Identification & Description:
Deer mice are members of the family Muridae. The common deer mouse is classified as Peromyscus maniculatus.

Deer mice are the prototype for “field mice” with large, bulging eyes, big ears, a bicolored pattern and a long tail. They are larger than the harvest mice (Reithrodontomys), but noticeably smaller than woodrats (Neotoma) and cotton rats (Sigmodon).

The deer mouse ranges from around 6 to almost 8 inches in total length including the tail, which is about 1/2 of its total length. Adult deer mice are generally colored a tan or brown above with a white underbelly and throat while young deer mice are colored gray above with white underparts. The ears are large, rounded and mostly hairless and the eyes are also large and bulging. In overall appearance, the deer mouse at first glance looks almost identical to the White-footed mouse with which it may share the same general habitats. They can generally be told apart by looking at the tail. The tail of a deer mouse is bicolored, the top half being slightly darker than below.

The deer mouse coloration is described as “bicolored”, meaning it has a distinctly darker upper body coloration compared to the white undersides. The body color varies from a yellowish or reddish brown to grayish above, with pure white undersides and feet. The bicolored body and distinctive large ears distinguish the deer mouse from the introduced house mouse (Mus musculus), which is uniformly gray. The house mouse is found in city and urban areas while the deer mouse prefers natural habitats. You can get either where development pushes against undeveloped lands.

Range and Habitat

Deer mice are commonly found in BC, California and Mexico. Throughout their range, they are found in nearly all ecological communities and life zones from the desert floor to the high mountains. They can be highly abundant, numbering as many as ten per acre. It is primarily active at night when it emerges from its nest to feed on many types of seeds, berries, acorns, fruits, and even insects and other small invertebrates. Deer mice usually make their nest in a cavity found inside a tree, stump, under logs and sometimes even in abandoned squirrel nests. They line their nest with an accumulation of moss, shredded tree bark, leaves, and other material. Their young can be born from early March on into November with as many as 4 litters a year and 2 to 7 young per litter. Young deer mice grow rapidly and are weaned by the age of 3 weeks and can breed at the age of 5 to 6 weeks. A nocturnal species, they are very energy efficient, reducing their body temperature when in their burrows. Lowering their metabolism means they need less food. Deer mice do not hibernate during the winter.

Reproduction:
Deer mice breed during the spring, and fall, and to some extent midsummer. Females can have up to four litters per year with an average of three to six young per litter. The gestation period is from 22 to 25 days long. Deer mice do not burrow but build their nest from grasses in protected areas above ground beneath debris, in tree cavities, in rotting logs, or in abandoned burrows.

Young mice may remain with the mother for sometime after they are weaned. Although deer mice establish home ranges, there is not an aggressive defense of a home territory. Home ranges are from three-quarters to ten acres in size and are shared with breeding animals of both sexes and a few immature individuals.

Diet:
Food selection is dependent on both habitat and season. Deer mice feed heavily on larvae from lepidopterans (includes moths and butterflies) and other insects in the spring. They can eat large volumes and are capable of ridding an area of many insects that may be detrimental to trees. In the fall, seeds become a major food source and are stored in caches for use during the winter.

Because of their abundance, deer mice are a major food source for almost every bird and mammal predator. When the predators are reduced or absent, the mice can become pests.

Natural History

Deer mice live in burrows they have made, abandoned burrows of other animals, beneath rocks, in stumps, in soil cracks, in debris, or in any other protected location. They build a cup-shaped nest of finely shredded plants and fur. Because they are nocturnal, deer mice use their nest for sleeping during the day. It is also used for raising their young and protecting themselves against winter weather.

Deer mice are active year round and do not hibernate during the winter. However, they still prepare for winter by putting on body fat and storing seeds. Weed seeds form the bulk of their diet during the winter months. Although we often think of mice as eating only plant matter, deer mice, like humans, are omnivorous . In the summer they eat seeds, small fruits and berries, beetles, caterpillars, grasshoppers, leafhoppers, and an underground fungus called endogone. What is eaten will depend upon what is available. When available, insects may provide more than half of their diet.

Because deer mice are abundant and found nearly everywhere, they are a major food for such carnivores as coyotes, foxes, bobcats, weasels, skunks, badgers, grasshopper mice, snakes, owls, and other birds of prey.

Home range is the area an animal normally lives in. In order to survive their many predators, deer mice must be thoroughly familiar with all the details of the landscape in which they live. Their home range c an be as small as a back yard or up to ten acres, depending upon the availability and competition for food. Within their home range , they usually travel along the most convenient routes, which can be the trails of other animals or trails they have created over time. Mice have even been known to trim the plants along their runways. In the winter tracks can sometimes be seen on snow. More often their runways are transformed into tunnels under the layer of snow. When the snow melts in the spring, these runways are often visible in the vegetation and can form fascinating mazes.

Deer mice are able to breed throughout the year, but most often breed in spring and early fall. Gestation is about three weeks and the litter size ranges from 1 to 9 babies with an average litter size of 4. Each newborn weighs about l to 2 grams or about half the weight of a penny. Newborn mice are blind, deaf and have no hair except whiskers. Their skin is so transparent that you can actually see the milk flowing into them when they nurse during their first 24 hours. By the second day their skin gains color and is no longer transparent. Newborn deer mice nurse almost constantly and grow rapidly. By 4 days they will begin to have fur. In one week they will have doubled their weight. And in 2 weeks their eyes open and they begin to move around. At about 7 weeks the females are able to reproduce and at about 8 weeks the males are sexually mature.

Deer mice populations are probably greatest in the fall when females of spring litters have produced their own young. The deer mice population can widely fluctuate from one time of the year to another, from year to year, and from place to place. Factors affecting populations include food supply, predators, and weather. Local flooding in the spring also frequently drowns spring litters.

Concerns
Deer mice may be carriers of Hantavirus. When present, this virus is spread through the rodent’s urine and feces. Although the mice do not become ill from the virus, humans can become infected when they are exposed to contaminated dust from the nests or droppings. We are advised not to camp or sleep where mouse droppings are abundant, and to clean indoor areas where they live using appropriate precautions.

Warning The droppings of the Deer Mouse have been associated with a sometimes fatal illness in humans called hantavirus. Never vacuum or sweep mouse droppings; thoroughly wet the area with a disinfectant, then carefully wipe up the droppings with a wet cloth.