Day of the Dormouse

Animals, 11 April 2019
Hazel dormouse (Muscardinus avellanarius)

Dormice are nature's professional sleepers. A sleeping dormouse doesn't wake up for anything, not even if someone picks it up.

The name comes from the Latin word "dormire", to sleep. But on warm nights, they're the acrobats of the trees. They're good climbers that can go straight up a tree trunk or a stone wall.

Dormice sleep all day, and they become torpid during cold, wet weather. Torpor is like hibernation but lasts only a few hours. During that time, the animal's temperature, respiration and heart rate may drop by 90%. Dormice that live in colder regions such as northern Europe hibernate every winter for six or seven months.


Remarkable fact: Some dormice snore very loudly when they're asleep.
Try a YouTube search on "snoring dormouse". How cute is that!


Habitat, diet and reproduction

Fat dormouse (Glis glis)

Dormice are rodents. Mice, rats and squirrels are also rodents, but dormice are a separate family. There are nearly 30 species of dormouse, and they live mainly in Europe, Africa, central Asia and Japan. The main European species are:

The hazel dormouse (Muscardinus avellanarius). Very small, loves hazel nuts. See top of page.

The edible dormouse or fat dormouse (Glis glis). Quite a large dormouse. Unpopular with home-owners if it gets into their house and chews things like electrical wiring. See left.

The garden dormouse (Eliomys quercinus). Small, has a bandit mask like the Lone Ranger. See bottom of page.

Dormice usually live in open woodland, or on the edge of a forest. They're very active at night, when they explore bushes, trees and cliffs. Their favourite food is nuts, but they also eat snails, caterpillars and insects, from small insects like aphids to large ones like grasshoppers and butterflies.

They eat the nectar, pollen and anthers from flowers in the spring and summer; seeds in summer; and fruit, nuts and berries in autumn, so they like a place with lots of biodiversity. A big garden is a good habitat for them, as long as it has lots of variety and no cats.

Dormice love to sleep all day. In hot weather they may sleep in a pile of rocks or in the roof of a building, but usually they sleep in their own nest. This is a ball of grass, bark and leaves, about as big as a grapefruit. It's often lined with hair or wool for warmth and comfort. The summer nest is usually in a tree or a hedge.

Their hibernacula (hibernation nests for winter) are at ground level, because the warmth of the earth protects them from extreme low temperatures. To hibernate all winter, they need a cool place or they will lose too much fat. They also need a place with high humidity, or they will lose too much moisture. The hibernaculum is usually under a pile of dead wood or dead leaves, or in a hole in the roots of a tree.

A female dormouse is able to breed (have babies) at one year old, and will typically have 4 babies per year for the rest of her life, which can be 6 or 7 years. That makes the dormouse a K-strategist like humans. A pair of dormice (the same male and female) may stay together for two years or more. One advantage of being a K-strategist is that you have time to educate your children. A dormouse has a lot to learn about food and where to find it, and young dormice typically stay with their mother for two months.

(Mice are R-strategists; they produce huge numbers of disposable offspring. A female mouse will probably live less than a year but is able to breed at 4-6 weeks old and produces frequent large litters. A litter is one set of babies. In a single year, one pair of mice can theoretically produce more than 100,000 children, grandchildren, great-grandchildren, great-great-grandchildren, etc.)

Does all this sound like a good survival strategy for dormice? Surprisingly, yes ... well, maybe not the snoring!


A good survival strategy

Sleeping, torpor and hibernation save energy, so a dormouse doesn't need to eat every day.

Dormice are not often killed by predators. Cats, owls, stoats, weasels and snakes will eat a dormouse if they can, but they usually can't. This is because of the dormouse's arboreal lifestyle. They are usually in a tree or bush, where predators can't see them, especially because they are active only when the trees and bushes are covered in leaves.

Also, they can jump from one branch to another like a squirrel.

Dormice have been very successful. They are an ancient family, much older than mice and rats. They were one of the first mammals on the planet, back in the Eocene epoch when you could still walk from Europe to north America, and from Australia to Antarctica.

It's even possible there were dormice living in the epoch before that, the Palaeocene, at the same time as the last dinosaurs. Dormice have been on the planet a thousand times longer than humans.


Big trouble for the dormouse

Dormice have been very successful for 55 million years, but that's all changed in the last 20 or 30 years. They're in big trouble because of human activities. In the last hundred years, hazel dormice in Britain have disappeared from more than half their range. It's getting worse. Just between 2000 and 2016, the number of hazel dormice in Britain decreased by about 40%. There's a similar situation in the rest of Europe.

Aerial view of intensive farming

The problem is human population density. Farms, towns, industrial estates, motorways and roads divide big areas of woodland into many smaller areas. Dormice are not very mobile, so the population in one small wood becomes separated from the population in the next wood. If the dormouse population in one wood is too small, it dies out. For the hazel dormouse, the minimum sustainable size is a wood of about 20 hectares (50 acres), which might have anything from 50 to 120 dormice. (In the same area of woodland you might also find 800 wood mice and 2,500 bank voles.)

Intensive farming and modern forestry practices are another problem for dormice.

Until very recently, every field on a farm had a hedge around it. A hedge is a barrier, typically about 1.5 metres tall, made of tree and shrub species such as hawthorn (Crataegus monogyna), blackthorn (Prunus spinosa), hazel (Corylus avellana), field maple (Acer campestre), ash (Fraxinus excelsior), holly (Ilex aquifolium), rowan (Sorbus aucuparia), wayfaring-tree (Viburnum lantana), elder (Sambucus nigra) and some surviving elm (Ulmus minor "Atinia" or Ulmus procera), all combined with many other species such as the dog rose (Rosa canina), honeysuckle (Lonicera periclymenum), traveller's joy or old man's beard (Clematis vitalba), and brambles (Rubus fruticosus).

For a dormouse, that's not a barrier, that's a combination of home, highway and dinner. Some hedges in Britain are thousands of years old and contain hundreds of different plant species with flowers, seeds, nuts, berries and insects at different times of the year. Dormice spend nearly all their time off the ground, so a hedge is also a great dormouse highway from one wood to another. It's a kind of "wildlife corridor". Unfortunately, many farmers now prefer metal fences, so hedges are no longer maintained by hand. Instead, they're cut once a year by a farm tractor with a "flail hedge trimmer". This is a brutal process, and if it's not done carefully it will slowly kill the hedge.

Farm pesticides poison the things dormice eat, and monoculture (growing only one species of plant in an entire field, or an entire wood) means that there is lots of food at one particular time of year, but not much to eat for the rest of the year.

Mammal Society dormouse booklet

Woodland is either not maintained, so you get a lot of big trees with not much understorey of brambles, bushes and smaller plants; or maintained too well, so that all the trees in the wood are the same kind, and they all produce seeds and fruit in only one month of the year.

The hazel dormouse (Muscardinus avellanarius) is the only native species of dormouse in Britain. It's a small dormouse, it's ridiculously cute, and it spends most of its time in trees. It usually stays within 70 metres of its nest. If it feels sleepy, it may have a nap in an old bird's nest.

It once lived in most places across England and Wales, but now lives only in southern England. Even there it's in big trouble, although it is protected by law. It's a criminal offence to disturb, capture or kill a dormouse; or to damage, destroy or block access to its nest, resting place or breeding site.

Many other British species of plant, bird, animal and invertebrate have decreased at the same rate. However there is hope for the hazel dormouse in Britain, because people love it. It is regularly reintroduced to woods in many parts of the country, with success.

Reintroduction takes thought and care, because every wood and garden is different. If you just take a dormouse and drop it in a new place, it may die before it learns where to find food in that place and at that time of year.

Several wildlife organisations (charities, zoos and individuals) are interested in dormice, especially the People's Trust for Endangered Species. See the Links page. The PTES runs the National Dormouse Monitoring Programme, and provides training.

If you want to work with dormice where you live, you need a licence from Natural England. To get a licence, you will need to have specific training. If you want to photograph dormice, you probably need a licence. If you want to do a dormouse population survey, or capture and weigh them so you can see which ones may need help to survive the winter, you certainly need a licence.


How do I know if I have dormice?

Hazel nuts and mouse cache

If you live in an area occupied by the fat dormouse or the garden dormouse, you may see them in your garden shed, or even in your home, at different times of the year. Unlike mice and rats, they're not interested in your kitchen, but they naturally live on cliffs and in caves, and your house looks a bit like that to a dormouse. And your cat may bring them home, alive or dead.

If you're a gardener, you may sometimes find torpid or hibernating dormice in piles of leaves, especially under rocks or tree roots. If you do, put them right back where you found them. A hibernating dormouse needs the right conditions - not too dry, not too moist, not too hot and not too cold.

Unless you are a professional naturalist, you probably won't see hazel dormice. They are not often active in daylight, except in autumn when the nights are cold, and they don't often move around on the ground. However you may see signs of them, especially empty hazel nuts. Animals don't eat the whole nut, they usually make a hole in the nutshell and eat the contents. If you find a hazel nut that has been split in half, it was done by a squirrel (Sciurus vulgaris or Sciurus carolinensis). If it has a shattered hole ("to shatter" is to break like glass), it was done by a squirrel or a bird. If you find a hazel nut with one end missing, it was probably done by a squirrel or a bank vole (Myodes glareolus).

If you find a hazel nut with a small circular hole, it may be a hazel dormouse, but it may be a mouse (either a wood mouse, Apodemus sylvaticus, or the very similar yellow-necked mouse, Apodemus flavicollis). Mice often take nuts to a "cache" in a safe place, and eat them all in one place, so you find a pile of empty nutshells as in the photo. Dormice don't make caches; they eat nuts where they find them, often up in the tree canopy, and they drop one nutshell here, another there.

Also, voles and mice make a lot of little tooth-marks around the edge of the hole. A hole made by a dormouse has a smooth edge, as if it was made by a spoon.


Serious sleeping

Garden dormouse, Vallouise

It's always hard to wake a sleeping dormouse. I often used to see a fat dormouse (Glis glis) asleep at the Vallouise municipal campsite, in the Hautes-Alpes of France. The high Alps and other mountainous regions of Europe are dormouse strongholds. (A stronghold is a place where there are large numbers of an animal that has big problems in most other places). Winters in the Alps are tough, but there is no intensive farming and the human population density is low.

In the campsite shower block there were two pieces of wood in the roof, just under the roof ridge, with an 8mm gap between them. In hot weather, this dormouse liked to spend the whole day sleeping between the two pieces. Often his tail or one of his legs hung down through the gap.

I lived for two years in an apartment near Vallouise, in an old stone building. It was about 1100 metres above sea level. The first snow at that altitude was at the start of October, and the local dormice hibernated from October to April. In early April we had the first few days of hot weather, and the dormice started to wake up. This garden dormouse (Eliomys quercinus) woke up, climbed a 7-metre vertical wall in the cellar under my apartment, came in and started exploring my bedroom.

But then he felt sleepy ... very, very sleepy ... and he fell asleep on the floor by my bed. At first I thought he was a tennis ball. I picked him up in a drinking glass. No reaction. I put him in a bowl with some water and cereal. No reaction. I waited an hour for him to wake up, but he didn't, so I took him out to the garden shed and left him there.

Garden dormouse (Eliomys quercinus)





Try some harder English

Dormouse Conservation Handbook cover

The Dormouse Conservation Handbook, 2nd edition, 2006.

This free 76-page booklet has lots of practical information about the hazel dormouse, and it's a good introduction to scientific or academic English - because it's not too academic. Click here to read more about academic English.

There are lots of places where you can download it. I got a copy from the People's Trust for Endangered Species, using this link.

Authors: Paul Bright, Pat Morris, Tony Mitchell-Jones.
Illustrator: Sarah Wroot.

Publisher: English Nature.

ISBN-10: 1857162196
ISBN-13: 9781857162196



Picture credits

1: Hazel dormouse in torpor, Fraser Combe, Creative Commons BY 2.0
2: Fat dormouse by Andreas Waltschek
3: Fat dormouse by Cristina Sanvito, Creative Commons BY 2.0
4: Hazel dormouse, Cristina Sanvito, Creative Commons BY 2.0
5: Intensively-farmed landscape, Micha Sager
6: Book cover
7: Hazel nuts, image by Myriams-Fotos
8: Hazel nut mouse cache, Dr Mary Gillham, Creative Commons BY 2.0
9: Mouse/vole/dormouse nut graphic, Nicholas of OnePlanet and Linguetic, Creative Commons BY 2.0
10: Garden dormouse. Nicholas of OnePlanet and Linguetic, Creative Commons BY 2.0
11: Garden dormouse. Nicholas of OnePlanet and Linguetic, Creative Commons BY 2.0
12: Garden dormouse, José Belzunce, Creative Commons BY 2.0
13: Book cover