Bank vole  (Clethrionomys glareolus)

Not legally protected in the UK. No conservation designations (3).

The bank vole has a small, stocky body and a blunt, rounded muzzle (2). The upperparts may be yellowish, reddish or brown in colour, the flanks are greyish and the rump is whitish-grey (2). The short tail is usually slightly bushy at the tip (2).

Common in Britain, including a number of the islands. On Skomer Island off the Pembrokeshire coast of Wales, there is a subspecies known as the Skomer vole (Clethrionomys glareolus skomerensis). The bank vole is also common throughout much of western and central Europe (4).

This adaptable species (2) inhabits broadleaved woodlands, scrub, parks, hedgerows and banks where there is plenty of herbaceous cover (4).

Bank voles are active during both the day and night, although they become increasingly nocturnal during the summer (4). They do not hibernate in winter, but are active throughout the year (4). They have a broad diet, which is mainly herbivorous, including fruit, soft seeds, leaves, fungi, roots, grass, buds and moss. They may also occasionally take invertebrate food such as snails, worms and insects (4), and the odd bird egg may be eaten (2). Breeding typically takes place between April and October, but when conditions are suitable, births may occur throughout the year (4). Ovulation by the female is stimulated by the presence of a male, possibly via certain scents that males produce (5). Gestation takes around 21 days, but may be a short as 17 days in this species if conditions are optimal (5). Between 4 and 5 litters are produced in a year, each one consisting of 3-5 young (4). Females are able to conceive again whilst still suckling the previous litter; under these circumstances the gestation period will be longer, up to 24 days (5). The young voles are born in a nest, which is usually located underground (5). Males do not assist with rearing the offspring. Females are very protective of the litter; if any young leave the nest the female locates them and carries them back to the nest (5). The young become sexually mature at around 4.5 weeks of age, however those born later in the year will not start to breed until the next spring (4). Predators such as owls, kestrels, foxes and weasels take their toll on vole populations; the maximum life span for this species is 18 months. The numbers of bank voles varies greatly throughout the year, being high towards the end of summer and plummeting through the winter to a low in April (4).

Where they occur in close proximity to roads, bank voles may be at risk from lead exposure. In agricultural areas, pesticide drift, and exposure to molluscicides and rodenticides may all pose threats. The fragmentation of woodlands, removal of hedgerows and overgrazing of herbaceous ground cover by deer reduce the chances of bank voles persisting in an area (4).

There is no conservation action targeted at this species.

For more on this species see: Macdonnald, D. W. & Tattersall, F. T. (2001) Britain's mammals- the challenge for conservation. The Wildlife Conservation research Unit, Oxford University. Available from
www.wildcru.org

• Hibernation: a winter survival strategy characteristic of some mammals in which an animal's metabolic rate slows down and a state of deep sleep is attained. Whilst hibernating, animals survive on stored reserves of fat that they have accumulated in summer. In insects, the correct term for hibernation is 'diapause', a temporary pause in development and growth. Any stage of the lifecycle (eggs, larvae, pupae or adults) may enter diapause, which is typically associated with winter.
• Ovulation: in female mammals, the release of a ripe egg from an ovary (one of the paired reproductive organs).
• Subspecies: a different race of a species, which is geographically separated from other populations of that species.

1. National Biodiversity Network Species Dictionary (May 2002) http://www.nhm.ac.uk/nbn/
2. Burnie, D. (2001) Animal. Dorling Kindersley, London.
3. The Environment Agency (1998) Species and Habitats Handbook. The Environment Agency, Bristol.
4. Macdonnald, D. W. & Tattersall, F. T. (2001) Britain's mammals- the challenge for conservation. The Wildlife Conservation research Unit, Oxford University. www.wildcru.org
5. Macdonnald, D. W. (2001) The New Encyclopedia of Mammals. Oxford University Press, Oxford.