Mus musculus can live in an incredible variety of different habitats. Commensal animals can live in all types of human-made structures including houses, buildings, barns, haystacks, ruins, and in coal mines, 1,800 feet below the ground. The possibilities are virtually unlimited — animals have been found in climates as different as frozen-food lockers and central heating ducts (Bronson, 1984). Feral animals can live in agricultural fields, meadows, and scrublands (Sage, 1981). In most places, they do not normally live in woodlands or forest, but even this is possible in areas, such as islands, where natural predators do not exist (Berry and Jakobson, 1974; Berry et al., 1987). The survival of feral mice is often dependent on the production of nests and burrow systems which act to ameliorate the prevailing air temperatures (Sage, 1981). Both sexes construct nests which can range from very simple to highly complex enclosed structures used for food storage as well as nesting. Feral animals can display a highly developed homing behavior and are capable of returning to their nests after long distance (250 m) displacement (Sage, 1981). Mice can eat almost anything — cereals, grass, seeds, roots and stems of various plants, adult insects, and even larvae (Rowe, 1981). Animals can also subsist with very little water, especially if their food is high in moisture content (Grüneberg, 1943). In many locations, the morning dew can probably provide much of the daily water requirement (Rowe, 1981). These traits provide the house mouse with great adaptability and have played an important role in their dispersion among many different habitats, both commensal and feral.
The paradigm population structure for animals living under commensal conditions is that of independent, relatively stable demes, or families. The classic deme will have a single dominant male who patrols a well-defined home range and sires most of the young; up to ten breeding female members of the deme will confine their own ranges to that of the single dominant male.
Different dominant males will have mutually exclusive territories. Males will tolerate their own offspring, but will kill offspring born to females that belong to other demes. In highly structured populations of this type, the level of interdemic migration is very low, even between nesting sites located within a few meters of each other (Sage, 1981).
In reality, the picture of demes presented above is an idealized situation that may actually define the structure of some populations but not others, and at some points in times, but not others. In the presence of an ample food supply and in the absence of predators or competitors, populations appear to retain a higher level of structure. However, demes can vary in size from two animals to at least 100; the amount of interdemic migration can vary between none and all; and the detailed structure of a population can change drastically in response to changes in the environment.
Under optimal environmental conditions with plenty of food and nesting material, commensal mice living inside temperature-controlled buildings can breed throughout the year (Rowe, 1981; Sage, 1981). In strictly feral populations in temperate climates, breeding activity tends to be seasonal, from spring to early autumn (Rowe, 1981). The average litter size has been found to vary from as few as three pups to as many as nine. Although mice from some laboratory lines can survive as long as three years, free-living wild animals are likely to die much earlier from disease, competition, or predators.
The usual structure of feral populations may be very different from that of commensal populations in that animals living outdoors appear to move over much larger distances, and deme structures appear to be much less stable (Berry and Jakobson, 1974). However, the ability and desire of mice to migrate over long distances is complex and highly variable. Many animals appear to live their entire lives in very small and well-defined home ranges (defined as the area in which an animal spends the vast majority of its time) of less than 10 m across. Others will move constantly over much larger distances, traveling kilometers daily, and some will migrate long distances between home ranges that are very small. All possible permutations are possible, and the distribution of animals in each class varies greatly among different populations. The lifestyle of the house mouse has been described aptly by Berry: "The house mouse is a weed: quick to exploit opportunity, and able to withstand local adversity... A consequence of the repeated formation of new populations by small numbers of founders is that every population is likely to be unique." (Berry, 1981).
The incredible adaptability of M. musculus to new environments can be accounted for almost entirely by the enormous plasticity that exists in its behavioral traits (Bronson, 1984). In the case of nearly all other species, specific behaviors are highly defined by genomes, and adaptability to new environments can occur only slowly with changes in behavior as well as physiology and/or morphology driven by natural selection. In contrast, the house mice can disembark from ships in sub-antarctic islands or in equatorial Africa, and adapt immediately to survive and prosper. "To be introduced into a radically new environment is one thing; to be able to reproduce there and so to establish a new population is quite another. The planet-wide spread of the house mouse in both manmade and natural habitats suggests an extreme reproductive adaptability, probably the most extreme among the mammals" (Bronson, 1984). Only humans are as adaptable (some would say less so).
Thus, the defining characteristic of the species M. musculus is the decoupling of genetics and behavior. At some point during evolution, the ancestral house mouse population broke away from its previous behavioral constraints and once this occurred, the success of the species was assured. With men and women as chauffeurs and guides, the global conquest of the house mouse began.