Like it or not, it’s now winter here in the northern hemisphere. Having spent several portions of my life in colder climates, I’ve often marveled at the idea that all manner of animals — from bears to groundhogs to box turtles — are nestled snugly somewhere beneath the surface of the earth, sleeping the long winter months away. Nature is amazing, and hibernation is one of the most fascinating biological adaptations in all of nature.
I recently became obsessed with learning about the science of hibernation, and I thought you all might be interested to learn a bit about it as well. For those who hate winter, the idea of curling up in a cozy den to sleep for weeks or even months at a time might sound appealing. But what is hibernation, exactly? Which animals actually do it? And is there any way humans could hibernate — or at least learn from it? Let’s find out …
1. Officially, hibernation is “a hypothermic condition when animals save energy during cold seasons by lowering body temperature, metabolism, breathing, and heart rate.” Why would animals do that? Well, mainly to reduce metabolic activity — and thus conserve energy — at the times when food is least available. That’s what “makes it possible for an animal to survive […] on a fat deposit that would otherwise not last the winter.”
But hibernation is not really one thing. It’s actually a spectrum of distinct adaptations that are generally lumped together, including “true hibernation,” denning, daily torpor, brumation, aestivation, diapause, and more. (The science can get very confusing!) In other words, the degree to which different species “hibernate” varies widely.
2. Most species known as “true hibernators” are small mammals like ground squirrels, dormice, hedgehogs, groundhogs, chipmunks, and bats. By one estimate, “The average hibernator weighs only 70 grams.”
But while all hibernators undergo substantial changes, some are what you might call elite hibernators. For example, the heart rate of some bats drops in hibernation from 300 beats per minute to as few as 10, while that of dwarf lemurs can fall to fewer than six. Both species can go minutes without taking a breath. Amazingly, hibernating hedgehogs can go two hours without a breath, as their metabolic rate falls to 2% of normal.
No less impressive, the body temperature of groundhogs (a.k.a. woodchucks) in hibernation can fall from 99°F to as low as 33°F, while that of arctic ground squirrels can even drop to sub-freezing levels. But the prize for the longest hibernator goes to European edible dormice, which — during a bad year for their food — were found to hibernate for an astonishing 11 months, the longest duration ever measured in the wild.
3. While lots of small mammals don’t hibernate, most do decrease their activity in winter. Rats and mice, for example, spend more time nestled safely in their burrows (or in your attic). Some mammals — including raccoons, badgers, and skunks — enter light hibernation-like states involving a combination of denning and mild torpor. Other warm-blooded species, including most birds, simply migrate to warmer climates. Only one bird species, the common poorwill, is known to be a “true” hibernator.
4. Most large mammals, such as foxes, wolves, dogs, cows, horses, deer, or humans, don’t hibernate at all, and remain active throughout the winter. But what about bears? Despite their reputation as elite hibernators, bears weren’t classified as “true hibernators” until recently because their body temperatures only fall a modest 7–12 degrees Fahrenheit in hibernation. In addition, not all bears hibernate. Those in warmer climates like Greece, Croatia, Mexico, or Florida don’t hibernate — or do so only briefly — because they don’t generally need to. Surprisingly, polar bears don’t hibernate either, because of the availability of fat-rich seals in the arctic.
However, it is now recognized that most species of black, brown, and grizzly bears in colder climates are in fact very efficient hibernators, despite their stature. While their body temperatures don’t drop dramatically, their heart, breathing, and metabolic rates all drop up to 75% for months at time. Amazingly, black and grizzly bears can hibernate “up to 7 ½ months without eating, drinking, urinating, or defecating.” That sounds intense!
5. Of course, warm-blooded animals (a.k.a. endotherms) aren’t the only ones that hibernate. Many cold-blooded reptile, amphibian, and invertebrate species (a.k.a. ectotherms) also enter a hibernation-like state in winter called brumation. Many likewise exhibit similar feats of extreme endurance. The hearts of wood frogs, for example, actually stop beating in hibernation and 35–45% of their bodies freeze over, making them appear stone dead.
But there’s one big difference between endothermic and ectothermic hibernation: for warm-blooded animals, hibernation is an active response to both internal cues and external cues like temperature change and resource availability. If resources are plentiful, most mammals won’t hibernate, even if it’s cold. Brumation, on the other hand, is a more passive response to decreases in outside temperature, meaning that changes in the body temperatures of cold-blooded species simply correspond to those in their surrounding environment.
6. Whether cold- or warm-blooded, all hibernating animals take steps to prepare for winter. For starters, this means finding safe hibernating spaces, or hibernacula, which can be just about any crevice that is warm and hidden from predators. Most small mammals, reptiles, amphibians, and hibernating insects either burrow underground or hide in rock crevices, hollow trees, or under logs, brush, or leaves. Some nest under porches or attics. Bats seek out caves or tunnels, while bears carve out dens. Many aquatic species, including some frogs and turtles, hibernate in the mud at the bottom of ponds. Perhaps most notably, some species, such as snakes or skunks, winter in communal dens. Snake dens have been found to contain more than 8,000 snakes at a time … EEK!
7. While hibernation is often seen as a cold weather phenomenon, some species in hot climates also enter a form of hibernation in periods of drought or extreme heat called aestivation. Most aestivating species — such as crabs, beetles, tortoises, crocodiles, salamanders, toads, frogs, and lungfish — are cold-blooded, and typically bury themselves in the ground to beat the heat. With built-in hibernation units, many snails simply seal up their shells with a layer of mucus and aestivate for several months to outlast extreme weather. Only a few mammal species are known to aestivate. Echidnas, for example, sometimes hibernate after wildfires, while the Madagascan fat-tailed dwarf lemur is known to hibernate 3–7 months when water is scarce.
8. One common misconception about hibernation is that it’s just extended sleep. In fact, normal sleep and hibernation are quite different. In normal sleep, bodily functions are only slightly suppressed, whereas in hibernation animals typically undergo dramatic and prolonged decreases in body temperature, metabolism, heart and breathing rates. In addition, hibernating animals don’t experience REM or the other restorative benefits of normal sleep, and find it much harder to wake up. As a result, scientists actually believe that hibernation often leaves animals with a “sleep-debt” that makes them lethargic for weeks after they awaken. As scientist Matteo Cerri explains, hibernation is not “rest. It’s suspension. You are retiring from the game of life for a little bit.”
9. Another common misconception is that hibernation is constant. Many hibernators, especially mammals, actually undergo periodic arousals of 12–24 hours throughout hibernation. But why? Scientists have a range of theories. Some species obviously wake up in order to eat, drink, or expel waste. Others may wake in order to temporarily warm up their bodies or kick-start their immune systems. Ironically, some may wake up in order to sleep. A few species, like hedgehogs, actually relocate their nests. Amazingly, female bears typically wake up in January or February in order to give birth and nurse their cubs. But all hibernators have to be careful not to wake too often — about 80% of the energy used during hibernation is spent during these periodic arousals.
10. In its essence, hibernation is an extreme survival technique. Not surprisingly, therefore, species that have some capacity to hibernate tend to have better long-term survival prospects. According to a 2009 study based on a database of 4,500 living mammal species, the 443 species that engaged in at least one “sleep-or-hide behavior” were less likely to show up on a “Red List” of endangered species. That said, there are potential risks associated with hibernation. For one thing, hibernation suppresses the immune system, leaving some animals vulnerable to disease. In addition, hibernation can be so deep for some species that they won’t awake even when being attacked. In Europe, for example, great tits have been found to slowly eat the brains of bats in deep hibernation.
11. So what about us? Can humans hibernate? Or could we, like Han Solo, perhaps be frozen in carbonite?
The short answer is no. Notwithstanding some intriguing historical anecdotes, no humans we know of have ever truly hibernated — and for good reason. We face some basic physiological barriers. As one New York Times article put it, if you “[g]ain a few hundred pounds and lie around in bed for months […] you are likely to develop a host of ailments, from diabetes and heart failure to muscle loss, osteoporosis and bedsores.” Yet, animals that hibernate, like bears, do exactly that without suffering long-term health effects. Even more astonishing, some hibernators go months without expelling waste. Humans have no such capacity.
Likewise, our bodies can only stand narrow changes in temperature before they begin to fail, not to mention becoming more vulnerable to disease or infection. It’s even unlikely we could endure the sleep deprivation.
There are lots of reasons why true hibernation is probably out-of-reach for humans, but this hasn’t stopped some from reaching for it. Since the 1950s, scientists have been trying to find safe ways to induce hibernation-like states in humans, with “an eye to all kinds of applications, such as preserving pulseless trauma victims while critical injuries are repaired, deep-space travel, and altering metabolic rates to help people lose weight.” Key advances have been made, particularly in medicine. It’s now common practice for doctors to use therapeutic hypothermia in surgeries or in cases of cardiac arrest or neonatal asphyxia. However, these treatments typically last no more than 1–2 days, a far cry from the months it would take, say, to enable long-distance space travel.
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Even if we can’t truly hibernate, what might we learn from this extraordinary biological adaptation? Well, for starters, it’s a reminder that nature is awesome. It’s the kind of thing that might just help instill in us, as the Book of Common Prayer puts it, “a reverence for the earth as [God’s] own creation.”
Hibernation is also a reminder that — like many other non-hibernating mammal species — it’s still okay to slow down a little in colder seasons, perhaps spend more time huddled together in our dens, and just generally embrace our winter lethargy rather than constantly trying to overcome it. If we do that, heading back to work in the first week of January might just feel a little less like a much-needed vacation from our winter vacations.
Finally, even though we can’t physically hibernate, there are times when we do experience what the School of Life refers to as “a mental midwinter.” As the School of Life explains,
[O]ur minds too have cycles. We cannot be permanently fruitful or creative, excited or open. There are necessary times of retrenchment when, whatever we might desire, there seems no alternative but to stop. We can no longer be productive; we lose direction and inspiration.
The general hibernation we observe in nature during the winter months can be reassuring in these contexts, because it teaches us hope — the mysterious engine that helps us endure the fallow periods in our own lives. Yes, life’s normal processes may be suspended and it may look as if nothing’s happening, but below the surface, “nutrients are being gathered, the groundwork for future ebullience and dynamism is being laid down, another summer is very slowly collecting its strength.” The “deadness” we may be experiencing is not an end, but “a prelude to new life.”
Loathe as we are to admit it, we may just need some time in hibernation mode. Let us, therefore, “make our peace with our own midwinters — and lean on nature’s wise accommodation.”
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