The change of the seasons is often depicted by quaint nature scenes: the leaves changing color and bears bedding down for their winter-long hibernation; then flowers blooming in spring and the very same bears emerging from their den. This period of hibernation is a known fact to many, but its details remain a mystery to most. What do the bears do? Why do they do it? What is hibernation?
1. Hibernation is a state of torpor.
This state means that the animal’s metabolism is working at minimal levels: less than 5%. It’s a way to conserve energy. During this time, their body temperature drops dramatically, their breathing and heartrate slow considerably, and they aren’t fully conscious.
2. Hibernation is not the same as sleeping.
The two are very different, though from an outside observer it’s hard to tell. Some great proof of this comes from a study of Madagascan dwarf lemurs. Researchers observing the tiny primates’ hibernation recorded periods in which the lemurs were naturally roused from their hibernation – only to start sleeping. During these mid-hibernation naps, periods of R.E.M. and non-R.E.M. sleep would occur. From these naps, the lemur would then re-enter a state of hibernation. In some cases of hibernation, brain activity in the animal becomes virtually undetectable! Whereas with sleep, unconscious brain activity still occurs.
3. Hibernation is an innate behavior; animals don’t learn to do it, they just know to do it.
One factor that could inspire hibernation is a shortage of food supplies. Another is protection. Hibernation doesn’t just keep animals safe from predators, though – it can also protect them from the seasons. It serves as an alternative to migration for small animals who aren’t able to travel long distances. The most obvious trigger is seasonal change: dropping temperatures hint at animals that it’s time to hunker down.
Obvious hibernators are chipmunks and squirrels. They don’t spend all that time storing food for nothing. When the go into hibernation, they regularly wake up out of that stasis to eat, urinate, and raise their body temperature before returning back to their torpor. Bears are by far the largest hibernators on the planet, but all other mammals known to do so are a fraction of their size. On average, mammals who hibernate weigh only 2.4 ounces.
4. Hibernation can be mistakenly viewed as a winter rite, but there are animals in balmier climates who do it, too.
Their hibernation is called estivation. As mentioned, dwarf lemurs of Madagascar estivate, hiding from 90° heat in the summer. Conversely, in the Arctic, ground squirrels hibernate by cooling their bodies down to sometimes subfreezing temperatures. Groundhogs, too. Neither can stay freezing for long, though, let alone through their entire hibernation. These tiny animals will use most of the little energy they have in hibernation to wake themselves up in order to warm up, then slip back into hibernation.
It isn’t always about temperature, though. One animal that goes into hibernation while waiting for more food is the echidna, an egg-laying mammal in Australia. If wildfires recently wiped out its food supply, it will hibernate to wait out the natural resupply.
Mammals aren’t the only part of the animal kingdom who hibernates. There are several amphibians, birds, fish, insects, and reptiles who do so. In cold weather, a nocturnal bird called the Common Poorwill will spend days to weeks hibernating. The original – and appropriate – name given to it by the Hopi tribe meant “the sleeping one.” One hibernating fish is twice talented: the lungfish hibernates for months when its habitat dries up because this primitive fish still has lungs that allow it to breathe air. The lungfish passes its hibernation in the mud of Africa and South America.
5. There is no set hibernation period that applies to nature overall. Every species is different.
While many do tend to hibernate in the winter, meaning they’re out from November through March, these aren’t fixed months. Obviously, animals who practice estivation go into their state of torpor during the blazing summer months to protect themselves from unbearable, dry heat. The time will vary for every species.
6. The length of time an animal goes into hibernation depends on the reason for which they are hibernating.
Some other factors influence this length of time, too, such as the animal’s biology (its muscle mass and fat content, its waste disposal system, etc.) or its environment (if it hibernates in a a den or underground, versus in the open air).
7. Some animals hibernate for mere days, some months – bears can hibernate up to 100 days.
They can spend this long in stasis without waking up from it once for several reasons. Because of their thick coats of fur that insulate them and because they live off the fat they’ve built up over the summer are just a couple. Much smaller than a bear, the edible dormouse – a small but plump rodent – hibernates for 11 months of the year. The big brown bat potentially has the longest record. In one controlled study, it hibernated for 344 days in a refrigerator.
The shortest hibernators can do so overnight, though it’s debatable if that’s true hibernation or just daily torpor. The black-capped chickadee is a bird who enters torpor for a few hours overnight.
In some species, the length of time spent in hibernation depends on the gender. Female brown bats come out of hibernation early to give birth and begin raising their young while the males continue being out cold.
8. For the most part, waking an animal out of hibernation would more than likely kill it.
As with every other element of hibernation, the effects of waking an animal from this stasis depends on the species. Such a rude awakening from such a dramatic state would shock its low-functioning system. It wouldn’t have enough energy to wake itself up out of hibernation quick enough to raise its body temperature to normal levels. Hibernation is a slow, low-level process that animals spend the year planning for. Disrupting it (though difficult, in many cases, as some animals are close to impossible to rouse) would be unnatural.
Being in hibernation is protection in and of itself. If it can make an animal’s brain waves almost undetectable to sensitive scientific equipment, you can bet that it’ll make the critter almost invisible to any potential prey. Without its body functioning at usual levels, a hibernating animal won’t give off any betraying smells and, since it’s not the same as sleeping, they won’t have any foot-thumping dreams that’ll catch a predator’s keen ear. They remain safe in stasis, especially if they’re hidden away in a hibernation den.
9. The most common things animals do, coming out of hibernation, is find food.
It could have been months since they last ate and, as they start to move again, their body will need a fresh energy source. Many will need to expel the waste that has been building up over the weeks or months. Believe it or not, some need to sleep! Though they were functioning at minimal-levels during hibernation, their body didn’t get any of the rest that comes with sleep. They will come out of the hibernation sleep deprived. Black bears give birth during hibernation, so coming out of it is a time to rear one’s young and teach them how to climb. This isn’t the case for all, though, especially those simply going into a daily torpor.
10. Humans don’t need to hibernate because we’ve evolved beyond the need to.
We do not depend on erratic food supplies, growing them ourselves, and have developed ways to survive extreme temperatures, both hot and cold. Plus, in the event of struggling with either of these two issues, we can travel far distances in search of what we need.
11. During hibernation, the male Arctic ground squirrels stop testosterone production and shrink their testes in order to reduce their metabolism.
This mean that every spring, when they emerge from hibernation, they go through puberty again. While this doesn’t mean that they’re not aging, scientists believe that many hibernating mammals do have a lifespan disproportionate to their size, suggesting that their longevity is thanks to hibernation. The general theory is that the conservation of energy and lack of strain on the body during hibernation delays senescence (the aging process) and slows the aging of cells. Scientists are not completely sure what precisely gives hibernation its anti-aging effect. Two hypotheses are because of the slow regeneration of cells and because of less oxidative stress on the animal.
There are so many more animals who hibernate – other than just bears – and they do so on every continent for a variety of reasons. Hibernation is a more complex process than sleep. Several different functions of the body need to change dramatically in order to keep the hibernating animal alive. Those who do go through this torpor are naturally inclined to do so for biological and environmental factors. In some cases, studying hibernating animals can teach us more about the world and its natural defenses.