Wood mouse

Latin name
Apodemus sylvaticus
Class
Mammals
Group
Mice, Voles & Shrews

Wood mice are one of the most common rodents in Britain and Ireland and frequent visitors to gardens, even in the middle of large cities, although they’re infrequently seen as much of their activity happens under the cover of darkness. Their comical bouncing gait makes them unmistakable for anything other than the closely related but less common yellow-necked mouse (Apodemus flavicollis) from which they were taxonomically separated in 1894.

Apodemus is a member of the largest rodent family, the Muridae; two murids are found in Britain and, more widely, five across Europe. Apodemus derives from the Greek apodēmos, meaning “away from the home” and it’s scientific name translates roughly to the “the wild mouse living away from the house”, alluding recognition by early naturalists that this rodent was distinct from the house mouse (Mus musculus). While not commonly found indoors, it may enter wall cavities and loft spaces, particularly during prolonged cold or wet weather.

According to Stefan Buczacki, in his 2002 opus Fauna Britannica, the word “mouse” is an ancient one:

It reached modern English from Old English but has a pedigree extending back through Latin and Greek to Sanskrit [a sacred Hindu language dating back to the mid- to late second millennium BCE]. It is said to have originally meant ‘thief’, its Sanskrit roots having to do with stealing.”

That which follows is a summary of wood mouse natural history. The detailed article for this species will follow in due course.

A wood mouse (_Apodemus sylvaticus_) in a city garden. - Credit: Marc Baldwin

The Wood mouse at a glance

Size: Grow to 8-11cm (3-4 in.) with tail as long as body (in some cases, slightly longer) and long hind feet (~2.5cm / ~1 in.). Weigh 13-27 grams (0.5-1 oz.), although heavily pregnant females may exceed 30g.

Colour/Appearance: Small mouse with large black eyes and large protruding rounded ears. Fur is dark brown on back, merging to paler (almost yellow-brown) hue on flanks and white on belly. Juveniles greyer in colour cf. adults. Almost identical in appearance to slightly larger (i.e. 12cm / 45g) Yellow-necked mouse; latter can usually be distinguished by continuous v-shaped band of yellow fur across chest, although this typically requires in-hand inspection.

Distribution: Widespread across Europe, although rare in the “coniferous belt” (e.g. Scandinavia). Found throughout British Isles, excluding most Scottish islands; patchily distributed in upland western and northern Scotland. Found on Orkney islands (excl. Hoy and South Ronaldsay) and Shetland islands. Widespread in Ireland, although distribution patchy in south-west of RoI.

Habitat: Found in wide range of habitats, including woodland, mixed and arable farmland, gardens (even small gardens in cities), parkland, bramble and bracken scrub, and on sand dunes. Sometimes found in heather moorland, particularly where field voles (Microtus agrestis) absent, but are rarely found on high moors and scree, unless there are stone walls and buildings to provide cover.

Longevity: Probably no more than 2 yrs in wild. In captivity, individual of unknown sex at Universität Erlangen (Germany) lived for 6 yrs 4 months.

Sexing: Impossible to sex from brief glimpse or from above. During breeding season testicles descend and can be obvious externally; equally, descended teats indicate lactating female although unlikely to be noticed at distance. Outside breeding season testicles held internally and female reproductive opening can appear penis-like, making distance between genital opening and anus only reliable method of sexing.

Activity: Primarily nocturnal, although can be active during the daytime, particularly during cold autumns and winters. May also be more prone to diurnal activity in quiet gardens where supplementary food is provided and that have comparatively low predation risk and/or good cover. Peak of activity at dawn and dusk during autumn and winter. Less active at full moon and during very cold (<4C / 39F)/wet weather. Use a complex system of underground and ‘litterzone’ (i.e. within the leaf litter and deep within long grass) tunnels and above-ground feeding stations. During periods of heavy snow cover, mice remain active in subnivian zone, moving around in a series of tunnels and pockets under the snow. Does not hibernate.

Territory/Home Range: Home range size varies with resource availability (e.g. larger in arable fields vs. woodland). Males tend to range over larger areas cf. females; in woodland, average home range of female ~2,000 sq-m (half acre), while male can range over three times that area. Despite small size, both sexes can move 2km (1 mile) in single night looking for food/mates. Apparently place leaves, twigs and other conspicuous objects around to use as way markers, highlighting sites of interest. Maintain good mental maps of environment and ranges relative to neighbours.

Show very limited territoriality. Females defend exclusive core (breeding) range from others and males more aggressive in spring, but males overlap ranges during winter and may nest communally with individuals of same or opposite sex.

Diet and Feeding: Heavily influenced by season and habitat, but nuts and seeds predominate. Tree seeds (esp. acorns and hazel/beech nuts) major component in autumn/winter and frequently found in caches. To lesser extent also take buds, fruit (esp. blackberry), fungi, moss and galls, as well as some animal prey including insects, spiders, snails, earthworms, centipedes and larval butterflies/moths. Reports of frogs occasionally taken. In gardens, will readily accept sunflower seeds, peanuts, mixed bird seed and sunflower hearts. Feed in open more than voles, particularly under bird feeders. In more wooded habitat, food carried into a sheltered spot close to nest entrance for consumption. Disused birds’ nests sometimes used as feeding platforms, becoming littered with discarded husks.

Reproduction: Can breed year-round if mast crop bountiful, but typically March to October. First pregnancies evident by April, peaking during July/August and tailing off by autumn. Gestate for ~3 wks (19-20 days), though if female still lactating when she conceives (some females can produce 6 litters a year) a period of delayed implantation may follow, extending gestation by up to 10 days. Short oestrous (<4 hrs), but highly promiscuous with some litters having 4 different fathers. Litters typically 4-7 pups (up to 9), each weighing ~1g (0.04 oz.). Pups born blind and furless; buff-grey fur starts growing shortly after birth. Pups weaned at 18-22 days old and start leaving nest shortly before (i.e. 15-16 days old, weighing ~6g/0.2 oz.). Parental care from female only. Early litters often breed in summer of first year; those born later breed in following spring – 12g (0.4 oz) seems critical weight threshold for pregnancy.

Nests built below ground (usually from finely shredded leaves, moss and grass) and used for resting and breeding, with chambers for the caching (storage) food.

Behaviour and Sociality: Generally non-aggressive and frequently feed together during autumn/winter. Communal nests (3-4 individuals, both sexes) found during winter. Male ranges overlap year-round, females overlap outside lactation period. Emit high pitched squeaks and ultrasonic calls when fighting, exploring, mating, chasing or just chatting. Well-developed scent glands in both species. Smell and hearing acute; vision good but seems largely movement-based. Very agile and climbs well to forage in trees and on bird feeders. Possibly orientate with magnetic fields. Frequently groom themselves and each other, although males groom females more than v/v.

Predators: Have wide range of predators, including cats, foxes, badgers, kestrels, stoats, weasels and owls. Tawny owl (Strix aluco) probably most significant predator in Britain (30% diet in English woodland) and long-eared owl (Asio otis) in Ireland (70% diet). Barn owl significant predator (50% diet) where bank voles (Myodes glareolus) absent.

Threats: Abundant and adaptable species not considered threatened or provided with any specific protection in Britain. Some indication introduced bank vole and greater white-toothed shrew (Crocidura russula) having detrimental impact on wood mouse population in Ireland. Abundant yellow-necked mice may also reduce number of breeding male wood mice. Very sensitive to agricultural pesticides and heavy metal contamination.

Wood mouse in detail


Bibliography

Collins Field Guide: Mammals of Britain and Europe - by David MacDonald and Priscilla Barrett
HarperCollins Publishers -- 1993 -- ISBN: 978-0002197793

Mammals of the British Isles: Handbook, 4th Edition - by Stephen Harris and Derek Yalden (eds)
The Mammal Society -- 2008 -- ISBN: 978-0906282656
The natural history "bible" covering all British mammals with detailed coverage of their biology, behaviour, ecology and taxonomy written by experts in the field and referenced to the primary literature.

The Encyclopaedia of Mammals - by David MacDonald (ed.)
Brown Reference Group -- 2006 -- ISBN: 978-0199206087

Urban Mammals: A Concise Guide - by David Wembridge
Whittet Books -- 2012 -- ISBN: 978-1873580851

Urban Wildlife - by Peter Shirley
Whittet Books -- 1996 -- ISBN: 978-1873580233

Wild Animals of Britain & Europe - by Helga Hofmann
HarperCollins Publishers -- 1995 -- ISBN: 978-0007627271