A torpid dormouse, handled under licence.
Fact file
Latin name
Muscardinus avellanarius
Threat status
Least Concern (Global)
Least Concern (Europe)
Vulnerable (UK)
Identifying features
Sandy coloured fur (though they can be black), which extends all the way down the tail.
Habitat
Broad-leaved woodland, plantation woodland (including some confider woodland), hedges
Numbers
Dormice live at a low density in the UK at about 3-6 per hectare, which means they need a large area to support a viable population (minimum around 20ha of continuous woodland). The current estimate of population size is 930,000 (95% confidence interval = 389,000–2,640,000) from the Mammal Society Review of the Population and Conservation Status of British Mammals. They are classified as Vulnerable in the UK as they have declined by between 50% and 72% since the 1990’s.
Distribution
Dormice are found throughout Europe and into Russia. They are at the northern end of their range in the UK.
Diet
Dormice rely on high value food, i.e. food that is easy to digest with high calorific content. Their diet includes flowers, nectar, insects, fruit, pollen, and nuts (particularly hazelnuts). Unlike most other UK rodents, dormice do not have a large caecum, a part of the gut which in other animals contains bacteria to help digest plant matter. This means that dormice cannot digest things like leaves or grass.
Size
Adult dormice typically weigh between 15g and 26g, but can get up to 45g in preparation for hibernation! Their bodies are also between 6cm and 9cm long, with 5.7cm to 6.8cm tails.
Life span
Up to 7 years wild, and 9 in captivity.
Breeding
In the UK, dormice typically only have 1 litter a year between June and October, though they can have up to 3 in captivity. They are also able to have 3 litters a year in Europe, likely as they have more time to forage for food, due to the longer summers. Dormice have 4 to 5 young per litter, which take about 6 to 8 weeks to become independent. They exhibit quite a lot of parental investment in their young, and so will not eat their young if disturbed, like other rodents!
A hazel nut nibbled by dormice!
Survey techniques
Dormice cannot be disturbed without a licence from Natural England, which is needed for some survey techniques. These methods include putting up nest boxes for long term monitoring projects, such as the National Dormouse Monitoring Programme, or using nest tubes for short term studies. Dormice can also be detected using footprint tunnels, as they have recognisable triangle shaped toe pads! Finally, you can identify hazel nuts that have been opened by dormice because they have a unique way of opening them. This does not require a licence.
Legislation in the UK
Dormice are heavily protected in both UK and EU legislation, including the Wildlife and Countryside Act (1981) and the Conservation of Habitats and Species regulation (2017). They are also a UK Biodiversity Action Plan (BAP) species, which means they have been identified as threatened and in need of conservation. This protection means it is illegal to disturb, capture, injure or kill a dormouse, disturb its resting place or possess/sell live, dead or parts derived from a dormouse. Exceptions to these protections can be made through a licence from Natural England, which allows scientific research and building/development to take place.
Hibernation and torpor
Torpid dormouse in a nest box, surveyed under licence.
One of the key features of dormouse ecology is that they hibernate throughout the winter and often torpor throughout the rest of the year. Hibernation is a state of deep sleep, where the animals metabolic rate, breathing and body temperature decrease to conserve energy. This usually occurs in winter as a way of coping with reduced food resources. For dormice, hibernation occurs from October to April, depending on the local temperature conditions. They hibernate in nests close to or just under the ground, as this provides a more constant temperature, and in humid places such as under leaf litter or in deadwood piles, as this prevents the dormouse from losing too much water as they breathe. During hibernation a dormouse’s body temperature will decrease from around 37°C to near ambient temperature (around 2-6°C!), though they will wake themselves up every 10 days or so to stop organ failure and defecate.
Torpor is similar to hibernation but is shorter, and the animal does not decrease its metabolic rate etc. as much. For example, during torpor a dormouse will only drop its body temperature to around 15°C, and it only lasts up to 9 hours. Torpor is used by dormice during the spring and summer to conserve energy during periods of food shortage or bad weather (wet and/or cold).
Agile Climbers
Dormice are arboreal (tree-climbing) specialists, that rarely come to the ground. Therefore, they have several adaptations that make climbing easier for them. They have incredibly sticky feet, due to a secretion on their pads, and opposable digits (1 on their hind feet and 2 on their front feet), that makes it easier to grip onto bark. They also have double jointed ankles, like squirrels, which means their ankles can rotate backwards so they can climb down tree trunks head first. They also have long tails for balance, and long whiskers that might be used to find their way along branches in the dark.