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Reithrodontomys megalotis - Western Harvest Mouse


The western harvest mouse is a slim, medium sized harvest mouse with a total adult length of 114-170mm and weighs 9.1-21.9g.  It has a brownish back, buff-colored sides, and a white underside.  There is an indistinct dark broad stripe along its spine.  The tail is 50-96mm, sparsely haired, dorsally grey while white underneath. The ears are naked giving them a prominent, flesh-color or a buffy-cinnamon color.  The western harvest mouse only has 4 digits on the forefeet.  The hind feet are 14-20mm in length with 5 digits. This specie has distinctly grooved upper incisors.  This characteristic is useful in distinguishing it from similar species.


The western harvest mouse is found throughout much of the western United States and extreme southwest Canada.  The range extends as far east as southwestern Wisconsin, northwest Indiana, northeast Arkansas, and western Texas.

Ontogeny and Reproduction:

The western harvest mouse has a high potential reproductive rate.  Females can breed at 4 months of age.  It births litters averaging 4 young, but ranging anywhere from 1-9 young.  The female is polyestrous and has a gestation period of 23 or 24 days.  It breeds throughout the year with the exception of late winter in the southern part of its range.  In northern parts of its range, this harvest mouse breeds only in late spring and summer (usually 2-3 litters).  Captive harvest mice have been known to produce 14 litters in a single year.  The young weigh 1-1.5grams, naked, pink, and blind at birth.  The mouse’s eyes open 10-12 days after birth and they are weaned 19 days after birth.  The western harvest mouse goes through 3 pelages: juvenile, sub-adult, and adult.  The juvenile pelage is rather wooly and dull grey.  The adult pelage is relatively brighter.  Molting starts on the ventral surface and spreads over the flanks to meet on the back.  A second point of origin is on the muzzle, the new coat spreads back forming a molt line behind the ears.  This specie molts once annually during the summer.

Ecology and Behavior:


Across its range the western harvest mouse inhabits sagebrush, steppe, and agricultural areas.  The western harvest mouse dwells in areas below 500meters. It forages in grasslands bordering riparian areas such as irrigation right of ways, coastal marshes, streams, or lakes.  The western harvest mouse is often considered an edge species.  It is important for the western harvest mouse to have shrub or grass overstory with tall lush herbaceous cover to conceal its nests.  The western harvest mouse may have more than one nest within its home range to use as rest sites.  The nests are about the size and shape of a baseball, consisting of grass lined with fine plant material.  These nests are located on the ground in clumps of grass, shrubs, or logs, or hanging from vegetation.  Each nest has a small entrance on its underside leading to a golf ball sized chamber lined with dandelion fluff or a similar material.  The western harvest mouse will not construct burrows but it will often use other mammal burrows for shelter.

Activity Pattern-

The western harvest mouse appears to be strictly nocturnal.  It is most active before midnight, on moonless or overcast nights.  Minimum activity occurs between 6:00 am and noon.  This mouse is active year round utilizing trails built by other small mammals such as field voles.  Captive harvest mice can be induced to enter a shallow torpor by exposure to temperatures below 10oC.  Torpor is essential to the mouse’s survival in northern parts of the range.


The western harvest mouse feeds primarily on the seeds of plants such as: blue grass, fescue, brome grass, oats, vetch, and fruits.  This mouse stores seeds in small caches of sectioned grass blades and stems, along runways occupied by the harvest mouse, for the fall and winter months.  The western harvest mouse consumes an average of 1.63grams of grain per day.  Most of the arboreal activity displayed by this specie is attributed to its diet of invertebrates and seeds.


The western harvest mouse is not a very sociable animal; it is described as ferocious, cannibalistic, not gentle, nervous, and dislikes being handled.  In colonies they seem to be rather sedentary, spending much of their time huddled together.  They are remarkably compatible in mixed colonies of house mice and deer mice.  The mice will even form integrated social hierarchies in the mixed group.


Predators include: snakes, owls, shrikes, weasels, skunks, and coyotes.

Literature Cited:

Bell, D.M., M.J. Hamilton, C.W. Edward, L.E. Wiggins, P.M. Martinez, R.E. Strauss, R.D. Bradley, R.J. Baker.  Patterns of Karyotypic Megaevolution in Reithrodontomys:  Evidence from a Cytochrome b Phylogenetic Hypothesis.  Journal of Mammalogy, 82(1): 81-91. 2001

Howell, A.H. 1914, Revisions of the American Harvest Mice (genus Reithrodontomys):  North America Fauna. 36. pp. 1-97

National Wildlife Federation. Western Harvest Mouse. <>.  Accessed Nov. 12 2003.  

Ritz, C.M., J.O. Whitaker Jr.  Ectoparasites of Small Mammals from the Newport Chemical Depot, Vermillion County, Indiana. Northeastern Naturalist: 10(1):149-158

Royal British Columbia Museum. <>   Accessed Nov. 12 2003.

Skupski, M.P.  1995.  Population Ecology of the Western Harvest Mouse, Reithrodontomys megalotis: A Long-Term Perspective.  Journal of Mammalogy, 76(2):358-367.

Whitaker Jr., J.O. 1994.  National Audobon Society. Field Guide to North American Mammals.  pp.466-467.

 Reference written by Joe Veverka, Biol 378: Edited by Chris Yahnke.
Page last updated 4-29-04.

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