Hazel dormouse ecology

Hazel dormice are about 8cm long in head and body, with a tail of about 6.5cm, relatively large eyes and small, round ears. Their weight, varying during the year as they fatten for hibernation, averages 19-20g. Average lifespan is three years but they can live up to five. They’re the only small mammal in the UK with a furry tail. 

Species: Hazel Dormouse Muscardinus avellanarius
Class: Mammalia (Mammals)
Order: Rodentia (Rodents)
Suborder: Sciuriomorpha (Squirrel-like rodents)
Family: Gliridae (Dormice)
Genus: Muscardinus

A year in the life of a dormouse

In the UK hazel dormice spend the winter in hibernation and will start to emerge during March. Dormice hibernate in winter, emerging from March, males about two weeks before females. By April about half the population is active. They reproduce May–October, our monitoring data suggesting just 0.69% of litters in May, 5.11% in June, but most in August (31.0%) or September (35.8%). By September dormice start to increase their weight for hibernation, continuing into October.


A day in the life of a dormouse

Dormice are strongly nocturnal and occasionally active during afternoon hours. Torpid dormice (with reduced metabolic rate and temperature) are found in spring and by May over 50% of dormice found in nest boxes are torpid.


Dormouse in torpor

Dormice hibernate to outlast a lack of food over winter. It’s a special state that the body undergoes when all vital functions are reduced to a minimum. Their hibernation is interrupted with periodic arousals (with a high energy cost), possibly to expel metabolic waste products. They hibernate on the ground where temperatures are more stable and conditions damper to avoid desiccation. They make small nests that they seal themselves into under log piles, the base of tree stumps, in soil depressions or under leaf litter.

Hazel dormouse in torpor snoring (video credit Lauren Alexander)



Pregnancy lasts for about 3.5 weeks with an average litter size of four typically born in July or August, but also as early as late May or early June. Young are weaned after four weeks but may remain with the mother as juveniles before they become independent and disperse. A single litter is usual, but early breeders may have a second. The young are sexually mature after 8-10 weeks, so young from early litters can breed in the same year. To survive the winter hibernation, dormice probably need to weigh 15-18g.


E Thomas photo credit -hazel-dormouse-nest-ecology-PTES
Hazel dormouse nest

Dormice construct different types of summer nests which are well insulated so that they can save energy to maintain their body temperature::

  1. Mixed nests of equal volumes of leavers and grass
  2. Grassy nests often found in conifer
  3. Foliage nests made from dry or fresh leaves
  4. Layered nests with and outer layer of leaves and an inner woven nest, used only by females for breeding

Honeysuckle is often an important component of nests. While the presence of green leaves doesn’t always signify dormouse presence, if those leaves were cut from a tree and carried to the nest from afar, it’s highly likely to be a dormouse nest.


Nut shell nibbled by a hazel dormouse

Dormice lack a caecum in their gut so they aren’t well-adapted to digest cellulose in leaves. They feed in succession on a range of other foods as they come available: flowers of oak, hawthorn, sycamore and willow in spring, followed by later flowering honeysuckle and bramble; caterpillars, aphids and wasp galls in summer; and fruits and berries such as blackberries and hazelnuts in autumn to fatten for hibernation.

Population density

Due to their specialised habitat requirements and their low breeding potential, dormice densities are lower than other rodents with an average of 4–10 animals in spring per hectare in ideal habitat. This drops to about two dormice per hectare in spring in oak dominated wood with hazel.

Home ranges

Dormice are relatively sedentary mammals. Male home ranges are larger than females, the range of one male usually partially overlapping the ranges of several females. Radio tracking studies suggest that a male home range is about 0.5 hectare and a female about 0.2 hectare.

We have been working to save hazel dormice in the UK for over 20 years. Find out about our campaigns and how you can help.

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