Reithrodontomys megalotis
western harvest mouse

Reithrodontomys megalotis is found over a wide portion of the western United States of America and central Mexico. It is broadly distributed from the Great Lakes to the Pacific Coast. It occurs at elevations from Death Valley, California (below sea level), to 4000 m on the Popocatepetl and Orozaba volcanoes in Central Mexico. (Ruff and Wilson, 1999)

Reithrodontomys megalotis is found in a variety of open areas, including grasslands, prairies, meadows, and marshes. It also inhabits more arid areas such as deserts, sand dunes, and shrublands. (eNature.com, 2000; Ruff and Wilson, 1999)

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)

Reithrodontomys megalotis is a polygynous species, in which the dominant male mates with females during their estrus period. (Webster and Jones Jr., 1982)

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)

Reithrodontomys megalotis is known to undergo a post partum estrus cycle, allowing rapid production of litters. As females reach the age of approximately 45 weeks, there is a reduction in litter size, signalling senility. (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)

Only a few individual reach at the age of 1 year. The maximum reported lifespan for this species is 18 months. (Nowak, 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. Reithrodontomys megalotis is also territorial. (Nowak, 1999; Ruff and Wilson, 1999; Webster and Jones Jr., 1982)

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, R. megalotis has been known to build nests in shrubs above ground level, or in burrows. (Webster and Jones Jr., 1982)

Like many rodents, R. megalotis 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, R. megalotis is able to find its way back. (Webster and Jones Jr., 1982)

Desity of these mice varies geographically. They have been measured at densities of 11.9 per hectare to only 4 per hectare. One measurement of about 60 mice per hectare was made when the gound vegetation improved and became very dense due to rains. (Webster and Jones Jr., 1982)

Home range size apparently varies with location. In California, home ranges of about 3,525 square meters were reported. (Webster and Jones Jr., 1982)

Communication patterns have not been reported for these mice. It is likely that they communicate with conspecifics with a combination of olfactory/chemical cues, vocalizations, and tactile communication, as these avenues of communication are prevalent in rodents.

The primary diet of this mouse is seeds. However, it eats anything available at the time, including new growth of plants and insects (grasshoppers and moths). These animals sometimes cache food in their nests. Reithrodontomys megalotis drinks water. (Nowak, 1999; Ruff and Wilson, 1999; Webster and Jones Jr., 1982)

Because of its small size and abundance, R. megalotis is an important prey species. There are many predators of the western harvest mouse, including owls, hawks, snakes, canids, mustelids, felids, and scorpions. (Royal British Columbia Museum, 1995; Webster and Jones Jr., 1982)

Because of their noctural activity, it is likely that these mice have the best opportunity of avoiding predation by nocturnal predators. These mice are most active on very dark nights, which may be a strategy for avoiding predation by animals that use vision to detect prey. (Nowak, 1999)

This species is essential to western ecosystems. It reproduces rapidly, and lives a very short time, even when removed from the threat of predation. This indicates that the species does not live long in the wild. The most likely source of mortality is predation. (eNature.com, 2000; Nowak, 1999; Ruff and Wilson, 1999; Webster and Jones Jr., 1982)

As a prey species, the availability of R. megalotis likely controls the populations of many predators which rely heavily upon this species in their prey base.

Also, because R. megalotis caches seeds, it probably helps in their dispersal. (Nowak, 1999)

There is no known benefit of this species for humans. However, because they are important in the food web, many of the higher profile animals that people enjoy watching, such as hawks, owls, coyotes, and foxes, rely on them.

There are no reports of these mice actually damaging crops. However, human agriculture has positively affected R. megalotis, allowing it to extend its geographic range eastward. (Nowak, 1999)

These mice are thought to be quite common, and not in danger. However, Canada considers R. megalotis vulnerable because it lives in grasslands. Grasslands are a threatened habitat. Also, there is little known about Canadian populations of Western harvest mice. (Royal British Columbia Museum, 1995)