Small British Mammals: Dormice

Hazel dormouse

The general body plan of looking like a mouse or rat is one that's proven very successful for rodents; a huge number of rodent species, not all of which are closely related, broadly fit this description. Both the mouse family and the "hamster family" (which includes deer mice, pack rats, and voles, among others) are considered to be part of a larger grouping of related rodent families called the Myomorpha. This roughly translates as "mouse-like rodents" and it's a group that includes, under the most common current system, seven different families.

Historically, the Myomorpha was defined based on the structure of the jaw muscles, so that it included some animals, such as jerboas, that don't necessarily look all that much like regular mice. But modern genetic analysis has shown that this anatomic feature isn't as unique as we'd thought, and seems to have evolved at least twice, perhaps in response to a similar dietary requirement in the two different groups. As it happens, the jerboas got to stay in the "mouse-like" group (if only just), but one family of animals that manifestly do look like mice turned out to be something else entirely, and about as unrelated to true mice as it's actually possible to be among living rodent species.

This was the dormouse family.

What the "dor" in "dormouse" was originally supposed to mean is unclear. The usual explanation is that the name of the animal is a corruption of the 17th-century French word dormeuse, meaning something like "sleepy one". However, many have argued that this is unlikely, since the word "dormouse" dates to at least the 15th century, and has antecedents before that. Counterarguments have included the claim that dormeuse may also have been Norman French, but that nobody in the next few hundred years ever thought to write it down, or, perhaps more plausibly, that it at least might be somehow related to the Latin dormire ("to sleep"), or similar words in other languages. At any rate, it's peculiarly English, with quite different words being used for the animal in French, German, and so on.

The animal usually referred to as "dormouse" in Britain is more specifically the hazel dormouse (Muscardinus avellanarius), sometimes also called the "common dormouse". It lives through most of Europe, apart from Spain, Portugal, and Scandinavia outside of Denmark and southern Sweden. At the eastern end of its range, it also lives along the north coast of Turkey, and its found through western Russia about as far as Volga River. There is some evidence that it might represent two different species, but the data is sufficiently confusing that we can't even be sure yet if there any subspecies, and, if so, how many.

The hazel dormouse is roughly the same size as a house mouse, with fur that's typically golden in colour, apart from a white patch on the chin and throat. It can also be distinguished by the fact that the tail, while much longer than in a vole, has a thick coating of fur, rather than the almost hairless scaly tail of a mouse or rat. Like harvest mice, but unlike the house mouse and others, dormice can lock the tendons in their feet to enhance their grip as they climb.

The reason that they need to do this is that hazel dormice inhabit woodlands and spend much of their time in the trees. Most types of temperate woodland will do, although there is a definite preference for those with rich undergrowth, such as brambles or honeysuckle. Where patches of forest are disrupted by agriculture, dormice are quite able to move between them by travelling along hedgerows; they can cross open fields if they really must, but they clearly don't like to.

The reason that they need this undergrowth, apart from it being somewhere useful to hide in, is that the mix of plants it provides ensure a decent supply of food throughout the year. In the spring, dormice feed on buds and flowers, switching to berries in the summer, and nuts in the autumn, when they are bulking themselves up for their winter hibernation. They are not, however, strict vegetarians, and will eat insects, or even bird eggs, at certain times of the year when suitable plants are less available. Notably, unlike voles, they tend to avoid tougher foods such as leaves or grasses, preferring food that is soft and easily digested.

Although this may be affected somewhat by the state of the weather, dormice are almost entirely nocturnal, sleeping in nests in trees or bushes from shortly before sunrise until slightly after sunset. Even at night, they tend not to travel very far from their nests, and they lead a largely solitary life outside of the breeding season. Population densities of one or two dormice per hectare (2.5 acres) are typical, although, naturally, it does depend on the amount of food that's available in a given habitat.

The hazel dormouse is the one referred to in Alice's Adventures in Wonderland, which nicely illustrates the feature for which they are probably best known - their sleepiness. In fact, during spring and autumn, dormice often enter torpor for a few hours each day - a state of extreme sleepiness when their metabolism slows down well below normal sleeping levels and that's closer to a state of hibernation. This doesn't apply if they're pregnant, and it's also more common in autumn if they've already fattened themselves up enough to be almost ready for the true winter hibernation.

That can last for up to six months, although it's worth noting that this depends on how bad the winter is. Indeed, dormice at the southern edge of their range, in hot Mediterranean climates, don't hibernate at all, evidently being able to find suitable food through the mild winters. When they do hibernate, dormice select a nest site near to the ground, and cover it with grass or leaves to conceal themselves. Conceivably, this may be a less harsh environment than spending winter up in the trees, although it does run the risk of the dormouse being eaten by foxes or other predators if it is discovered sleeping.

Like most mammals, dormice don't spend the entire hibernation period literally asleep. Although they never leave the nest, once every few days they do at least partially wake up, if only for a few hours, before going back to sleep again... even though the kidneys largely shut down during hibernation, it's probable that there are only so many days the dormouse can stay asleep without needing to pee. The liver partially shuts down too, and there are also changes to a type of cell called a brown adipocyte, whose usual function is to burn fat to help keep the body warm.

In these, and to a lesser extent liver cells, unusual microscopic structures begin to appear in the cell nucleus when a dormouse hibernates, vanishing again when it fully wakes in the spring. Most of these structures are composed of RNA, and may have something to do with preserving this vital genetic material through the winter and preparing for the burst of activity it will need when the animal eventually wakes. Others, however, are composed of protein, and have no obvious purpose that we've yet thought of.

Litters of around four young are born in the summer, after a pregnancy lasting no more than 25 days. The young are weaned just seven weeks later, and, where the summer is long enough, it is possible for a mother to give birth to two litters in the same year - or to become a grandmother before the year is out. Surprisingly, it's only relatively recently that we have really analysed the mating habits of hazel dormice; it turns out that both sexes mate with as many partners as they can find, and that a given litter therefore often has multiple paternities.

Edible dormouse

The hazel dormouse, however, is not the only species of dormouse living wild in Britain. The other goes by the somewhat unfortunate name of edible dormouse (Glis glis). This comes from a Roman tradition of eating the animals, roast and glazed, and even today, they are apparently considered a rare delicacy in Slovenia and Croatia.

More recently, the alternative name of "fat dormouse" is often seen, which is probably an improvement, if hardly flattering. (For what it's worth, it's "grey dormouse" in French, and "seven-dormouse" in German, on account of the supposed length of its hibernation in months).

With the possible exception of an obscure species from southern Africa, this is the largest of all dormice, with adults reaching 18 cm (7 inches) plus tail, and weighing up to 140 g (5 oz.) - before they've fattened up for hibernation. Indeed, they are sufficiently large, and their tail so bushy, that, when they were first scientifically described in the 18th century, it was assumed they were a kind of squirrel.

Edible dormice live in much the same geographic area as the hazel sort, but also extend into northern Spain and parts of western Asia around the Caucasus Mountains and northern Iran. They are not, however, found anywhere in Scandinavia, or along the coast of the North Sea. Until recently, they weren't found in Britain, either.

Far from being native to Britain, edible dormice have only been living wild on the island since 1902, when some escaped from a private collection. They just about managed to establish a breeding population, but only because their long lifespans compensated for the fact that they could only manage to raise litters about once every other year. As a result, they never spread far, and, even today, can only be found on the southern edge of the Chiltern Hills, with a total estimated population of around 10,000.

Like the hazel species, edible dormice are forest-dwelling animals, preferring mature forests with plenty of oak and beech, although they also live in more shrubby terrain in the Mediterranean. Their diet is generally similar, and they feed on a particularly large amount of nuts when they fatten up prior to hibernation. Probably for much the same reason, they prefer forests with good undergrowth, but otherwise, they tend to inhabit forests with older, taller, trees than hazel dormice do. This is likely partly because they are more arboreal, and like to travel between the branches of densely packed trees, and also because such trees are more likely to have hollows in which they can nest.

Edible dormice spend even more time hibernating than the hazel sort, managing at least five months even in the Mediterranean, and seven or eight further north. Not only that, but, in years where there is relatively little seed production in the spring (for reasons of weather, or whatever), edible dormice can "decide" to start hibernating as early as July, in some cases spending a staggering eleven months of the year in torpor.

This is longer than any other mammal and, since they hibernate in relatively secure underground chambers, it may (unlike the hazel dormice) actually protect them from predators. Of course, it also means that they can't breed in that year, since there simply wouldn't be the time for both pregnancy and child-rearing. This skipping of entire years without reproduction is unusual for rodents, but they seem well adapted to it... which may help explain why the crappy English weather isn't such a problem for them as one might think. Indeed, most females probably only breed about once or twice in their entire lives, and, in non-reproductive years, the males never fully develop their testicles, ensuring that they don't waste any effort trying it on with uninterested females.

Being larger, edible dormice live at lower population densities than hazel dormice do, with the males, in particular inhabiting relatively wide areas. This ensures that they can meet up with as many females as possible and, as with the hazel species, it seems that when they do breed, both sexes do so with as many partners as they can. Females, in contrast, drive away rivals of the same sex and can therefore stay put in a smaller area, waiting for any passing males to come to them.

On the other hand, while males only huddle up with others of their sex when it's cold and they aren't competing for females, pairs of females have been observed sharing the same nest, and using it to raise their young. They even go as far as to nurse one another's young, something that's particularly surprising, given that female dormice are otherwise quite anti-social. It turns out that, when this happens, the females tend to be closely related - often mother and daughter - and that, while the individual benefits don't seem to be very high, they may at least be sufficient not to actively discourage such behaviour.

Compared with the mouse and "hamster" families, the dormouse family does not have many living species. Only around thirty are currently recognised, with just five species in Europe. The garden dormouse (Eliomys quercinus) inhabits continental western Europe, and scattered areas to the east, being distinguished by black markings around the eyes and a pale belly. The forest dormouse (Dryomys nitedula) lives in eastern Europe and south-central Asia out to Mongolia, and looks somewhat similar, while the Bulgarian mouse-tailed dormouse (Myomimus roachi) is found only in the eponymous country and a few parts of Turkey. As dormice go, none of these are exceptionally closely related, although they do have close relatives elsewhere in Asia, as well as more distant ones in Africa.

As I mentioned at the start, in evolutionary and genetic terms, the dormice are not "mouse-like" rodents, belonging to a separate (and, as it turns out, much older) lineage. But if they aren't mouse-like rodents, what are they? Well, it turns out that one of the families of rodents thought to be closest to the dormice also has representative species living in Britain, and it is to those that I will turn in a couple of weeks...

[Photos by Zoë Helene Kindermann and "Svíčková", from Wikimedia Commons. Cladogram adapted from Montgelard et al. 2003 and Mielka et al. 2018.]