This mouse is slender, long-tailed, and has large, naked ears. These mice range in length from 118 to 170 mm. The tail is shorter than the body, measuring between 50 and 96 cm. Western harvest mice typically weigh between 8 and 17 g. The upper incisors have distinct lengthwise grooves. There is no apparent difference in size or coloration between males and females.
The color of the fur on the back ranges from pale-gray to brown, and the fur on the belly ranges from white to deep gray. There is a dark stripe down the middle of the back and along the forehead. There are 3 pelages categories: juvenile, sub-adult, and adult. The juvenile pelage is relatively short and woolly, with grayish brown color. Sub-adult pelage is longer, thicker, and brighter than that of a juvenile. Adult pelage is characterized by one of two patterns. The summer pelage is short and sparse, with brown above and grayish below. The stripe down the back is not clearly demarcated in the summer pelage. The winter pelage, in contrast, is thicker, longer, and paler than the summer pelage. (Keienburg, et al., 1990; Ruff and Wilson, 1999)
Few individuals live more than a year. As would be predicted from this short lifespan, young reach sexual maturity early, at about 1 month of age, and full maturity is reached at about 4 to 5 months. This species breeds from early spring to late autumn, foregoing reproduction only in the most severe winter weather. (eNature.com, 2000; Nowak, 1999; Ruff and Wilson, 1999)
Females have a high reproductive potential, having early sexual maturity and short gestation period of 23 to 25 days. The average litter size varies geographically, but is around 4, and as many as 9 pups can be born at one time. (Webster and Jones Jr., 1982)
Newborns are born naked, pink and blind. Neonates weigh 1 to 1.5 g, are 7 to 8 mm in length, and are totally helpless. They have a slight coating of fur by the time they start to crawl, around 5 days of age. Their incisiors erupt around this time. The eyes and ears are open by around 11 days of age. The young are weaned by 24 days. Young are reported to leave their natal nest around 3 weeks of age. (eNature.com, 2000; Ruff and Wilson, 1999; Webster and Jones Jr., 1982)
Females care for their young in a nest made of grass, nursing them for up to 24 days. The young are born blind and helpless, but grow quickly. The young can leave their natal nest as early as three weeks of age. Males apparently play no role in parental care. (Ruff and Wilson, 1999; Webster and Jones Jr., 1982)
This solitary mouse is nocturnal and mostly active before midnight. Activity is greater on moonless nights or rainy nights. They are sedentary, basing their activity at a nest, and using known runways. (Nowak, 1999; Ruff and Wilson, 1999; Webster and Jones Jr., 1982)is also territorial.
It is known that this mouse undergoes torpor when it meets cold temperatures; however, it is unknown if it undergoes true hibernation. (Royal British Columbia Museum, 1995)
Western harvest mice cache food in their nests, and use runways to navigate their environment. They are not opposed to using the runways of other species, such as Microtus and Sigmodon. (Nowak, 1999; Webster and Jones Jr., 1982)
Western harvest mice build spherical nests. These are usually located directly on the ground, beneath the cover of grasses, bushes, weeds, or logs. These nests are about 125 mm in diameter. The inside of the nest is usually lined with softer, finer plant material, whereas the outer portion has coarser plant fibers. Access to this nest is provided by one or more holes in the base of the sphere. Occasionally, (Webster and Jones Jr., 1982)has been known to build nests in shrubs above ground level, or in burrows.
Like many rodents, (Webster and Jones Jr., 1982)seems to be able to get its bearings when it is placed outside of its normal home range. When displaced up to 1000 feet from its normal range, is able to find its way back.
Desity of these mice varies geographically. They have been measured at densities of 11.9 per hectare to only 4 per hectare. One meas