Western Harvest Mouse are adaptable, widespread, and abundant, especially in meadows, prairies, old pastures, stream valleys, and marshes. They eat seeds, insects, and plants. They rarely live for more than a year, but under optimal conditions, a female can produce more than 50 young in her lifetime. Their nests are built of plant material, usually on the ground, but sometimes in burrows or in vegetation slightly above the ground. Each mouse may have several nests, which it uses at different times. The Mice are nonterritorial and show a great deal of tolerance for one another, even huddling together when it is cold. Such intimate contact carries risks: they are afflicted with many parasites, including protozoans, worms, fleas, chiggers, mites, and lice. They are a vector for a hantavirus that can cause acute respiratory illness and hemorrhagic fever in humans.

Links:
Mammal Species of the World
Click here for The American Society of Mammalogists species account

Known from portions of southwestern Canada, south through the western and mid-western United States, to southern Mexico (Musser and Carleton 2005). Found from below sea level in Death Valley to 4,000 m in central Mexico. (Webster and Jones 1982). There is evidence that the range within the U.S. is expanding to the east as a result of increased habitat alteration for agricultural use (Ford 1977)

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.

Biogeographic Regions: nearctic (Native )

occurs (regularly, as a native taxon) in multiple nations

Canada

Origin: Native

Regularity: Regularly occurring

Currently: Present

Confidence: Confident

Type of Residency: Year-round

United States

Origin: Native

Regularity: Regularly occurring

Currently: Present

Confidence: Confident

Type of Residency: Year-round

Global Range: Southern British Columbia (Okanagan Valley), southeastern Alberta Similkameen Valley), North Dakota, and southern and western Wisconsin south to northern and central Baja California (Alvarez-Casteneda and Rios 2003), Oaxaca, Veracruz, western Texas, western Oklahoma, Kansas, and northeastern Arkansas; east to Indiana (recently invaded); from below sea level in Death Valley to above 3960 m in southern Mexico (Webster and Jones 1982). See Nagorsen (1994 COSEWIC report) for details on distribution in Canada. See Mercado-Morales (1990) for recent records from Mexico.

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.

Range mass: 8 to 17 g.

Range length: 118 to 170 mm.

Average length: 140 mm.

Sexual Dimorphism: sexes alike

Average basal metabolic rate: 0.13 W.

Length: 17 cm

Weight: 22 grams

Sexual Dimorphism: None

Length:
Average: 140 mm
Range: 118-170 mm

Weight:
Range: 8-15 g

See Hoofer et al. (1999) for information on discrimination between R. MEGALOTIS and R. MONTANUS using cranial characters.

Habitat and Ecology

Systems

• Terrestrial

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.

Range elevation: -77 to 4000 m.

Habitat Regions: temperate ; terrestrial

Terrestrial Biomes: desert or dune ; chaparral ; forest ; scrub forest ; mountains

Wetlands: marsh

Other Habitat Features: urban ; suburban ; agricultural ; riparian

Comments: Old fields, meadows, weedy roadsides, agricultural areas, grassy situations within pine-oak forest, and riparian borders. Prefers dense vegetative cover. Also may be found in shrubby arid regions. In Canada, ideal habitat includes dry gullies with dense shrub cover bordering grassland and shrub-steppe rangeland (Nagorsen, 1994 COSEWIC report). Climbs in vegetation. Uses runways made by voles. Spherical nests usually are constructed on the ground under heavy vegetation or in shrubs.

Non-Migrant: Yes. At least some populations of this species do not make significant seasonal migrations. Juvenile dispersal is not considered a migration.

Locally Migrant: No. No populations of this species make local extended movements (generally less than 200 km) at particular times of the year (e.g., to breeding or wintering grounds, to hibernation sites).

Locally Migrant: No. No populations of this species make annual migrations of over 200 km.

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.

Animal Foods: insects

Plant Foods: leaves; seeds, grains, and nuts; fruit; flowers

Foraging Behavior: stores or caches food

Primary Diet: herbivore (Granivore )

Comments: Prefers seeds of weeds and grasses, but also eats some herbaceous material. May climb into tumbleweeds, apparently searching for seeds (Hoffmeister 1986).

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.

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.

Ecosystem Impact: disperses seeds; keystone species

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.

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.

Known Predators:

• snakes (Serpentes)
• owls (Strigiformes)
• shrikes (Lanius)
• squirrels (Sciurinae)
• weasels (Mustelinae)
• skunks (Mephitinae)
• foxes (Vulpes)
• raptors (Falconiformes)
• coyotes (Canis latrans)
• short-tailed shrews (Blarina)
• cats (Felidae)
• scorpions (Scorpiones)

Reithrodontomys megalotis is prey of:
Strigiformes
Serpentes
Accipitridae
Sciuridae
Lanius
Blarina
Mustela
Canis latrans
Vulpes vulpes
Vulpes velox
Urocyon cinereoargenteus
Mephitinae

This list may not be complete but is based on published studies.

Reithrodontomys megalotis preys on:
Insecta

This list may not be complete but is based on published studies.

May forms mixed-sex social units dominated by a male.

Home range usually is about 0.5-1.5 acres.

Density commonly may be about 5-10 per acre, up to 60/acre in optimum habitat (Gray 1943, Whitford 1976). In Canada, Nagorsen (1994 COSEWIC report) reported 1-7/ha. In Arizona, density was highly variable over 12 years, ranging from local extirpation to about 13 per ha; density averaged about 5-6 per ha in winter, about 1 per ha in late summer-early fall (Skupski 1995). In Wisconsin, autumn densities varied from about 0.75/ha in sandy fields to 45/ha in an abandoned field with a dense cover of low vegetation (Jackson 1961, Svendsen 1970). (Skupski 1995). Populations may decline during peaks in vole abundance. In Kansas, populations declined following grassland fire (Kaufman et al. 1988); moved to unburned area (Clark and Kaufman 1990).

Long-distance movements (up to 3200 m) were recorded in Kansas; vast majority moved less than 300 m (Clark et al. 1988). Movements of up to at least several hundred meters occurred in Arizona, but distance moved from one month to the next usually was less than 90 m (Skupski 1995).

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.

Communication Channels: visual ; tactile ; acoustic ; chemical

Perception Channels: visual ; acoustic

Comments: Primarily nocturnal; most active on moonless, rainy nights. In California, starts moving along vole runways about 1/2 hour after sunset, ends activity in runways about 1/2 hour before sunrise.

Only a few individual reach at the age of 1 year. The maximum reported lifespan for this species is 18 months.

Range lifespan

Status: wild: 18 (high) months.

Typical lifespan

Status: wild: 12 (high) months.

Observations: Little is known about the longevity of these animals, but one specimen lived at least 3.6 years in captivity (Richard Weigl 2005).

Reithrodontomys megalotis is a polygynous species, in which the dominant male mates with females during their estrus period.

Mating System: polygynous

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.

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.

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.

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.

Breeding interval: Breeding interval varies geographically, with animals in mild climates breeding approximately once per month, year round.

Breeding season: Wild western harvest mice breed from early spring to late autumn, foregoing reproduction only in the worst of winter weather..

Range number of offspring: 1 to 9.

Average number of offspring: 4.

Range gestation period: 23 to 25 days.

Average weaning age: 24 days.

Average time to independence: 3 weeks.

Average age at sexual or reproductive maturity (female): 4 to 5 months.

Average age at sexual or reproductive maturity (male): 4 to 5 months.

Key Reproductive Features: iteroparous ; seasonal breeding ; year-round breeding ; gonochoric/gonochoristic/dioecious (sexes separate); fertilization ; viviparous ; post-partum estrous

Average birth mass: 1.33 g.

Average number of offspring: 3.3.

Average age at sexual or reproductive maturity (male)

Sex: male: 80 days.

Average age at sexual or reproductive maturity (female)

Sex: female: 107 days.

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.

Parental Investment: altricial ; pre-hatching/birth (Provisioning: Female, Protecting: Female); pre-weaning/fledging (Provisioning: Female, Protecting: Female); pre-independence (Provisioning: Female, Protecting: Female)

Breeds year-round but mostly early spring to late autumn. In southeastern Arizona, breeding occurred year-round, with a peak in August when food resources likely were high (Skupski 1995). Gestation lasts 23-24 days. Litter size is 1-6 (mean 4). Young are weaned in slightly less than 3 weeks. Sexually mature in 2-4 months. May produce multiple litters annually.

Barcode of Life Data Systems (BOLDS) Stats
Public Records: 0
Species: 9
Species With Barcodes: 1

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.

US Federal List: no special status

CITES: no special status

IUCN Red List of Threatened Species: least concern

Canada

Rounded National Status Rank: N2 - Imperiled

United States

Rounded National Status Rank: N5 - Secure

Rounded Global Status Rank: G5 - Secure

Population

Population Trend

Major Threats

Comments: In Canada, limiting factors include habitat disturbances from livestock grazing, hay mowing, fire, cultivation, and especially habitat loss and fragmentation from urban development (Nagorsen, 1994 COSEWIC report).

Conservation Actions

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.

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.

Comments: Several of the 16-17 nominal subspecies listed by Webster and Jones (1982) and Hall (1981) have not been subjected to a modern taxonomic analysis; the validity of these subspecies is open to question. A morphometric and allozyme analysis by Collins and George (1990) indicated that the southern coastal California subspecies catalinae, limicola, and santacruzae do not merit taxonomic recognition and should be relegated to synonymy under R. m. longicaudus, which name would apply to all populations in southern California west of the foothills of the San Gabriel, San Bernardino, and San Jacinto mountain ranges, as well as to those on the Channel Islands. Hoffmeister (1986) found no significant cranial or external differences between subspecies megalotis and the nominal subspecies arizonensis, and he regarded the latter as a synonym of the former.

Reithrodontomys zacatecae of Mexico formerly was included in R. megalotis (see Musser and Carleton, in Wilson and Reeder 1993, 2005). In a mitochondrial DNA analysis, Bell et al. (2001) confirmed that Reithrodontomys zacatecae is indeed distinct from R. megalotis, but those two species along with R. sumichrasti formed a monophyletic clade.

Substantial chromosomal variation in R. megalotis suggests that other species may be lumped in this taxon (Musser and Carleton, in Wilson and Reeder 1993, 2005).