Two species of harvest mice inhabit Colorado, the western harvest mouse and the plains harvest mouse. These are dainty, brownish mice. Plains harvest mice are only five inches long (of which half is tail) and weigh three-eighths of an ounce; western harvest mice are about an inch longer and half again as heavy. The animals are so small that they often huddle for warmth. The two species are hard to distinguish, but the plains harvest mouse has a narrower black stripe on its tail and tends to have a blackish patch on the back.
Range: Both Coloradoan species of harvest mice live on the eastern plains, and the western harvest mouse also inhabits the valleys of western Colorado and the San Luis Valley.
Habitat: Plains harvest mice live in open grasslands, whereas western harvest mice like a diversity of ecosystems, including shrubby grasslands, weedy borrow pits and riparian woodland. Harvest mice are smaller than deer mice and have grooves on the faces of their incisors.
Diet: Harvest mice eat mostly seeds of grasses and forbs, but like many granivorous animals they also eat some insects as available. They are active year-round. They fatten in fall but do not store food. Virtually all nocturnal, carnivorous animals feed on harvest mice; but the mice are too small to be a dependable source of food.
Reproduction: Reproduction occurs throughout the warmer months. Females produce several litters of three to five hairless, blind young after a gestation period of about three weeks. At three weeks old they are weaned, and they may breed at four weeks of age.
By David M. Armstrong
Department of Ecology & Evolutionary Biology
Environmental Studies Program, University Museum of Natural History
University of Colorado-Bouldermausmann@aol.com