The western harvest mouse (Reithrodontomys megalotis) is a small neotomine mouse native to most of the western United States. Its range extends from southwest British Columbia and southeast Alberta continuously to west Texas, northeast Arkansas, northwest Indiana, southwest Wisconsin, and the interior of Mexico to Oaxaca. Many authorities consider the endangered salt marsh harvest mouse to be a subspecies, but the two are now usually treated separately.
Description and comparison with similar species
They have brownish fur with buff sides, a white belly, and an indistinct white stripe on the fur along the spine. Adults grow up to eleven to seventeen centimeters in length with a tail length of five to ten centimeters. Their height (from the ground to the highest point of their back) is between 1.5 and 2.0 centimeters. A mature mouse weighs anywhere from nine to twenty-two grams.
It is a nocturnal, with particularly intense activity on very dark nights. This mouse is particularly resourceful, making use of the ground runways of other rodents. It is also a very agile climber. Its primary food source is seeds, but springtime dining is augmented with new plant growth. In June, July and August the mouse is known to consume certain insects, especially grasshoppers and caterpillars. It stores seeds and other foodstuffs in underground vaults. Its many predators include the fox, weasel, coyote, hawk, snake and owl species.
Similar species are the plains harvest mouse, which has a more distinct but narrower stripe on its spine, and the fulvous harvest mouse, which has a longer tail. Also similar is the salt marsh harvest mouse, which has an underbelly fur that is more pinkish cinnamon to tawny. Finally, the house mouse has incisors without grooves, unlike those of the western harvest mouse.
Breeding nests are spherical constructions woven from grass or other plant material. A nest is approximately 13 centimeters in diameter and lined with a more downy material of fibrous plants. A nest may have one or more entrances near its base. Most commonly, the nest is built on the ground in a protected area such as within a shrub or beside a fallen tree; however, the mouse will occasionally place the nest aboveground within a shrub.
It breeds from early spring to late autumn, with reduced activity at midsummer. The gestation period is 23 to 24 days. Repeated fertilization often occurs immediately after giving birth. It is not uncommon for a female to have ten to fourteen litters per annum, with a typical litter size of two to six individuals; however, litters of up to nine offspring can occur. Thus an annual production of forty to sixty young per female is normal. The newborn mice weigh approximately 1.0 to 1.5 grams.