WOOD MICE: SILENT FORAGERS
The wood mouse is one of our commonest wild animals, yet it rarely attracts our attention as it scuttles noiselessly through the undergrowth in search of seeds and insects.
The wood mouse can easily be distinguished from the house mouse by its larger ears and eyes. Its warm brown coat cannot generally be confused with the dull greyish coat of the house mouse, although unusual colour variants and the grey fur of the young of both species may make identification more difficult. The wood mouse does not have the distinctive odour associated with the house mouse.
Wood mice have soft, smooth fur which is sandy or orange brown on the head and back, yellowish on the flanks and white on the belly. There is usually a small streak of yellow pigmentation in the otherwise white fur of the chest. The tail is almost as long as the body and has a sparse covering of black hairs. The tops of the feet are covered by short, white hairs and each toe – four on the forefeet and five on the hind – ends in a sharp, curved claw.
Mainland British and Irish mice do not vary much in size but forms 50-100% larger occur on Rhum, St Kilda, Fair Isle and several other islands. Mice probably reached these islands with the assistance of the Vikings and other settlers who brought livestock with them. The wood mice of mainland Britain, on the other hand, may have originated from southern stock which survived the last advance of the ice sheet. These groups of mice re-invaded the major part of England, Wales, Scotland and possibly Ireland as the ice retreated 10,000 years ago.
The wood mouse is undoubtedly the most widespread of our small mammals. Although more numerous in some habitats than others, it occurs in most places from coastal sand dunes to mountain tops and from mixed deciduous woodland to the increasingly prevalent forests of conifers. It is also found in towns and cities, lurking in suburban gardens, unkempt graveyards and railway embankments. Researchers who examined mice caught near a railway station in Manchester found that they lived on a diet of biscuit crumbs, presumably from packets discarded by passengers.
Hedgerows, stone walls and ditches are frequent haunts in farmland, and wood mice also enter disused outbuildings and farm-houses. They avoid open pasture or grassland, where the field vole is present. Highest densities occur in mixed woodland. Here, the mice use extensive runway systems which follow tree roots, fallen branches, banks, rocks or any other feature close to the ground. Wood mice prefer areas with some low vegetation. Brambles are particularly favoured.
Wood mice are primarily seed-eaters (granivores), relying to a great extent on the seeds of trees such as oak, beech, ash, lime, hawthorn and sycamore. Every year they eat a high proportion of the annual seed crop and it seems likely that only small quantities of seeds therefore survive to germinate the following spring. The mice are efficient seed gatherers and when there is a plentiful supply on the ground, they carry them back to the nest for storage.
The seeds of bushes and herbs are also eaten, as well as fruits, seedlings, buds and sometimes fungi. Small invertebrates, particularly small snails and insects, may be eaten throughout the year, but are particularly important sources of food in late spring and summer. This is the time of year when seeds are least available and larval and adult insects are abundant. Moth caterpillars, which fall from the upper canopy of trees to pupate in the soil, are a common food in summer. In Britain a number of predators eat wood mice but only two, the tawny owl and the weasel, kill large numbers. A third, the domestic cat, may have a major effect on mice living close to human habitation.
The density of population fluctuates markedly, following an annual cycle. Numbers are greatest in autumn and remain high throughout the winter. During which survival is determined by the size of the autumn seed crop. When seed production is very great, winter breeding may occur but generally breeding is restricted to spring, summer and early autumn.
Numbers do not increase with the first appearance of young in spring. In most years, survival of both young and adults is poor during the first half of the breeding season and the population declines rapidly before reaching a stable, low level in early summer. Survival of young mice improves as the last of the winter generation die off. And numbers increase during the late summer and autumn to reach a peak in the first half of winter. Juveniles which are born early in the season and which survive may breed in their first summer. Females may conceive at only seven or eight weeks, when they weigh 12gms (less than 1oz).
Wood mice have weak eyesight, but their hearing and sense of smell are both acute. It is therefore not surprising that they are largely nocturnal animals (although in captivity they become increasingly active in daylight.) The burrow systems which they excavate for themselves may have one or two nest chambers in which they spend most of the day and to which they return many times during the night. Wood mice do not hibernate but during cold weather their movements may appear slow and lethargic as they enter a temporary torpor, or they may huddle in groups to retain warmth. In familiar areas, mice run or scurry along quickly. In exploration, they move slowly and deliberately as they are nervous and timid. They are good climbers and use low branches – where running is silent and does not attract the attention of predators – as freely as the runways at ground level.
The structure of wood mouse society is still not well understood but in the breeding season it is likely that dominant males patrol large areas which include subordinate males, females and young. Mice may sleep and forage for food in family groups. In captivity, fights are rarely seen as dominant relationships are quickly established. Subsequently most social contact is amicable and contrasts markedly with the more aggressive lifestyle of the house mouse.