Diminutive, nocturnal and one of Britain’s true hibernators, the hazel (or common) dormouse is easily overlooked. Originally classified as Mus avellanarius by Carolus Linneaus in the 10th edition of his System Naturae, published in 1758, it was subsequently split into its own genus based on the publication of German biologist Johann Kaup’s work on the taxonomy of European animals in 1829. Its binomial name is a beautiful description of the animal’s niche, translating roughly to ‘mouse of the hazel trees’.
A phylogenetic study published in 2012 found evidence for two highly diverged lineages of dormice in Europe—one in western Europe and one in central-norther Europe, the Balkan peninsula and Turkey—but it is still currently considered a single widely distributed species.
The English name dormouse is of somewhat obscure origin, but probably derives from the French verb dormir, “to sleep”; the French word dormeuse, meaning “a sleeper”, appeared at some point during the 17th century. Indeed, this enigmatic animal has long been associated with slumber and in Act 3, Scene 2 of Shakespeare’s Twelfth Night, Fabian tells Sir Toby Belch that Olivia was flirting with the count’s serving-man to ‘awaken his dormouse valour’ and spur him into action. Meanwhile, in Luis Carroll’s Alice in Wonderland, the dormouse was only feigning sleep so he could eavesdrop on the conversations at the tea party table.
Once thought to have once been widely distributed across the whole of England and Wales, the hazel dormouse has become extinct in 12 counties in the last 100 years. The Mammal Society currently estimate there to be about 930,000 dormice in England and Wales and the population is thought to be declining. The Society estimate numbers to have fallen by 48% between 2005 and 2014, although the range appears to have been reasonably stable over the same period. A decline in traditional forestry management (specifically long-cycle coppicing) in many countries is likely to be a core reason for the decline, along with continued habitat fragmentation.
In Britain, the hazel dormouse is classified “Vulnerable” by the IUCN, while more widely in Europe it is a species of Least Concern. Currently protected in the UK under the Wildlife and Countryside Act (1981), a “Priority Species” under the Biodiversity Framework and listed as a European Protected Species under the Conservation (Natural Habitats &c.) Regulations 1994 (aka The Habitats Directive) and included in Appendix III of Bern Convention. In short, it’s an offence to kill, capture or disturb a dormouse or damage/destroy its breeding or nesting sites without a dormouse disturbance class license from Natural England.
That which follows is a summary of Hazel dormouse natural history.
Size: Head-body length 6-9cm (2.4-3.5 in.) with 5-8cm (2-3 in.) tail, although false tail autotomy (shedding tip of tail to escape predator) common in this species. Weigh 15-43g (0.5-1.5 oz.), although typically ~17g (0.6 oz.) and rarely >30g (1 oz.) – heaviest during late autumn prior to hibernation.
Colour/Appearance: A small mouse-like mammal with long tail fully covered with short fur. Short nose with long (up to 3cm/1 in.) whiskers and relatively large black eyes. Prominent ears up to 1.4cm (0.6 in.) protrude above head fur. Dorsal fur is orangey-brown or sandy; off-white underneath. Juveniles greyer cf. adults and melanistic animals recorded in Germany. Small number (~10%) have white-tipped tail. Feet have four digits and specialised pads that aid climbing.
Distribution: In UK confined to England and Wales. Bulk of population in southern and southwest England (incl. Isle of Wight) plus southern and eastern Wales. Very sparsely distributed through west Wales, the Midlands and northern England as far north as Newcastle. Globally, species is found from Cornwall/Brest in west, east into central Russia, north to southern Sweden and Estonia, and south to Athens and Sicily.
Habitat: Predominantly a species of deciduous woodland (81% UK population); hazel coppice of particular importance, but also found in oak, holly, birch and oak/ash woodland and mature mixed hedgerows. Occasional use of coniferous and sycamore forest. Optimal habitat requires abundance of fruiting trees, with vigorous unshaded shrub layer. Long coppice cycles appear to benefit species most and woodland >20ha (49 acres) required for sustainable population.
Longevity: Long-lived for mice. Mark recapture suggests 6-7yrs in wild, same as captivity, although most wild individuals survive max. 4yrs. Estimated 60-70% population dies in hibernation annually.
Sexing: Impossible at distance. In the hand, genital and anal opening closer in females (3-4mm) vs. males (6-7mm). Testes descend and are obvious during breeding season.
Activity: Primarily nocturnal, emerging ~1hr after sunset and returning in early hours. Juveniles may be encountered during day in late summer when dispersing. Adults more prone to daylight activity during autumn when preparing for hibernation. Spend most of active time in trees/shrubs, 2-10m (6.5-33 ft.) off ground, and are extremely agile. Reluctant to cross large open spaces. Smell, hearing and vision appear acute.
Hibernates October/November to April/May, during which time heart rate and breathing drops to ~10% active level and body temperature may hover around 0C (32F). Body temperatures of -2.9C (27F) recorded in Europe where ambient dropped to -18C (-0.4F). Preparation appears to be driven by photoperiod and weight; heavier animals hibernate first. Arouse periodically, esp. when temperatures fall below ~-5C (23F). Individuals <20g (0.7 oz.) at onset are unlikely to survive hibernation. Males emerge from hibernation 2-3 wks ahead of females, which may not emerge until mid-May. May undergo brief bouts of torpor during summer if food in short supply or conditions inclement (e.g. wet/windy), but this delays breeding season.
Territory/Home Range: Sedentary with permanent home range. Typically range within area of 0.3-0.9ha (0.7-2.2 acres), with males having larger ranges cf. females. In rich habitat may move over only 700 sq-m (837 sq-yds). Tend to forage within 50-70m (164-230 ft.) of daytime nest. Males display territoriality during breeding season and maintain range overlapping several females. Live at densities of max. 10 per ha (ave. 3-5 adults per ha).
Nests: During summer may build woven nests in low vegetation, but also use variety of other sites – tree holes, bird nests, squirrel dreys, behind loose bark. Tree holes appear preferred summer nest sites and may limit population. Readily use nest boxes for resting and breeding, but not hibernation. Females begin construction of nests ~3 days prior to parturition. Young moved to alternative nest if female perceives danger. Tightly woven nests of leaves and grass built at ground level, under moss, leaf litter, brushwood or among tree roots for hibernation. Hibernacula must remain humid and this perhaps explains why dormice don’t hibernate in nest boxes; dormice susceptible to desiccation if nest dries out.
Diet & Feeding: Specialist feeders taking high-quality foods including flowers (esp. protein-rich anthers), nectar, nuts, berries and insects. Diet varies with season and plant community around nesting sites. Spring hawthorn and honeysuckle flowers commonly taken. Honeysuckle nectar and bramble pollen consumed during summer, although often hiatus between end of spring flower and onset of late summer/autumn fruiting season that might trigger summer torpor. Hazelnuts eaten while ripe but still on tree, as husk softer and easier to penetrate than once nut has been shed; takes ~15 min per nut. Chestnuts and acorns also taken and rare reports of eggs and nestlings in diet. Diet in conifer forest is poorly understood but thought aphids and sap may predominate. Will sometimes visit garden feeders during the autumn while preparing for hibernation.
Reproduction: Breeding season runs May-September with two birth peaks – late June/early July (south) and early August (north). Some young born as late as October. Gestate for ~23 days and produce 1-2 litters of 2-8 pups (ave. 4-5) per year. Altricial young born hairless and blind. Hair begins growing at ~7 days and have covering of short, velvety grey fur by ~14 days. Ears and eyes open at 14-17 days. Young leave nest with mother at 3-4 wks and 6-11g (0.2-0.4 oz.). Weaned around 4-5 wks and disperse at 8-10 wks – sexually mature at 1 yr.
Behaviour and Sociality: Poorly understood. Typically solitary, coming together to breed, although marking of individuals suggest long-term pair bonds, with same male and female using same nest box over successive years. Spend several minutes at nest entrance prior to emerging, apparently checking for danger; washes thoroughly once emerged before beginning to forage. May “freeze” for 30+ mins if disturbed before creeping away. Vocalisations include mewing and purring, whistling (esp. juveniles), and a ‘zeeep’ noise when disturbed in hibernation or torpor. Likely to emit ultrasonic calls, but this is poorly studied. When angry will adopt aggressive stance and flick tail akin to a squirrel.
Predators: Extensive predator base covering at least 30 species. Tawny owl (UK) and Tengmalm’s owl (Europe) probably most significant predators, although most owls and raptors will take them occasionally, including little owls, buzzards, sparrowhawks and even golden eagles. One record of pheasant excavating and eating hibernating animal. Martens, weasels, cats (wild and domestic) and foxes probably most significant mammalian predators. Some suggestion wild boar actively search for hibernating animals and pannage pigs may also take them.
Threats: Habitat loss and fragmentation considered biggest threat. Decline in coppice management results in overgrown stands that shades out understorey, making habitat less suitable for dormice. Coppice rotations of <15yrs typically unsuitable for dormice owing to paucity of flowers, fruits and nuts. Short coppice rotations of <10 yrs tend to be practiced today in effort to promote butterfly and flower abundance. High rabbit and deer densities may also retard coppice development to detriment of dormice. Removal of mature hedgerows also a significant problem owing to elimination of food, nest sites and “highway” between small woodlands.
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 Dormouse (2nd Edition) - by Paul Bright & Pat Morris
The Mammal Society -- 2005 -- ISBN: 978-0906282533
The Encyclopaedia of Mammals - by David MacDonald (ed.)
Brown Reference Group -- 2006 -- ISBN: 978-0199206087
The Hazel Dormouse - by Rimvydas Juskaitis & Sven Buchner
Westarp Wissenschaften -- 2013 -- ISBN: 978-3894322595
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