Reithrodontomys megalotis (Baird, 1858)
Western Harvest Mouse
Basis for Listing
The Western Harvest Mouse (Reithrodontomys megalotis) occurs across the western United States, from the Rocky Mountains and the Upper Midwest south to Mexico. In recent years, they have extended their range as far east as Indiana. The current range of the Western Harvest Mouse in Minnesota is the southern third of the state (Prairie Parkland and Eastern Broadleaf Forest provinces), where they occur in small isolated populations. The Western Harvest Mouse thrives in open prairie habitats, but agricultural practices in southern Minnesota have restricted their habitat (Hazard 1982). Due to this lack of prairie habitat, limited populations are found in “waste areas” such as fence rows, grassy fields, and unmown roadsides in the state. The Western Harvest Mouse was listed as a species of special concern in Minnesota in 2013.
The Western Harvest Mouse is one of the smallest rodents in Minnesota, with an average total length of 13.6 cm (5.4 in.) and an average weight of 11.8 g (0.42 oz.) (Hazard 1982). They are brown, with a whitish gray underbelly. They have prominent ears and a long slender tail. Similar species include the House Mouse (Mus musculus) and Plains Pocket Mouse (Perognathus flavescens); however, the House Mouse is more uniformly colored and the Plains Pocket Mouse is lighter brown, with external cheek pouches (Hazard 1982).
The Western Harvest Mouse prefers upland prairie habitats. Within Minnesota, the species is typically found in ditches, fence rows, and grassy areas due to habitat destruction and fragmentation (Minnesota's Remaining Native Prairie).
Biology / Life History
Very little is known about the life history of the Western Harvest Mouse in Minnesota; however, it has been studied in other states. Western Harvest Mice shelter in burrows. Their nests may be in burrows but are usually above-ground, attached to a clump of grass or located at the base of a shrub. The nest is spherical and is composed of woven grasses, with softer plant matter on the inside of the nest. An opening is located near the base, and any damage to the nest is immediately repaired (Webster 1999). Western Harvest Mice have a high reproductive potential, with females reaching sexual maturity as rapidly as 4-weeks of age (Webster 1999). They are known to have several litters a year, with gestation lasting 23-24 days and a maximum litter size of eight. The young begin to crawl after five days, and their eyes and ears open on day 11-12 (Webster and Jones 1982); young are completely weaned by day 24. In warmer climates, Western Harvest Mice will reproduce year-round; whereas, in Minnesota they are only sexually active from spring to autumn. Western Harvest Mice are not territorial and are extremely tolerant of each other. They are often found huddled together in a nest.
Western Harvest Mice are strictly nocturnal and forage for food at night. They are mainly granivorous, eating the seeds of prairie grasses and forbs, but they are opportunistic eaters and will also eat invertebrate larva and herbaceous material. They are agile climbers and are able to scale shrubs and vegetation to forage on seeds.
Conservation / Management
Western Harvest Mouse populations are in decline due to a lack of habitat. They prefer prairie habitat and thrive in open grasslands. Historically, their range consisted of the prairies of southern Minnesota, but agricultural practices have destroyed much of the prairie (Minnesota's Remaining Native Prairie). Hence, very little suitable prairie habitat exists for Western Harvest Mice, and what remains is fragmented. They currently occupy restored prairies, waste areas, and ditches. The establishment of prairie corridors would help connect the populations and provide more habitat for the species.
Conservation Efforts in Minnesota
Surveys targeting the Western Harvest Mouse by the Minnesota DNR’s Minnesota Biological Survey have documented a number of locations for this species. Other surveys conducted in western counties by Bruns Stockrahm (1991) failed to find any individuals of this species. Additional surveys are needed to locate new populations, and known populations should be monitored to determine the current status of this species in Minnesota.
References and Additional Information
Hazard, E. B. 1982. The mammals of Minnesota. University of Minnesota Press, Minneapolis, Minnesota. 280 pp.
Stockrahm, D. M. B. 1991. Distribution of small mammals in grasslands of western Minnesota with special emphasis on the Prairie Vole (Microtus ochrogaster), the Northern Grasshopper Mouse (Onychomys leucogaster), the Plains Pocket Mouse (Perognathus flavescens), and the Western Harvest Mouse (Reithrodontomys megalotis). Final report submitted to the Minnesota Department of Natural Resources. 53 pp.
Webster, W. D. 1999. western harvest mouse (Reithrodontomys megalotis). Pages 558-560 in D. E. Wilson and S. Ruff, editors. The Smithsonian book of North American mammals. Smithsonian Institution Press in association with the American Society of Mammalogists, Washington, D.C. 816 pp.
Webster, W. D., and J. K. Jones, Jr. 1982. Reithrodontomys megalotis. Mammalian Species 167:1-5.