It is a feeling many, if not all, of us know a little too well; it’s been a long weekend, you've not gotten enough sleep and you just don't feel ready for the week ahead. It is cold and wet outside and to top it all off, there’s not even any food in the fridge. You just want to head back to bed and wake up in a few months when it’s all nice and warm again. Sound familiar?

Bank vole, photo by Elizabeth Dack

For those of us not fortunate enough to have a second home in the Bahamas, the prospect of such a super sleep might sound appealing but unless you have unlocked the secrets behind slowing your heart rate by 99%, stopping your bones from wasting away and finding the means to avoid the effects a very full bladder, then hibernation is probably not for the likes of me or you. But as we meander through autumn, with winter yet again around the corner, many of our favourite British species will be facing the prospect of a long winter slumber.

Historically it was thought that many of our species hibernated through the frigid months. Even swifts were at one time believed to hibernate. Now, although we know that hibernation and migration are essentially different ways to overcome the same problem, it is maybe surprising how much we still have to learn about hibernation. Firstly not all extended winter sleep is hibernation. There is something called torpor. Torpor can broadly be defined as a short-term reduction of body temperature on cool days. Hibernation differs mainly in that it is an extended version of torpor. Whereas hibernation is typically associated with changes in day length and altered hormone levels, torpor is more closely linked to more immediate environmental temperatures and food availability. Other than these differences, both share numerous similarities such as an often-drastic decrease in body temperature (often to within 1-2C of the surrounding temperature), a major drop in breathing rate and oxygen consumption and a fall in both heart rate and metabolic function. Basically, the whole system goes on standby and the animal hovers between life and death, for hours, weeks or even months. It’s a little like when your TV goes into standby mode... but with more jeopardy.

So, which of our favourite critters opt for the big sleep? While there are a few you might expect to see, there are a few surprise names also. Whereas a range of species such as rabbits, shrews, mice and voles all go in for a spot of torpor, the only mammals to show true hibernation in the UK are hedgehogs dormice and bats. Squirrels are well-known hibernators but in actual fact they do not hibernate and are instead active throughout the winter. Badgers are also less conspicuous during the colder seasons and instead of hibernating, opt for periods of sleep which can last for 29 hours at a time (a feat exceeded only by human teenagers).

Brimstone butterfly, photo by Chris Mills

Leading the hibernators rather surprisingly are the butterflies, with many of these beautiful ephemeral insects sheltering in our homes throughout the cold dark months. The brimstone, peacock, comma, small tortoiseshell and red admiral all overwinter, so make sure you watch out for them and leave them well alone, as they’ll re-join the awake masses in April or May.

Maybe the best example of a true hibernator is the humble hedgehog. We all sort of know it prefers to sit out the worst of the winter but few of us truly appreciate just how difficult this is. Firstly, the average heart rate for a happy hog is 190 beats per minute but this slows to a mere 20 bpm during hibernation. In order to stand any chance of surviving hibernation (because there is a frighteningly high mortality rate), a hedgehog needs to forage and bulk up before the big sleep. As much as doubling in weight, half of this maybe lost before spring. In severe years, a hedgehog will sleep between October and April, only waking periodically to use the loo (many readers will appreciate the need to get up at least once during the night, so imagine a six month sleep) or to look for the occasional snack. We still have no real idea as to why hibernating animals sporadically wake up like this. Theories abound, from the benefits of sudden bursts of activity and the increase in body temperature to possible advantageous immune responses. The truth is that we still have a long way to go before we fully understand this life-saving phenomenon.

One unexpected area of hibernation research was revealed through the study of blood from hibernating animals. When this ‘sleeping blood’ was experimentally introduced to animal hearts (removed from their bodies) in laboratory conditions, results showed that the heart tissue survived well past the expected range, with scientists predicting that human hearts destined for transplants could survive for much longer periods thanks to a better understanding of hibernation. So although we may not fully understand how a hedgehog hibernates, unlocking this fairy-tale deep sleep might have real implications for transplant patients around the world.

With the effort and energy needed to undergo this monumental sleep, then we really should do all that we can to help our hibernating species through the coming winter. Putting food out for hedgehogs, allowing butterflies to rest undisturbed and even investing in bat boxes are all ways that you can help our natural neighbours prepare for the sleep of their lives.

NWT Ambassador, Dr Ben Garrod
Header image by Tom Marshall.

This article appears in September's EDP Norfolk magazine
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