregon, home ranges of adult males were significantly greater (p < 0.001) than home ranges of females. Reproductively active adult males had significantly (p < 0.05) larger home ranges than adult males with unenlarged testes. In black greasewood (Sarcobatus vermiculatus) habitat, however, there were no significant differences between male and female home ranges or between home ranges of reproductive and nonreproductive adult males.
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