Hibernation, winter dormancy, winter torpor: What's the actual difference?

👉 The key facts from this guide

A few years ago, someone asked me: "Martin, do foxes actually hibernate?"

Me, completely convinced: "Of course. They have a den. They surely sleep there all winter."

Wrong.

Foxes are completely active in winter. They even mate in January and February. Right in the middle of the coldest time.

I had simply stored it away like that at some point – and never questioned it. Hibernation, winter sleep, winter torpor. It all sounds similar. But it isn't.

And I believe many people feel the same way. We quickly say: "It's hibernating." But what does that actually mean? What happens in an animal's body?

That's exactly what today is about. Three strategies, three entirely different mechanisms. And by the end, you'll know what to answer on your next winter walk when your child asks: "Is the squirrel sleeping now?"

Hibernation: The Little Death

Hibernation is easy, right? Animal lies down and falls asleep for a few months.

Hm, unfortunately no. That would be too simple.

In reality, it is one of the most radical survival strategies nature has produced. Some call it the "little death" – and when you read what happens in the body in a moment, you'll understand why.

True hibernators are endothermic animals. Mammals that normally have a constant body temperature. Like us humans. But during hibernation, they shut everything down. Everything.

The Hedgehog: An Example

In summer, a hedgehog has a body temperature of about 36 degrees Celsius. Its heart beats 180 to 250 times per minute. It breathes 40 to 50 times.

In hibernation?

• Body temperature: 5 to 6 degrees. Sometimes down to 1 degree.
• Heartbeat: 8 to 20 times per minute.
• Respiration: 1 to 2 times per minute.

From 250 heartbeats to 20. From 50 breaths to 2. The entire metabolism runs on the back burner. For months. Four to six months, depending on the weather.

During this time, the hedgehog does not eat. It lives completely off its fat reserves. That's why it's so important for hedgehogs to find enough food in the fall.

An adult hedgehog should weigh at least 1000 grams before winter. Young hedgehogs need at least 500 to 600 grams by the beginning of November. Below that, it becomes critical.

Waking Up Costs Lives

Important: Hibernation is not sleeping through the whole time.

Even true hibernators have short waking phases. They change their sleeping position, occasionally pass urine. But they do not eat. And these waking phases are rare – every few weeks or so.

The problem: Every waking phase costs an enormous amount of energy.

The body has to warm up again. The heart has to ramp up. This drains the fat reserves. That's why it is so dangerous when we disturb hibernating animals. Every unintentional awakening can decide whether the animal survives the winter or not.

So, if you have a hedgehog pile in your garden in winter – leave it alone. Really. Your urge to tidy up can be a death sentence.

Other True Hibernators

The Edible Dormouse – it isn't called "Siebenschläfer" (seven-sleeper) in German for nothing. It sleeps for seven to eight months at a time. From September to May or June. And here it gets crazy: the short waking phases every 20 to 29 days consume 70 to 80 percent of its total winter energy. Starting up costs more than sleeping.

Bats hang upside down in caves or attics, wrap themselves in their flight membranes, and lower their temperature drastically. In flight, their heart beats 600 to 1000 times per minute – in hibernation, only 10 to 20 times. If they are disturbed, they need 30 to 60 minutes just to be able to fly again. Every disturbance costs them as much energy as weeks of sleep.

Therefore: Never enter or light up bat quarters in winter. Not even to "just take a quick look."

Marmots practice social hibernation – up to 20 animals in one burrow, warming each other. They even seal their burrow with a plug of earth and stones up to two meters long. Six months. Without food. Completely from fat reserves.

The Common Hamster – which has unfortunately become very rare here – is also a true hibernator. Its special feature: it builds up to 4 kilograms of stores in the fall and wakes up every 5 to 7 days to eat from them.

Winter Sleep: The Light Sleep

Winter sleep is often confused with hibernation. But it is something completely different.

Imagine winter sleep as a lighter sleep that is repeatedly interrupted. The animals sleep, yes. But not as deeply. And they wake up regularly and eat.

The biggest difference: the body temperature is not lowered, or only slightly. It remains largely normal. Heartbeat and respiration go down, but nowhere near as much as in true hibernation.

The Squirrel

The best-known example. And a wonderful one, because you can actually observe it in winter.

Squirrels build up stores in the fall. Nuts, seeds, acorns – hidden and buried everywhere. In winter, they sleep in their drey but wake up regularly and go out to look for their stores.

By the way: The old story that squirrels forget their hiding places and only find the nuts by chance – isn't true. They have a good spatial memory and also use their sense of smell to find nuts under the snow.

So if you see a squirrel hopping around in the snow in winter: That's not a confused animal.

It's doing exactly what it's supposed to do. Perhaps you'll also discover animal tracks in the snow – an exciting topic for your next winter walk.

The Brown Bear: Not a True Hibernator

This is where it gets interesting. Most people think bears hibernate.

They don't. Not in the classical sense.

Their body temperature drops only slightly – from about 37 degrees to 33 or 34 degrees. But they lower their metabolism by up to 75 percent.

And now comes the fascinating part: during the entire winter sleep – three to six months – bears do not pass urine or feces. Not at all.

They recycle the urea in their bodies and use it to build new amino acids. That's why bears lose hardly any muscle mass in winter. A phenomenon that medicine is very interested in.

Even more fascinating: brown bear females give birth to their young in January or February. Right in the middle of winter sleep. The cubs are tiny – about 500 grams – and the mother nurses them for months without eating or drinking herself.

Other Winter Sleepers

• Badger – retreats to its set, sleeps a lot, but is active in between
• Raccoon – also practices winter sleep
• Raccoon dog – the only wild dog that practices winter sleep

Winter Torpor: When the Body Freezes

The third strategy. And it is completely different again.

Winter torpor (cold rigor) affects ectothermic animals. Amphibians, reptiles, insects. That is, animals that cannot regulate their body temperature themselves.

With us endothermic mammals, it's like this: our body actively maintains a certain temperature. No matter if it's 30 degrees outside or minus 10 – we have about 37 degrees.

Ectothermic animals cannot do that. Their body temperature adapts to the environment. When it gets cold outside, they get cold too. And when it gets really cold, they literally freeze.

The body becomes rigid. The metabolism is almost completely shut down. No movement. No food intake.

The Crucial Difference

A hibernator can wake up on its own.

An animal in winter torpor cannot.

It is completely dependent on the outside temperature. Only when it gets warmer does the rigidity dissolve. The animal has no control over it.

Antifreeze in the Blood

Now comes the fascinating part: many of these animals survive temperatures below freezing.

How?

They have a kind of built-in antifreeze. Wood frogs, for example, accumulate glucose (grape sugar) or glycerin in their body fluids. These substances lower the freezing point and prevent sharp-edged ice crystals from forming and destroying the cells.

An ingenious invention of nature.

Animals in Winter Torpor

Frogs and Toads – bury themselves in the mud, look for mouse holes, or crawl under leaves. Some overwinter at the bottom of bodies of water and cover their minimal oxygen needs through skin respiration.

Lizards and Slow Worms – look for frost-free crevices, cracks, or holes in the ground.

Turtles – also bury themselves.

Insects – ladybugs, wasps, beetles. They look for tree crevices, cracks in masonry, or under bark. If you are interested in tracking, you will also learn to discover such hiding places.

Why Leaf Piles Save Lives

Even though these animals can survive frost: they still look for places that are as frost-free as possible. Antifreeze has its limits.

That's why leaf piles in the garden are so valuable. Or deadwood. Or stone piles. These are winter quarters.

If you "tidy up" the garden in the fall and blow away all the leaves and remove every branch – you might be taking away these animals' overwintering quarters. And thus their lives.

And the Fox?

It neither hibernates, nor practices winter sleep, nor enters winter torpor.

The fox is completely active in winter. It looks for food, hunts – and even mates in the middle of winter.

The mating season, known as the "Ranz" in German, falls in the coldest months: January and February.

If you hear a strange, hoarse barking at night in January – it could well be a fox looking for a partner. During this time, foxes are particularly active and are also seen more frequently during the day than usual.

So: Not every animal sleeps in winter. Some just keep going. If you're wondering what emergency food you can find in winter – the fox knows it instinctively.

Summary: The Three Strategies

Hibernation:

• Body temperature drops drastically (down to a few degrees)
• Heartbeat and respiration at a minimum
• No eating (or only from built-up stores)
• Can wake up independently
• Examples: Hedgehog, edible dormouse, bat, marmot, common hamster

Winter Sleep:

• Body temperature remains largely normal (drops only slightly)
• Lighter sleep with regular waking phases
• Eats in between
• Examples: Squirrel, badger, brown bear, raccoon

Winter Torpor:

• Affects ectothermic animals
• Body temperature adapts to the outside temperature
• Animal becomes rigid, CANNOT wake up on its own
• Many have antifreeze in their blood
• Examples: Frogs, lizards, turtles, insects

What You Can Do

On your next winter walk: pay attention to it.

Where could a hedgehog be sleeping right now? Where has a squirrel buried its stores? Which insect is waiting under the tree bark for spring?

You don't have to see the animals. It's enough to know they are there. That is connection to nature in the best sense.

And in your own garden: leave leaves lying. Leave deadwood lying. Stack up branches. Create winter quarters.

It costs you nothing. But for a frog or hedgehog, it can mean the difference between life and death.