Small British Mammals: Field Mice

Wood mouse

While the common house mouse is likely one of the most familiar of all animals living wild in Britain, it is only one of four species of mouse native to the islands. (France and Germany, for comparison, have six each, although they aren't the same six). Of the other three, the closest relatives of the house mouse are the two species of field mouse, which are also found widely on continental Europe.

Field mice have benefited less from the presence of humans than house mice, since they tend to avoid human dwellings, and there are rather a lot of those in Europe. Given their common name, however, one might suppose that they have at least benefited from the spread of agricultural land. This, however, only true of one of the British species. This is the wood mouse (Apodemus sylvaticus), sometimes also known as the "long-tailed field mouse". In Britain, when the term "field mouse" is used without qualification, it typically means this one.

Wood mice are found across almost the whole of western and central Europe, living as far east as (very roughly) the Russian border. In the north, they reach southern Sweden and Norway, but not most of the Baltic states or Finland. Otherwise found only in Europe, they also live in the wild along the Mediterranean coasts of Morocco, Algeria, and Tunisia, likely having followed humans there in historical times. They are also common on islands, including not only many of those in the Mediterranean, but even Iceland, where it is one of a very small number of native land mammals. Again, especially in the case of smaller or less hospitable islands, this might be due to human activity, rather than an ancient, prehistoric, presence.

Wood mice are similar in size to house mice, and their bodily proportions, such as the length of the tail, are not drastically distinct, either. They have a tendency to have redder fur, although in some parts of their range, they tend more towards the duller grey of house mice. However, while both species have paler underparts, these are more distinct in wood mice, and the pale colour also extends onto the underside of the tail. Another distinction, albeit one that's hard to evaluate at a casual glance, is that females have only six teats, rather than ten, limiting the number of young they can rear at a time. They are also said to have less of an odour.

The native habitat of wood mice is more accurately described as the edges of woodland than the forest proper, and they are particularly common in such places towards the eastern parts of their range. In western Europe, however, they are commonly found in agricultural land, and even in suburban parks and gardens. While they are clearly attracted to farmed fields by the ready availability of edible crops, they actually prefer more of a variety of foods, and therefore are most plentiful in fields with a greater number of weeds.

The problem with living in crop fields, of course, is that once every year, humans come along and cut everything down. This seems to have less immediate effect on the wood mice than one might think, perhaps because they have a tendency to live in the hedgerows around the field rather than amongst the crops themselves. However, it does reduce the amount of cover that they can use to hide from predators such as owls, so that wood mice are far less likely to be found in fields in winter than they are in the summer - as much due to them fleeing the area as to the owls getting lucky.

Wood mice are nocturnal, spending the day sleeping in grass-lined burrows, or in trees - they are skilled climbers, and better at jumping than house mice. Their preferred foods are seeds of various kinds, with grass seeds (such as crops) and acorns being particular favourites. Nonetheless, they are omnivorous, eating a wide range of foods, including a fair amount of insects and worms. One report from the Netherlands even describes them feeding on hibernating bats, gnawing away at their soft parts and leaving the skin, bones, and wings.

For most of the year, individual wood mice live alone, avoiding one another as far as possible. The size of the area which any given individual inhabits varies depending on the availability of food, but is generally smaller for females than for males, presumably because the latter, being larger, need to eat more. During the breeding season, this difference in scale becomes even greater, as males search for suitable females to mate with. During winter, small groups of three or more adults gather together, perhaps for mutual protection, constructing larger, communal, nests for the purpose.

In all cases, however, the mice do not inhabit one single burrow, but move between various nests across their area, which doubtless makes them harder for predators to find. Nests are surrounded by a network of trackways, through which the mice move to locate areas of rich food and so on. It has been shown that, in order to help their navigation, the mice drop leaves or small twigs in conspicuous locations, apparently as artificial landmarks. It may be that these are less apparent to predators than scent marks, which one might otherwise expect the mice to employ.

Breeding takes place through many of the warmer parts of the year, the exact time varying with the local weather as much as with wider geographic effects - in some years, they may mate well into a mild winter. Since pregnancy only lasts three weeks, and it is only another three weeks before the young are weaned, there are multiple births throughout a year, and young born in the spring can have children of their own before the year is out.

Females seem to have some control over reproduction, withholding sex from males that fail to give them a satisfactory grooming session beforehand, and with the population growth in a particular year having more to do with their breeding success than with anything the males might be doing. Even so, mating is promiscuous, with both males and females having multiple sexual partners, and it being quite common for females to be pregnant by two or three different males at the same time.

Yellow-necked field mouse
(apparently; it's not like I can tell the difference...)

Likely rather less well-known, the yellow-necked field mouse (Apodemus flavicollis) is a very close relative of the wood mouse. First identified as a separate species in 1834, it looks almost identical to its relative, with the only real distinguishing feature being the presence of a yellowish bib or collar of fur at the base of the neck. This is such a minor point of discrimination that, until at least the 1950s, some scientists regarded the two as belonging to the same species, but nobody has ever got them to interbreed, despite them having ample opportunity to do so.

Like the wood mouse, the yellow-necked mouse is found throughout much of western and central Europe, but it is also found further east, in western Russia, Finland, and the Baltic states. However, it is less common in the west, being absent from Portugal, all but the northern parts of Spain, most of the Low Countries, and the northern and Atlantic coasts of France. In Britain, it is found only in central and southern England, and in Wales, its spread further north apparently being limited by cool summer temperatures. Likely this is because of the sorts of food plants cooler summers encourage, rather than the mouse's own temperature preferences.

During the Ice Ages, both kinds of field mouse (and others besides) were forced south, unable to cope with the spread of the tundra south of the ice sheets proper. Whereas the wood mouse seems to have survived by hiding out in Spain and Portugal, yellow-necked mice retreated to southern Italy and the Balkans, spreading out again across Europe once the climate warmed. A second population of yellow-necked mice, living in what is now Turkey, parts of the Levant, and western Iran, survived with less of an issue, and they are still there today, genetically distinct from the populations on the other side of Bosporus, although not sufficiently so to qualify as a separate subspecies.

Despite their similarity to wood mice, yellow-necked mice have different habitat requirements. Indeed, it is they, not the wood mice, that are more likely to be found in wooded country, and they tend to avoid the open fields. While this may have something to do with the relative amount of cover as well, the main deciding factor seems to be that yellow-necked mice prefer the sorts of large seeds that trees such as oak and hazel produce over those of grass and wheat. As a result, while they may occasionally approach human dwellings in the winter, they tend to stick to the woodlands, preferring those with a good mix of trees and relatively little human disturbance.

While they are more ready to climb trees than wood mice, in most respects, yellow-necked field mice have very similar habits. One biological oddity, however, is that while, like wood mice, they normally have 48 chromosomes (for what it's worth, house mice have 40) it is quite common for them to have up to three additional ones as well. These are not mere broken fragments of the normal chromosomes, but entirely separate structures, bearing genetic information that is simply missing in other individuals of the same species.

While common in insects and plants, the presence of these "B chromosomes" is unusual in mammals, and, while they are very occasionally seen in wood mice, too, some studies estimate that they may be present in 40% of yellow-necked mice. In many animals, such chromosomes consist primarily of densely packed DNA that does not appear to be in active use, but those in yellow-necked mice show every sign of having functional genetic material, and to be clearly doing something. Quite what that is is unclear, especially since so many mice in the same species do perfectly well without it. They may somehow affect the functioning of other genes, possibly providing some minor, but not essential, survival advantage.

There are four other species of field mouse native to Europe, with the Alpine field mouse (found only in the namesake mountain range) being perhaps the closest relative to the two described above. However, many other kinds of field mouse also exist, in places such as Russia, China, and northern India; there are even two species unique to Japan. They, and the group containing the house mouse, are just two of the many branches of the wider murine family tree, but most of the others are not found in Europe. There is, in fact, just one other species of murine that is, and it so happens that it is also native to Britain...

[Photos by "BlueBreezeWiki" and Vojtěch Dostál, from Wikimedia Commons.]