British Wildlife of the Week

Hazel Dormouse

The dormouse has a famously sleepy disposition. It’s an image that was cemented over 150 years ago when a very tired dormouse appeared in the well-known tea-party scene in Lewis Caroll’s Alice’s Adventures in Wonderland. And unlike many animal reputations worldwide, this one doesn’t need exaggerating. No other British mammal sleeps for such a high proportion of the time – it can spend over half of the year asleep, from October through to April or May. Even during the summer, when it is supposed to be awake, it may enter smaller periods of inactivity called torpor if weather conditions or food supplies are poor. In all, a dormouse may spend as little as a quarter of its life awake.

Of course, most of that isn’t really sleep – it’s hibernation. Hibernation is not so much a strategy to escape the cold (some animals hibernate during warm months, too, although this is called aestivation); rather, it’s an adaptation to food shortages in which a very low body temperature reduces energy consumption. Animals that would really struggle to find the necessary food during winter switch all metabolic functions into ‘standby mode’ and wait for more tolerable conditions to return next spring. It is often assumed that many of our small mammals hibernate over the winter months, but apart from several species of bat, only two British mammals go into a state of true hibernation: the hedgehog and the hazel dormouse.

With double-jointed ankles that enable it to get a good grip when clambering around and a long, furry tail aiding balance, this agile and acrobatic woodland dweller is perfectly adapted to an arboreal existence. It spends most of its time awake scurrying within thick, overgrown hedges, dense scrub, and the canopy of deciduous woodland. During the spring and summer, it feeds mainly on leaf buds, pollen, nectar and insects. But as autumn approaches and food becomes scarcer, the dormouse would have no chance of making it through the coldest months with what meagre scraps remain. It therefore switches to fruit, nuts and seeds – including hazelnuts, a particular favourite, after which this animal is named. It relies on this new autumnal diet of high-energy foods to gain weight and build up reserves of fat before hibernation.

Around mid-October, the hazel dormouse finds a hole or creates a spherical nest out of woven grass and moss, then tucks its head into its belly, wraps its soft tail around itself, and allows the heat of its body to slowly seep away. Its heartbeat drops to one-tenth of the normal rate. Its breathing becomes so shallow and infrequent that it is difficult to detect at all. With its metabolic processes reduced to an absolute minimum during this state of suspended animation, the body’s fuel demands are so low that the animal’s fat store is sufficient enough to keep all the essential processes ticking over for months.

The dormouse is programmed to awake when warmer weather returns in the spring. It may have lost as much as half of its body weight during its long slumber, and now it feeds urgently to satiate its huge post-hibernation appetite. Unfortunately, milder winters caused by climate change are affecting hibernators such as dormice, causing them to wake up earlier than they normally would do, when there is little food available.

The hazel dormouse was once so plentiful and widespread in Britain that it was often tamed as a pet. Beatrix Potter kept one, and Victorian schoolchildren sometimes swapped them in the playground. But the number of these endearing little rodents is estimated to have fallen by over half since the start of the 21st century. Habitat fragmentation has slowly broken ancient woodlands into smaller pieces, and as fields became larger during the intensification of agriculture, hedgerows upon which dormice often depend became scarcer. As its home and food supply began to die out, so did the dormouse itself.

But since 1993, thanks to a collaborative project led by the People’s Trust for Endangered Species (PTES), around a thousand captive-bred dormice have been reintroduced back into the wild, at many different sites around England. The dormice chosen for reintroduction are supplied by the Common Dormouse Captive Breeders Group and quarantined for about six weeks at the Zoological Society of London and Paignton Zoo, during which they receive detailed health examinations and are microchipped. Each release in June is a highly secretive affair to ensure that these shy mammals aren’t disturbed by human spectators.

With their tiny bodies, enormous eyes, bushy tails and chubby, hamster-like bodies, these honey-coloured creatures are irresistibly cute and adorable. It seems the hazel dormouse is as well-loved as rats are hated, even though the two, being both rodents, are related. There is nothing even remotely controversial about the dormouse, so it has been comparatively easy to raise money for its conservation and reintroduction. (It’s a completely different story for a rodent that is even rarer in this country than the hazel dormouse – the black rat – but we’ll save that tale for another time.)

Whereas a wood mouse can manage six litters in a single year, the dormouse, unusually for a small mammal, only produces one. It seems as though it can only fit in a single litter between the time it wakes up from hibernation and the time it returns to its deep slumber. But this low rate of reproduction is offset by the fact it lives a lot longer than most other rodents. Whereas mice and voles tend to breed fast and die young, seldom living beyond 12 months, dormice take a more leisurely approach to life and can reach the ripe old age of five.

In the next British Wildlife of the Week, we’ll be looking at an arachnid whose sightings, much to the distress of many arachnophobes, increases around this time of year – the much-maligned house spider.