A small, scaly-tailed mouse with a distinct notch in the cutting surface of upper incisors (seen best in side view). Their hair is short and ears moderately large and naked with pale brown skin. The upper-part of this mouse is usually reddish tinted brown, suffused with black, and their belly is white. Their tail is usually brownish with a black tip, not distinctly bicolor, but paler on underside. External measurements average: total length, 169 mm; tail, 93 mm; hind foot, 18 mm. Weight of adults, 17-25 g.
These mice are exceedingly prolific breeders; as many as 13 litters can be produced in one year. The number of young per litter averages about six. The gestation period is approximately 19 days, varying from 18 to 20. At birth the young mice are nearly naked with their eyes and ears closed. They develop rapidly; at the age of 3 weeks they are fully weaned and at the age of 4 weeks some of the young females are ready to assume family duties, although the average age of sexual maturity is about 35 days in females and 60 days in males. With commensals, breeding occurs throughout the year although it is somewhat curtailed in the colder months.
Although largely nocturnal, house mice are moderately active during the day, chiefly in their quest for food. In the wild they feed on a variety of plant material, including seeds, green stems, and leaves. Alfalfa hay, either in shocks or in stacks, affords an ideal source of food supply and, consequently, it is frequently infested with these mice. As commensals, house mice feed on practically any type of food suitable for the use of man or beast. They are particularly obnoxious around granaries, feed houses, and stores and may do considerable damage in destroying or contaminating food supplies intended for human consumption. In addition they will feed on such animal matter as insects and meat when available.
Although these mice are destructive when allowed to run free, they are widely used in laboratories as subjects for biological, genetic, and medical studies. When ranging free, however, they do a considerable amount of damage although they are not nearly so troublesome as the rat.