Western Harvest Mouse

This species is a slim, long-tailed mouse that has prominent, naked ears, a slender sparsely haired tail, grey above and whitish below. The harvest mouse has three pelages: juvenile, sub-adult, and adult. The juvenile pelage is rather woolly and dull grey. The adult pelage is the brightest. Moulting starts on the ventral surface and spreads over the flanks to meet on the back; then the new fur spreads fore and aft. A second point of origin is on the muzzle, the new coat spreading back to form a moult-line behind the ears. There is one annual moult each summer. It has four toes on the forefeet and five on the hind feet. 

A small murid rodent, the Western Harvest Mouse is a naturally rare species associated with the grassland rodent communities of western North America. It has been extensively studied in the United States.

Across North America, the Western Harvest Mouse inhabits sagebrush steppe and agricultural areas in areas below elevations of 500 m. It forages in grasslands bordering riparian areas such as irrigation right-of-ways, coastal salt marshes, streams or lakes; and in deciduous ravines of willow, rose and trembling aspen. 

The Western Harvest Mouse may have more than one nest within its home range to use as rest sites. The nest site is about the size of and shape of a baseball, consisting of grass lined with fine plant materials and can be found on the ground in clumps of grass, shrubs or logs or hanging from vegetation. There is a tiny entrance on the under side, which leads into a golfball-sized chamber, lined with the finest of plant material such as down or dandelion fluff. This species does not construct burrows, although it will use other small mammal burrows for shelter. 


The potential reproductive rate is high in the Western Harvest Mouse. The female are polyoestrous , breeding throughout the year in southern parts of the range with the exception of late winter. Towards the northern limits of its range , where the seasons are pronounced, it breeds in late spring and summer. Females are capable of breeding at about four months age and have three pairs of mammae: one pectoral and two inguinal. Pregnant or lactating females were captured between June and September. Although females may produce as many as 14 litters per year in captivity, wild females probably produce two or three litters. The gestation period is 23-24 days, with a litter size at birth averaging 2.6, but the number of young varies from one to nine. 

The young harvest mice weigh approximately 1.0 to 1.5 g at birth. They are pink, naked, and blind. Their lower incisors appear at four days; the pelage is visible at five days; the eyes open between the tenth and the twelfth day, and they begin to walk at about the same time. The young are weaned at 19 days and continue to develop rapidly 

There have been no comprehensive behavioural studies on this species. The available data consist of descriptive observations from field studies or captive individuals. Wild and captive R. megalotis appear to be strictly a nocturnal creature that is mostly active between before midnight, on moonless or rainy nights. Minimum activity occurs between 6 am and noon. It is active the year round, utilizing trails built by other small mammals such as field voles and its relatives. 

The information on the sociability of this species is rather conflicting. It has been described as 'ferocious,' 'cannibalistic,' 'not gentle,' and 'nervous,' and it has been reported that it dislikes being handled. In colonies, harvest mice appear rather sedentary and spend much of their time clustered together. On the other hand, they are remarkably compatible in mixed colonies of house mice and deer mice. They often cluster together with these other species and even form integrated social hierarchies in the mixed group. 

Captive Western Harvest Mice can be induced to enter shallow torpor by exposure to temperatures below 10C. The ability to enter shallow torpor presumably is an adaptation for conserving energy during periods of stress from food deprivation, water shortage, or cool ambient temperatures. Torpor may be critical for the survival of northern populations because they are at the extreme northern limits of their range, where they may be exposed to cool temperatures. It is unknown if this species is capable of hibernation. 

Diet or Growing requirements
Dietary data are based on studies done in California and the Great Plains of the United States. The summer diet of the Western Harvest Mouse consists primarily of seeds and invertebrates including: blue grass, fescue, bromegrass, oats, vetch, fruit and insects. Flowers, herbaceous material, and Endogone fungi are also consumed. Small caches of sectioned grass blades and stems may be found in runways occupied by harvest mice. In the late summer and fall, the diet consists mainly of seed from grasses and forbs. Some food is stored away against the arrival of inclement weather. Harvest mice also eat insects such as moths and grasshoppers. The average food consumption has been found to be 1.63 g of oats per day. The arboreal activity of R. megalotis in shrubs is probably related to foraging for seeds, flowers, and invertebrates. 

Rodents as diminutive as the Western Harvest Mouse must be on guard against a host of possible predators including: snakes, owls, and shrikes, and carnivores, such as weasels, skunks, and coyotes. 

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