Lifestyles and Adaptability of Wild House Mice
Shelter, Food, and Water
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, and ruins, as well as in coalmines 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).
Deer mouse, a common wild mouse in America
The survival of feral mice is often dependent on the production of nests and burrow systems that 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 behaviour and are capable of returning to their nests after long distance (250m) 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.
Population Structures and Reproduction
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 metres 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 time, but not others. In the presence of an ample food supply and 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 10m across. Others will move constantly over much larger distances, travelling kilometres 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).
Adaptability and Success
Thus, the defining characteristic of the species mus musculus
is the decoupling of genetics and behaviour. At some point during evolution,
the ancestral house mouse population broke away from its previous behavioural
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.
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