Edible dormouse

Latin name
Glis glis
Class
Mammals
Group
Mice, Voles & Shrews

The venerable Swedish naturalist Carolus Linnaeus was unsure what to make of the edible dormouse, confessing that it might’ve been alpine marmot or hamster. It was only thanks to the detailed description in a letter from Italian naturalist Giovanni Antonio Scopoli, in April 1763, that Linnaeus was able to include it among the squirrels, as Sciurus glis, in the 12th edition of his Systema Naturae, published three years later. The dormouse was subsequently reclassified into its own genus, Glis – glis from the Latin gliris, meaning ‘a dormouse’. There is still some argument among taxonomists over whether Glis is valid, or whether it should be moved into the Myoxus genus. The IUCN’s opinion is that it should remain as Glis, owing largely to how widely entrenched this name has become, although some authors continue to refer to it as Myoxus glis.

Unlike its cousin, the Hazel dormouse (Muscardinus avellanarius), the Edible dormouse is not a native to Britain’s countryside. The historical documentation we have suggests that some Glis imported from Hungary were released into Tring Park in Hertfordshire by Lord Rothschild “on or about February 4th, 1902”, although some naturalists have questioned this date, given that dormice hibernate until April. Regardless, the introduction was a success and the animals turned out to be quite prolific. By the mid-1920s they were a relatively common sight around the Park and, in 1926, 39 were caught in Pendley Manor near Tring. Sightings were made in Bedfordshire and Buckinghamshire in the early 1930s and by the end of the decade the dormice had made it into Worcestershire and Wiltshire. Despite having proliferated within the Home Counties and with some unconfirmed reports from elsewhere, including in the New Forest where they were apparently deliberately released, quite why the Edible dormouse hasn’t spread further remains a bit of a mystery, although it may be related to our increasingly fragmented woodlands.

The English name “edible dormouse” derives from the Roman’s habit of eating them. According to Dan Eatherley, in his Invasive Aliens, these rodents were kept in earthenware (terracotta) jars called gliaria or dolia and fattened up on walnuts, chestnuts, acorns and currants before being stuffed for cooking – roasted and rolled in honey and poppy seeds. The gliaria were often cooled with water to induce hibernation, therefore prolonging the “shelf life” of the contents. The Gauls and Etruscans apparently also considered this species a delicacy.

Population densities tend to be up to five animals per hectare, although as many as 30 per ha have been recorded. We don’t know how many currently inhabit Britain, although it’s thought to exceed 10,000 animals and some estimates suggest as many as one million. Habitat separation means that Edible dormice do not compete with Hazel dormice and have no obvious impact on their population or distribution. They’re included on Wildlife & Countryside Act (1981); Schedule 6 makes it illegal to trap/kill them without a license and Schedule 9 makes it an offence to deliberately release one into the wild.

That which follows is a summary of Edible dormouse natural history.

An Edible dormouse (_Glis glis_) photographed in a French kitchen. Introduced to Britain at the start of the 20th century, this species is now well established in the Home Counties. - Credit: Bertille de Fombelle (CC BY-SA 3.0)

Size: Largest dormouse species, at least 2x size (up to 10x weight) of Hazel dormouse. Head and body 12-19cm (4.7-7.5 in.) with 10-15cm (4-6 in.) tail. Sometimes present with truncated tail – autotomy occurs when tail is grasped/caught. Huge variation in weight with season, from 60g (2.1 oz.) post-weaning to ~300g (10.6 oz.) prior to hibernation in autumn. Mean ave. adult weight in summer ~140g (5 oz.).

Colour/Appearance: Similar to small squirrel. Uniform grey fur on head, back and tail with white stomach and chin. Coat becomes browner with age. Dark rings around large ‘bulging’ black eyes; dark stripes on legs and occasionally faint dark stripe along spine. Pinkish-grey nose and small rounded ears that protrude a short way above fur. Tail completely furred, but typically less bushy cf. grey squirrels. Single moult, during May.

Distribution: Escaped from Tring Menagerie at some point after 1902 and spread slowly through Chilterns. Bulk of population in north of Chiltern Hills, with isolated populations in south Kent and Essex in east and Buckinghamshire, Oxfordshire and Gloucester in west. Slow, often unsuccessful, reproduction, lack of dispersal and fragmented woodland probably explains very slow spread – very little increase since mid-1990s. Spread since introduction to Tring ~350 m (1,150 ft.) per yr. Found throughout central Europe, from northern Spain in west, east to Moscow, north to Lithuania and south into Sicily, Greece and northern Turkey.

Habitat: Generalist but with preference for beech and mixed deciduous woodland. Also found in pure conifer plantations, orchards and large gardens. More flexible than Hazel dormouse in not requiring shrub layer in woodland and readily entering buildings (lofts/attic spaces) even when occupied.

Nests: Nests often constructed in tree hollows and ground burrows (to depth of 60cm / 2 ft.), but will readily use nest boxes and wall cavities, although akin to Hazel dormice abandons them to hibernate in tree hollows or underground. Nests lined with individual green leaves (esp. beech) collected from canopy. In lofts may include paper and plastic in lining and sometimes use nest boxes without lining. Where nests are constructed outside hollow, they’re situated near trunk and looser than those of Hazel dormice. Reported hibernating in disused rabbit burrows, badger setts, under floorboards and in old drains. Hibernacula doesn’t usually include nesting material; animal curls up on bare ground.

Longevity: Wild individuals probably only live for 3 yrs (several studies suggest <10% population exceeds 3 yrs), although max. in wild and captivity is 9. Pat Morris found many survived to 5yrs in his UK study population.

Sexing: Impossible at distance. In the hand, genital and anal opening closer in females vs. males. Testes descend and are obvious during breeding season.

Activity: Largely nocturnal and almost exclusively arboreal. Hibernates for ~6 months, typically October-April but may start as early as late August and remain into May. Hibernation triggered by temperature and food availability and may be active into November in mild autumns. Multiple arousals during hibernation, but does not typically leave nest. Very agile climber; scales smooth tree bark and walls with apparent ease. Ascend to about 8m (26 ft.) into canopy to feed.

Territory/Home Range: Small home ranges (although larger cf. Hazel dormice) and limited dispersal. Typically, seem to move within ~100 m (330 ft.) of daytime nest, with nightly ranges in summer 0.1-0.6 ha (commonly <0.2ha) (0.25-1.5 acres). One study in Chilterns radio-tracked two animals over 13 nights; male moved ave. 523 m (1,715 ft.) per night within 250 m (820 ft.) of nest and female 111 m (360 ft.) within 100 m. Disperse 300-1,700 m (0.2-1 mile), with males moving further cf. females.

Diet & Feeding: Catholic diet, although distinct preference beech and hazelnuts (even unripe). Shows preference for acorns in some parts of Europe (e.g. Spain). Overall, diet includes nuts, fruit (esp. blackberries, elderberries and apples), buds, flowers, fungi, bark (esp. fruit trees and willow) and insects. Known to predate hole-nesting birds and eggs; suspected to take open-nesting species such as blackbirds and thrushes. Some evidence from Berkhamsted that they may take roosting bats. May strip bark in patches or spiral shapes during spring and early summer, possibly to get at cambium tissue or access sap.

Reproduction: Breeding season runs June-August with some late births into September – peak of births during August. Most courtship probably in July and involves male closely following ‘squeaking’ female who repeatedly drives him away. Perform circular mating ‘dance’ culminating in male mounting female and copulating while biting neck and shoulders. Females produce single litter of 1-13 altricial pups (ave. 7) weighing ~2g (0.07 oz.) after 31 day gestation, but fecundity strongly aligned with food availability and widespread reproductive failure associated with poor mast years. Very large litters may be crèches. Fur well developed by 16 days, eyes open ~22 days, and pups leave nest ~4 wks old. Pups grow rapidly, gaining ~1g per day as milk supplemented by partially chewed food taken from mother’s mouth. Cannibalism of nest mates strongly suspected as litter size decreases as pups grow and no females found to-date with >4-5 well-grown pups. Family unit breaks down ~6 wks and pups reach sexual maturity in second year, but highly unlikely to breed before second hibernation. Mothers and daughters may share nest and nesting/caring duties.

Behaviour and Sociality: Appear sociable and form loose-knit groups without hierarchy. Not uncommon to find multiple generations sharing nest box during summer and hibernating together. Reports of >60 animals trapped in one roof and Croatian hunters report hundreds migrating together to traditional overwintering locations. Males aggressive during breeding season; may adopt defensive postures and make explosive ‘churring’ vocalisation. Overall, a series of raucous squeaks, grunts and chittering used in communication. Particularly vocal at night, often calling from vantage point in trees. Scent probably also important although little studied.

Predators: Tawny owl probably most significant in Britain, although domestic cats may also be important, esp. where dormice enter houses. Also taken by stoats and weasels and may occasionally be dug out by foxes and badgers while hibernating. Hunted for meat and fur in Slovenia and Croatia.

Threats: Considered of Least Concern by IUCN owing to robust populations within south of native range, although may be declining in northern parts. Frequently trapped under license and killed in Britain as widely considered a pest. In buildings they chew cables, make holes in roofing felt and plaster and generate noise and mess at night. Strip bark from trees, typically around upper part of trunk that results in crown loss – particular problem in 15-30 yr old conifer plantations (esp. larch and Scots pine, but also birch and spruce) and to lesser extent in orchards (esp. apple and plum) where damage is more limited. May compete with both hole-nesting birds and other small rodents for food and nest sites. No indication control measures have significant impact on population.

Edible dormouse in detail


Bibliography

Atlas of the Mammals of Great Britain & Northern Ireland - by Multiple Contributors
Pelagic Publishing -- 2020 -- ISBN: 978-1784272043

Britain’s Mammals 2018: The Mammal Society’s Guide to their Population & Conservation Status - by Multiple Contributors
The Mammal Society -- 2020 -- ISBN: 978-0993567339

Dormice: A Tale of Two Species - by Pat Morris
Whittet Books -- 2011 -- ISBN: 978-1873580820

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

The Naturalized Animals of Britain & Ireland - by Christopher Lever
New Holland Publishers Ltd. -- 2009 -- ISBN: 978-1847734549

UK Mammals: Species Status & Population Trends - by The Tracking Mammals Partnership
JNCC/TMP -- 2005 -- ISBN: 978-1861075680

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