The most common species of Old World mice is the house mouse (Mus). A genus originating in southern Asia, the house mouse includes about 44 separate species; only one species is found in the United States. The body of the house mouse measures about 2.5-3.75 in (6-10 cm) long and is covered by brownish gray fur. Its tail, naked and scaly, typically measures about the same length as its body. Its ears and legs are fairly large.
One of the oldest known forms of domestic rodent pests, house mice have adapted their lives to human habitats. Often living in buildings and making nests behind paneling and beneath floorboards, house mice thrive in large cities and on farms. These mice are able to breed at three months old and have life spans of about four years. Typically, in a given year, a female house mouse can bear from four to six litters of four to eight young, although it is not unheard of for a litter to contain as many as 13 young. Like many other species of mice, the gestation period lasts three weeks and the young are born bald with their eyes shut. At about 13 days old, their eyes open, and fine hair covers their bodies. While the young initially start to feed on their own at around 17 days old, they nurse from their mothers until they are four weeks old. Interestingly, when a population of house mice grows too large for a given area, a form of natural birth control takes effect. Reproductive rates fall dramatically because adolescent females become infertile as their reproductive organs fail to mature or become inactive.
House mice live in family groups. The mice commonly groom each other, particularly on the backs of their necks where they are unable to groom themselves. Mutual grooming occurs daily in most mouse families. Within these family groups, the males have clearly defined ranks. These rankings are not indisputable, however. House mice fight and display threatening and submissive postures.
These social standings are directed at protecting the mice's territory, which the mice outline with their urine. Within the territory, the animals are able to live alone and build their nests, but they do not delineate their own smaller territories within the larger one. The house mouse territory provides the inhabitants with common escape holes and areas for urinating and defecating. The territory can be quite small as long as it provides the mice adequate food and shelter. In fact, the activities of the house mouse can be restricted to an area of only a few square yards. Every night, each mouse within the group typically investigates the entire territory to discover changes that have occurred during the day.
These mice are more active during the night, although they sometimes alternate between periods of rest and activity up to 20 times each day. Furthermore, they are able to move about in many different ways. Preferring not to go into the water, house mice are still able to swim, as well as to run, jump, and climb. Their sense of hearing is very good. They hear very high tones well, a useful ability when listening to other mice squeak, but are much less attuned to lower notes. Their sense of smell is also keen, enabling them to find food and know their territorial boundaries.
Because the many different subspecies have developed slightly different behaviors, they have been able to adapt to any place that man lives. Often, house mice live in hiding places near human food, even inside bags of grain. Because they live near man's own stores, these mice do not typically store their food. They gnaw their way into food storage containers, eating as much as they can stomach and spoiling even more. Although they prefer to eat grains and grain products, house mice can eat practically anything.
The most important European subspecies are the western house mouse (Mus musculus domesticus), found in northwestern Europe, and the northern house mouse (Mus musculus musculus), found in eastern, northeastern, and southeastern Europe. While these subspecies of house mice originated in Europe, they quickly spread as Europeans moved to other parts of the globe. The western house mouse lives almost exclusively in houses or other manmade structures, while the northern house mouse spends part of the year outside. Both subspecies are descended from wild subspecies.