Small British Mammals: Bank Voles

I had intended to wrap up voles a couple of weeks ago, but it turned out that there was so much to say about water voles in particular that it made more sense to add in an extra post to cover everything else. So, here we are with the fourth and final species of vole found in Britain: the bank vole (Myodes glareolus).

Although water voles are more closely related to common voles and their kin than the bank voles, it is the latter that have the closest physical resemblance. While most voles are broadly similar in shape, bank voles are also roughly the same size as the common and field voles, and can be distinguished (without close examination of the skeleton) by the fact that the fur on their backs typically has a distinct reddish tinge. A few instances of individuals with other colours have been noted - including a pure white one that had somehow survived to adulthood without being eaten - but these are rare.

Bank voles are widespread across Europe, being absent only from Lapland, Greece, Portugal, all but the north coast of Spain, and the various Mediterranean islands. Ireland also used to be entirely devoid of voles (of any kind), but a bank vole was seen there for the first time in 1964. The animal in question was found in County Kerry, and, since then, the voles have steadily marched across the island, at an average rate of 2.5 km (1.5 miles) per year, until they now occupy most of the southern half of the Irish Republic. Genetic analysis later showed that they reached Ireland from Germany, most likely sneaking across with some equipment for a hydroelectric construction project in the 1920s, and escaping notice for 40 years because they're rather small and nobody had been looking for them.

Outside of Europe, bank voles are also found wild in northern Turkey and in south-western Siberia as far east as the Yenisei River, broadly between the southern limit of the tundra in the north, and assorted inconvenient mountain ranges in the south. The common name might suggest that, like water voles, they are found along river banks but, while it's fair to say that they sometimes are, they are more accurately described as forest dwellers. (Indeed, their name in other languages more commonly refers to the reddish colour of their backs than to anything river-related). They are, however, found in many other habitats with decent vegetation cover, such as scrubland, farm hedgerows and the like.

Voles as a group have been successful because of their ability to feed on relatively tough plants, giving them an edge over the more omnivorous mice in places with the right kind of vegetation. Bank voles are no exception here, being far more purely herbivorous than mice are, and feeding primarily on seeds, with a side order of berries and fungi when seeds are in short supply. As a consequence, bank voles can have a significant effect on forests through means such as seed dispersal, and their own population can boom and crash partly in response to how well the forest is doing in any given year. While they do also occasionally feed on animal matter, their typical reliance on food sources that may be unpredictable may explain a spatial memory that's apparently superior to that of the otherwise similar common voles.

Bank voles are most active at dawn and dusk, although this may change at certain times of the year, or depending on the weather. Females, but not males, dig small nests and line them with grasses - these are perhaps most obvious along river banks, which may explain the common name. They are quite defensive of the territory around these nests, but really only when they are pregnant or suckling young; they are fairly relaxed at other times of the year. Females are, however, more tolerant of voles they already know (unlike the less sociable common voles), and, in particular, are willing to share some of their territory with their daughters - so long as they don't get pregnant and try and hog the resources.

Males are less aggressively territorial, but do mark their home range with urine scent marks. These mainly seem to advertise their dominance over other males, since females make most of the choices when it comes to sex, and greatly prefer the scent of a dominant male. In fact, the females don't actually come into heat, and are sexually active throughout the breeding season, perhaps ovulating only in direct response to sexual stimulation. Litters of about three or four young are most common, with the offspring being able to breed themselves in less than two months, allowing for multiple generations in a year. Like other voles, they typically don't outlast their second winter in the wild.

The closest living relative of bank voles is probably the mountain-dwelling Tien Shan vole (Myodes centralis) of central Asia. More broadly, it belongs to a group referred to as the "red-backed voles", most of which are found in relatively small localities in eastern Asia - Japan and Korea have a number of species between them, with most of the rest in China.

However, one species, the northern red-backed vole (Myodes rutilus), is found from Scandinavia across the whole of northern Asia, into Alaska and across northern Canada as far as the east coast of mainland Nunavut. This seems to be capable of cross-breeding with bank voles, although the two species live in separate areas now, and may not have done so for thousands of years. In fact, bank voles seem to have had a complex history during the Ice Ages, retreating to a number of different places, some of them surprisingly far north, resulting in quite a broad genetic diversity once they met up again afterwards.

The vole subfamily

From this northern retreat, red-back voles spread into other parts of North America, with two other local species being known, one across much of Canada and the northern US, and the other restricted to California and Oregon.

This is all part of the great vole success story. While the animals first evolved somewhere in Asia, they are now (unlike non-commensal members of the mouse family) found across many parts of North America, too. Perhaps because of their adaptation to eating tough grasses and similar plants, they have rarely been successful outside of the temperate zones, with just one species in Africa (on the Libyan coast), and a few in Mexico, although one of these is also found across the southern border in Guatemala.

Among the most interesting of the voles are a group known as the mole-voles, peculiar animals that spend almost their entire lives underground, feeding on plant roots and digging tunnels with their teeth, as mole-rats do. One of the things that's strangest about them is that, while some species follow the usual mammalian pattern, at least two have no Y-chromosome - in one, both sexes have the XX pattern normally seen only in females, while in the other, the second X-chromosome has vanished from females, too, leaving both sexes with just the one (and thus, an odd number of chromosomes, which ought to make reproduction difficult at best, but evidently doesn't).

You might think that the specific bit of the Y chromosome that determines maleness must be hiding somewhere else on the genome, but close analysis has proved that it really isn't. It's just not there. How on Earth either of these species determines sex remains a mystery.

Just as, at least in English, we refer to particularly large mice as "rats", so we also use different words to describe the largest kinds of vole. Lemmings are particularly large voles, adapted to living in cold, treeless wastes in both Eurasia and North America. There are two broad groups; the 'collared' sort, which turn white in winter, and the 'true' lemmings, which do not. Both are about twice the length of a water vole, with around 15 cm (6 inches) being fairly typical. The very largest member of the vole subfamily, however, is the muskrat, which is about the size of a brown rat, but much heavier, because of its bulky build.

Moving further afield, evolutionarily speaking, the voles form part of the Cricetidae. This name literally means "hamster family", but, in fact, the great majority of the species in it are either voles or look remarkably like mice. While voles are found across the Northern Hemisphere, the mouse-like forms are found only in the Americas (North and South) and include such familiar animals as deer-mice and pack rats. The hamsters themselves include only a comparatively few species, all native to Asia and eastern Europe.

The Cricetidae and Muridae (mouse family) are two of the largest of all mammal families, in terms of number of species. But a third, somewhat smaller, family of animals also includes some distinctly mouse-like animals, two of which do live in Britain. So it is to them that I will turn next...

[Photo by Evan James Shymko, from Wikimedia Commons. Cladogram adapted from Steppan and Schenk 2017.]