Hedgehogs are one of the few mammals that are true hibernators. During hibernation hedgehogs are not really asleep, instead they drop their body temperature to match their surroundings and enter a state of torpor. This allows them to save a lot of energy but slows down all other bodily functions making normal activity impossible.

Hibernation is a complicated and often perilous energy conservation strategy. It is not just ‘going to sleep.’ In sleep all bodily functions remain nearly normal but in hibernation the metabolism is almost at a standstill.

Like all other animals, hedgehogs need energy in order to maintain normal body activities, to grow and to move about. This energy comes from their food (slugs, beetles, caterpillars, snails etc). As the weather gets colder such food becomes more difficult to find and so they decide to slow down their use of energy.

They become immobile, their bodies cool and their normal physiological activities are slowed down e.g. their heart rate decreases considerably from approximately 190 beats per minute to a faint 20 beats per minute, the body temperature, which is normally 35°C drops to 10°c or less and respiration almost stops (they will breath about once every few minutes).

This is very dependent on the prevailing weather conditions and starts when the temperature in the immediate surroundings becomes and remains relatively low, which in Britain is usually November, December or even January. There also appears to be an internal readiness that brings about certain physiological changes.

Towards the end of autumn, hedgehogs consider the best places to build their hibernation nests (called hibernacula). Favourite sites are under hedges and roots of trees, in piles of brushwood, inside compost heaps or in old rabbit burrows and underneath timber buildings and sheds. The nest itself is ramshackle in appearance but well constructed from old dry leaves, grass and other vegetation and can be up to 50cm (20 ins) thick.

Hedgehogs will be relatively inactive over winter, especially when temperatures drop very low. However, on milder days it is possible that hedgehogs can wake up early and look for a quick bite to eat before returning to slumber. Research also shows that during the hibernation period, hedgehogs can move between nests every few weeks, and even use nests built by other ‘hogs.

Hedgehogs in warmer climatic zones appear to be able to manage quite well without hibernating. The descendants of European hedgehogs acclimatised in New Zealand only seem to hibernate for short periods in July and August. Even in Britain, if there is enough food and the air temperature is not too low, hedgehogs need not hibernate. This applies especially to those that may be kept indoors through winter – e.g. youngsters found in autumn, weighing less than 450g (1lb) and thus being vulnerable to problems during and just after hibernation.

Because hedgehogs are immobile during hibernation they are very vulnerable to climate and disturbance. Hence, the need for protective hibernacula. They are also at particular risk if the air temperature falls too low (e.g. below freezing point ice crystals may form in their blood). If the environment cools below this temperature, their body functions have to “switch on” again and this may rouse the animal to activity.

Because hedgehogs do not hibernate for the whole winter (some do not even hibernate) we recommend providing food and water all year round. If you’d prefer not to leave food out all winter, then on very mild days, or when you notice hedgehog activity in your garden, you may want to put some food out for a short time until it gets cold again. Please make sure water is available all winter though. Just make sure to never disturb a nesting hedgehog, so avoid opening up the hedgehog house or clearing the garden or leaf or log piles as they may have some sleeping ‘hogs inside!

If you are ever worried about the health or welfare of a hedgehog, and you live in Dorset, please call us on 01929 290112 or call the British Hedgehog Preservation Society on 01584 890801 and they will be able to give you advice, and put you in touch with a local rescue centre if necessary.