Why Do Hedgehogs Hibernate?

The seasons seem strange just now you can't predict what the weather will do from one day to the next. We still had bees in the garden in November, and the daffodils are already out in Cornwall in January.  Many people are seeing hedgehogs still out and about in December and wondering what's going on? Shouldn't they be hibernating now? Why are they still out? Will they be OK? So now seemed like a good moment to take a look at why hedgehogs hibernate see if we can find answers to some of those questions. 

Why Do Hedgehogs Hibernate?

There has been a lot of research done on hibernation, not least because of its benefits for us humans. But even after all this we still don't understand hibernation all that well. But before we get into the "why" of hibernation, let's first take a look at what hibernation actually is. 

What is Hibernation?

In the cold dark days of winter curling up and going to sleep for a few months can seem like a good idea. And hibernation is often described as going to sleep for the winter. 

But hibernation is very definitely not sleep. In fact, in some ways, it's pretty much the opposite. 

Sleep is a physiological necessity—a period when the body and mind undergo major maintenance and repair work. Without sleep, our physical and mental health suffer pretty quickly. Though we may not be aware of it, sleep is a very busy time for the body and mind. 

Hibernation, by contrast, is a period of shut-down. It's so different from sleep that studies have shown that many creatures need a nap as soon as they rouse from hibernation, or in the case of the hedgehog, rouse for periods during hibernation specifically so they can go to sleep. 

Hibernation is an almost science-fiction-like slowing down of the hedgehog's bodily functions. For example:

• Heart rate slows from over 200 beats per minute to less than 10
• Body temperature drops from around 35 c to under 10
• Breathing slows to the point that hedgehogs only need to take a few quick breaths once an hour. (Wildlife Online)
• The brain pretty much shuts down other than a small area of the hypothalamus with remains alert to dangers. (New Scientist)
• Growth stops. You can see when a hedgehog has hibernated by growth rings in its bones, just like those on a tree. (Pat Morris).

This all creates such energy savings that fat reserves which would only last an active hedgehog 16 hours can sustain her for 120 days during hibernation. 

Why Have Hedgehogs Evolved To Hibernate?

Hibernation seems to be a response to unfavourable environmental conditions. 

There is a "hibernation gene", and scientists have identified that all major mammal gourds have it (yes, that includes us humans). But obviously, not all mammals can or do hibernate. 

Creatures who do hibernate have evolved in ways that make hibernation possible. For example, the human heart (and many mammal hearts) cannot function if it's temperature falls below 28c. At that temperature, calcium starts to gather around the heart and causes heart attack. 

The hedgehog's body temperature falls below 10c during hibernation. Ther heartbeat slows, but it certainly doesn't cause a heart attack. Why is this? Well, a special pump to remove calcium from the heart has developed in hibernating animals that humans don't have. 

So, if so many creatures have the hibernation gene, what have some evolved to hibernate and some not?

Well, it's thought to be one response to unfavourable environmental conditions. 

Humans, for example, evolved in equatorial Africa. There is little variation in seasons here, and food is available year-round. 

Contrast this with the European hedgehog who has evolved in a climate with definite seasons and significant variations in the seasonal food supply. 

In British and European winters the weather is cold and the food supply limited. 

Mammals, including the hedgehog, don't use the sun's warmth to keep their body temperature up. Instead, they burn energy derived from the food they eat. 

As air temperatures plummet in winter hedgehogs need to generate more and more energy just to keep warm. But at the same time as temperatures are dropping, so is their fuel supply. As autumn turns to winter, there are fewer and fewer invertebrates about for them to eat. Their need for fuel is going up, just at the time the fuel supply is shrinking. 

Something's got to give. 

Birds fly south or adapt their diet to suit the seasons, foxes and badgers are superbly insulated with thick fur coats. Squirrels store up food in larders for the winter. 

The hedgehog clearly isn't flying anywhere. Spines are a fearsome defence mechanism, but do a dismal job as insulation. And hedgie's teeth and claws aren't really up to the job of cracking nuts and seeds. 

So taken in this context evolving to hibernate through the winter seems like a very reasonable energy-saving strategy. 

What Triggers Hibernation?

Hedgehogs start to prepare for hibernation weeks before the temperatures drop, and food becomes scarce. 

Hibernation nests or hibernacula are large, elaborate and very well insulated structures that take time to build. 

Hedgehogs preparing for hibernation also become hyperphagic. This means that they eat vast amounts of food to build up the energy reserves they need to see them through the winter. They also become more efficient at turning food into fat and have been observed to gain more weight from the same quantity of food during the autumn months than they do in the spring and summer. A hyperphagic hedgehog can eat 20% of her own body weight in one sitting.

But what actually sends them to bed for the winter? In the UK we used to think we could pretty much rely on hedgehogs hibernating in October/November and emerging in March/April. 

Things aren't so straight forward any more. Hedgehogs hibernate at different times every year, different individuals follow different patterns, and some hogs fail to hibernate at all. 

Some people put these changes down to climate change. This could certainly play a role, but so could human feeding. And It could just be that as we observe and study hedgehogs more, we realise that their behaviour is much more intricate than we previously thought. 

Broadly, a combination of food scarcity, falling temperatures and decreasing daylight hours seem to trigger hibernation. 

But we have no idea of the importance of each factor. So hedgehogs are observed visiting feeding stations later and later in our winters. This could be down to global warming and milder winters, but it could equally be due to the hogs having a continued abundance of food. 

And in any given year different individuals will hibernate and rouse at different times. Males tend to hibernate earlier than females and emerge earlier. This is thought to be because females, especially those who have a late litter, need longer to get up to a suitable hibernation weight than males. In the spring males emerge early to give themselves time to get up to peak form come the breeding season and stand the best chance in the mating game. 

Do Hedgehogs Need to Hibernate?

Given suitable environmental conditions - enough food (and possibly warmth), there is no need for hedgehogs to hibernate. European hedgehogs in the mild climate of New Zealand's North Island don't hibernate. And if you overwinter a hedgehog in a warm indoor environment, they are also likely to stay active all winter. 

Whilst sleep is a physiological necessity for repairing and maintaining the body and mind, hibernation is just an energy-conserving mechanism. And if there's no need to conserve energy, there is no need to hibernate. 

Some rescuers are starting to believe that given enough food, even small hedgehogs can survive the coldest winter in the wild and will not try to hibernate. Amazing Grace gives us the story of the Blackwater Babies, a mother and very young hoglets who stayed active and survived a snowy winter in the wild with the help of daily feeding. 

How Long Does Hibernation Last?

It varies. In most parts of the UK hedgehogs typically hibernate for 4 to 5 months of the year. They hibernate for longer in the north of Scotland, and the shortest typical hibernation periods are found in the south of England. In Scandinavia, where winters are much colder, hogs typically hibernate for over six months. 

Length of hibernation varies depending on the severity of the winter. But It can also vary by individual. Larger hogs hibernate for longer than smaller ones. 

Hibernation isn't continuous. A hog will rouse up to 20 times during the winter. We don't really know why. There is no evidence that they accumulate toxins during hibernation - so they're not getting up to wee and poo. Muscles don't atrophy during hibernation, so they are not getting up to exercise. Hogs will often feed when they rouse, but their act of rousing will often use more energy than they can get from the food they find. 

It's possible that they need to drink periodically during hibernation, It's also possible that they need to sleep. Many just rouse and go to sleep. Others leave the nest and look for a new one. Which could suggest that they are rousing as a safety precaution. 

Is Hibernation Dangerous?

Though hibernation is designed as a mechanism to help hedgehogs survive the winter, it is a risky strategy. 

We know now that around 335,000 hedgehogs die during hibernation each year. That's a third of the UK population. 

Some hogs will be taken by predators during hibernation, a number will be lost of weather events like flooding, and in very cold winters some will freeze. But by far the most significant cause of death is hogs being underweight and simply not having sufficient fat reserves to see them through hibernation and allow them to rouse in the spring. 

And Could Humans Hibernate?

It's not such a silly question. Scientists have been working on the possibility of hibernation for years. 

There are problems, We mentioned the heart earlier, and it's also thought that hibernation may impact memory. But the potential benefits are seen as being huge. 

As we've mentioned, the body doesn't seem to produce toxins during hibernation, and muscles and bones don't waste, even with no exercise. So there are great potential benefits, for example, for astronauts or people recovering for severe illness or major surgery. 

It was thought that although humans have the hibernation gene, we had never evolved the ability to hibernate. 

Then in summer 2020, an archaeological find in Spain made scientists question that. 

Bones of early humans, Neanderthals, found in the Sima Caves near Burgos in Southern Spain, showed the same annual "growth rings" as the bones of hedgehogs, Suggesting a period of annual hibernation. The researchers feel this makes sense, as the mountains of northern Spain would have experienced extremely cold winters with minimal food at that time. 

So, if we've hibernated before, who knows what the future holds. Watch this space!

Hibernation - Still Very Much a Mystery.

Hibernation is an amazing physical trick to pull off in response to challenging environmental conditions—just one more example of how incredible our hedgehogs are. 

There is much we still don't know about the why and the how of hibernation. But more research is assured because of its potential benefits for humans. 

The one thing we do know for sure is that hibernation is a perilous time for hedgehogs and one where we can genuinely help by offering regular food and shelter. 

We hope you've found this article interesting and informative. Do you have questions or suggestions? We would love to hear them, leave us a comment below.