Temporal range: Middle Miocene – Recent
The hazel dormouse or common dormouse (Muscardinus avellanarius) is a small mammal and the only living species in the genus Muscardinus. It is 6 to 9 cm (2.4 to 3.5 in) long with a tail of 5.7 to 7.5 cm (2.2 to 3.0 in). It weighs 17 to 20 g (0.60 to 0.71 oz), although this increases to 30 to 40 grams (1.1 to 1.4 oz) just before hibernation. The hazel dormouse hibernates from October to April–May.
The hazel dormouse is native to northern Europe and Asia Minor. It is the only dormouse native to the British Isles, and is therefore often referred to simply as the "dormouse" in British sources, although the edible dormouse, Glis glis, has been accidentally introduced and now has an established population.
The United Kingdom distribution of the hazel dormouse can be found on the National Biodivestity Network website.
The hazel dormouse has golden-brown fur and large, black eyes. It is a nocturnal creature and spends most of its waking hours among the branches of trees looking for food. It will make long detours rather than come down to the ground and expose itself to danger.
In winter (early October), the hazel dormouse will hibernate in nests either in the crooks of trees, nesting boxes or logs piles. Dormice are almost completely arboreal in habit, and rarely if ever become terrestrial. When it wakes up in spring (late April or early May), it builds woven nests of shredded honeysuckle bark, fresh leaves and grasses in the undergrowth. If the weather is cold and wet, and food scarce, it saves energy by going into torpor; it curls up into a ball and goes to sleep. The hazel dormouse, therefore, spends a large proportion of its life sleeping − either hibernating in winter or in torpor in summer.
Examination of hazelnuts may show a neat, round hole in the shell. This indicates it has been opened by a small rodent, e.g., the dormouse, wood mouse, or bank vole. Other animals, such as squirrels or jays, will either split the shell completely in half or make a jagged hole in it.
Further examination reveals the inner rim of the hole has toothmarks, which are at an angle to the hole for the dormouse. The toothmarks are parallel with rough marks on the nut surface for a wood mouse; the bank vole leaves parallel grooves with no rough marks.
The hazel dormouse requires a variety of arboreal foods to survive. It eats berries and nuts and other fruit with Hazelnuts being the main food for fattening up before hibernation. The dormouse also eats Hornbeam and blackthorn fruit where hazel is scarce. Other food sources are the buds of young leaves, and flowers which provide nectar and pollen. The dormouse also eats insects found on food-source trees, particularl aphids and caterpillars.
- Hedgerows – These are species-rich and connected to woodland. Ideally, they are three to four metres high, and left at least seven years before cutting, because many shrubs do not begin to fruit until that time period has passed.
- They usually only travel less than 70 m from their nest.
Protection status 
Plants of value to dormice 
- Hazel is the principal food source, supports insects, forms an understory of poles, especially when coppiced, which makes it useful for its arboreal activity. The hazel dormouse's Latin name avellanarius means 'hazel'.
- Oaks supply insect and flower food; the acorns are of little value.
- Honeysuckle bark is their primary nesting material, and flowers and fruit are used for food.
- Bramble flowers and fruits provide food over a long period. The thorns give protection for nests. Dormice thrive on blackberries.
- Sycamore supplies insects and pollen, and a habitat. However, they cast a dense shade which decreases the understory.
- Ash – seed keys whilst they are still on the tree
- Viburnum lantana – fruits and flowers
- Yew – fruits are a favoured food
- Hornbeam – seeds
- Broom – flowers (in early summer)
- Sallow – unripe seeds, supports many insects
- Birch – seeds
- Sweet chestnut provides an excellent foodsource, and the flowers are eaten, as well.
- Blackthorn – fruits
- Hawthorn flowers are an important food in the spring. The fruit is eaten occasionally.
- Predation from Eurasian badger, fox, stoat, weasel
- Trampling, e.g., deer, human
- Lack of food source, e.g., from too frequent hedge-trimming, or competition from other species, e.g., squirrels
- Destruction of forest and hedgerow habitats, or their diverse range of species, as a broad spectrum of food is required across the calendar year.
- Amori, G., Hutterer, R., Kryštufek, B., Yigit, N., Mitsain, G., Meinig, H. & Juškaitis, R. (2008). Muscardinus avellanarius. In: IUCN 2008. IUCN Red List of Threatened Species. Retrieved 29 October 2009.
- Mitchell-Jones, A. J., Amori, G., Bogdanowicz, W., Kryštufek, B., Reijnders, P.J.H., Spitzenberger, F., Stubbe, M., Thissen, J.B.M., Vohralik, V. & Zima, J. (1999). The atlas of European Mammals. London: Academic Press. p. 484.
- The Dormouse Conservation Handbook published by Natural England
- Dormouse: European protected species. Natural England Species Information Note SIN005 (19 October 2007)
- Hedgerows for Dormice. Ptes.org. Retrieved on 2012-12-28.
|Wikimedia Commons has media related to: Muscardinus avellanarius|
- The Mammal Society site with a Hazel dormouse fact sheet. There is also a book entitled The Dormouse available, by Pat Morris.
- Peoples Trust for Endangered Species site describing the hazel dormouse and its conservation
- Information and images from the BBC
- Extensive information and pictures
- Pet care
- A lot of facts, links and book reviews about the dormouse
- Dormouse nest-box construction