| 1 | The eternal yearly cycle |
Hibernation is part of who brown bears are. They can’t stop the process – it’s embedded in their bear DNA, and the reason is the severe lack of winter food in their frigid northern hemisphere habitats.
The yearly cycle starts in early summer, when a brown bear enters hyperphagia, the manic phase of insatiable hunger when it tries to pack on as many pounds as bear-manly possible. A bear might spend all its waking hours scavenging berries and hunting elk, or it might walk down to a local salmon river, catch up on bear friends from years gone by, fight a few others, and begin eating 30 fish per day. During summer, a brown bear can pack on 3 pounds in weight per day and eat 20,000 calories.
Slowly, as autumn draws in, salmon become less plentiful and berry crops wither and die. Having fattened up to walrus proportions, a bear will choose its denning site, and spend 4-7 months in lockdown, half asleep. Unlike a hibernating squirrel, the bear won’t eat or drink for the entire time. They live solely off their immense reserves of energy accumulated during the summer.
A bear will lose 15-30% of its body weight while hibernating, mostly fat. Most bears reappear in late March, but in unseasonably warm weather they might leave in February, or even late January, particularly male bears. Then they start the cycle of fattening anew, eating steadily in spring, before accelerating when the summer salmon season rolls around.
| 2 | What triggers hibernation? |
It’s not just the winter cold, but also the scarcity of food in November that triggers a bear’s hibernation instinct. Zoo bears don’t hibernate at all when given plenty of food, although they do become sleepier and more lethargic than usual. Dwindling light probably helps to trigger the instinct as well.
In Spring meanwhile, it’s definitely the warmer weather that triggers their awakening. January 2020 was a record breaker for warmth in the US, and that March, far more grizzlies were seen roaming Yellowstone park out of season. A study on western Canadian bears found that autumn food supply and spring temperatures were the number one triggers respectively.
Pregnant females are normally first to enter their dens in late November, followed by lone females, and then females with cubs. Males enter their dens last and are the first to leave. In warmer coastal regions, dominant males may not hibernate at all, having become extra fat due to dominating all the best fishing spots. In a study of Terror Lake (don’t worry, it’s just a name) on Kodiak Island, 25% of males had at least one year where they didn’t bother hibernating.
| 3 | They somehow preserve their body mass |
A hibernating bear loses vast quantities of body fat, but despite spending months immobile, they lose only 20% of their muscle. A sleeping bear’s muscles enter random cycles of contractions, to keep them active and strong.
Bears also have extensive systems for amino acid recycling, including conversion from urea, the toxic waste product of protein metabolism from the kidneys. Not all is understand, but scientists have identified a handful of muscle genes that suddenly rev up as bears hibernate. Meanwhile, it’s estimated that humans would lose 80% of their muscle if they lay around motionless for 4 months.
Their skeletons are a similar story. In humans, bones are constantly being reformed and destroyed by cells called osteoblasts. This cycle is affected by exercise – activity accelerates it, but doing nothing slows it. Long-term coma patients can have 25% thinner bones when they wake up, while astronauts are theorised to lose 2% of their bone mass for every month in space. Yet hibernating bears don’t have this problem – they have the ability to recycle the calcium in their bloodstream during hibernation. Over 3-5 months, their skeletons don’t weaken at all.
| 4 | Miscellaneous medical marvels |
Bears have been dubbed “super hibernators” by some scientists. During hibernation, an adult brown bear will take 1 breath every 45 seconds compared to its usual 6-10 per minute. The brown bear’s oxygen intake falls by 50%, and its metabolic rate falls by 50-60%, although this is actually less than a black bear’s at 75%.
Its heartbeat will fall from 40-50 beats per minute in the height of summer to just 8-19bpm. For 4-7 months, the bear won’t go to the bathroom once: that includes number 1 and number 2.
Under an x-ray, a hibernating bear would appear to be on the precipice of a life-ending heart attack, as its cholesterol doubles to a level where gallstones and clogged arteries would be guaranteed in humans. But the brown bear’s liver secretes a special substance which dissolves young gallstones, and their arteries stay mysteriously clear. Bears also become completely insulin resistant. Insulin is the energy summoning hormone found in humans and all animals which breaks down stored fat, meaning that insulin resistance makes a bear’s energy reserves last for longer. In essence, bears make themselves diabetic, for cunning strategical reasons.
Bear hibernation is such an impressive phenomenon that NASA is now studying the various tricks they use. The goal – to allow human astronauts to hibernate on the way to Mars!
| 5 | How bears time their digging |
The average brown bear spends 37 backbreaking days constructing its winter den, by relentlessly digging through the frozen soils of the mountainside, shifting tons of solid Earth. For this, they have specially adapted 5 inch claws, compared to the 2 inch claws of black bears (who prefer to hibernate in old tree logs). The increasingly sleepy grizzly will drag in wild bedding materials such as tree branches, grass, lichen and mosses, covering not just the entrance, but the floor too. This gives the bear both comfort and insulation, to trap its body heat as it dozes for 4 months straight.
Of course, that’s if a bear decides to make a den at all. Sometimes, a bear will hit the jackpot and find a comfortable cave or rock fissure to hibernate in, while others will lie down beneath a fallen spruce tree.
In Yellowstone Park, dens tend to be built on the colder north-facing slopes. You’d expect a hibernating bear to seek out warmth, but with less sun exposure, the deeper snow pack provides extra insulation. Bears know what they’re doing!
The weather also manipulates a bear’s thinking, as according to one study, northern Kodiak bears entered hibernation on November 5th, versus November 19th for the warmer southern reaches.
| 6 | Bears reveal their den digging secrets |
The typical bear den has a tunnel leading to the main chamber, which averages at just 2 feet tall and 3 feet wide (although there’s probably one somewhere with a palace fit for a king). Sometimes, bears dig their tunnels upwards through the soil, to allow warm air to rise upwards. Others take the lazier option and dig a downward sloping tunnel. Bears are commonly unhappy with the unfinished result, and dig more than one den until they’re satisfied.
From 2014 to 2017, Alaskan researchers tracked 33 bears using GPS collars. They noticed that the grizzlies preferred significantly steeper slopes for their dens, with an average angle of 31 degrees, most likely for stability and drainage (meltwater accumulation would not be fun).
The den’s entrances are normally high and thin, only just large enough for an adult bear to squeeze through. The bear’s goal is for the entrance to become covered with the winter snowpack, for perfect natural insulation. Every April, the newspapers have a ball when dozens of awakening brown bears poke their heads out of the snow and become the perfect photo opportunity. Nicole Grangon spent 8 years trying to capture some footage, before finally succeeding in March 2020. Part of a bear’s denning wisdom is taught by its mother, but another portion is pure animal instinct.
| 7 | Our bears are innovative and resourceful |
The style of dens varies by location. For example, Kodiak Island is weird for having almost no natural rock caves. So instead, the bears dig their dens into snowy hillsides or steep mountainsides. In the more mountainous north, the average den height is 665 metres above sea level, to guarantee hard frozen ground. But in the southwest, the average height is 447 metres, and the bears prefer thick bushes of alder, where the spider’s web network of roots stabilises the soil perfectly.
When summer rolls around, the unoccupied dens collapse in their hundreds, which is why bears nearly always dig a fresh den each year. However, they do return to the exact same mountainsides. Female bears dig their new dens an average of 2.3km away from the old one, while males average at 3.7m. Males are more adventurous than females, and some have extra adventurous personalities. One bear built a den 31km away from the previous winter’s, and the next year’s den was 45km away.
That said, some spots are too perfect for bears to ignore, such as Bear Cave Mountain in Canada’s Yukon province. These infamous slopes are home to 50 hibernating bears alone, winter after winter.
| 8 | The awakening process |
When spring arrives and the snow is finally melting, a bear doesn’t burst out of its den and charge around Alaska attacking elk instantly. The first two weeks are what’s called “waking hibernation”, when a bear is walking around, able to scan for food, but still sluggish, with a reduced body temperature and heartbeat. Most newly awakened bears return to their dens several times before leaving for good, particularly mothers with vulnerable cubs.
For two weeks, if a bear gets lucky and kills some prey, they’ll usually spend more time sleeping on the carcass than eating it, because their metabolism hasn’t fully revved up yet. The first thing a newly awakened bear looks for is the corpses of elk or bison calves which have died during the winter.
Very occasionally, bears might leave their winter den for no reason other than a relaxing stroll. It depends heavily on the climate and food availability. Inland grizzlies are much more dependent on berries and foliage which quickly withers and dies, and consequently, north Alaskan grizzlies can hibernate for 7 months. But on Kodiak Island, hikers have reported seeing bear tracks in the snow multiple times, as the weather there can get surprisingly toasty. Coastal bears usually hibernate for 3-5 months.
| 9 | Cubs and hibernation |
Brown bears have something called delayed implantation, where a mother is impregnated in May or June, but keeps the embryo in stasis for months without it growing. If a mother fails to pack on enough pounds during summer, then the pregnancy will spontaneously abort just prior to hibernation.
If she does eat enough, then the ovum will attach to the uterine wall in late November, and 2-4 cubs will be born during a mother bear’s hibernation, usually in January. The newborns will be the size of a squirrel, weighing 1 pound and completely blind, hairless and helpless. The cubs spend the rest of hibernation (and 1 year in total) drinking milk from the mother, who occasionally grooms and rearranges them too. 2-3 months later, when the cubs leave to lay eyes on the world for the first time, they normally weigh 6-8 pounds.
Leaving the den too early is a big no-no, as food is in short supply. Freakishly warm Marchs can lead to newborn starvation, as the awakening instinct gets confused. Nearly 50% of brown bear cubs don’t make it to adulthood.
After 2-3 years together, things turn ugly: the mother bear forces her offspring away from her denning turf. This is great because it prevents bear inbreeding, but it forces a lonely odyssey of several years where cubs have to find their own denning slope to call home.
| 10 | Differences to other animals |
Officially, bears are a hibernating animal, but they never enter the minimal consciousness of true hibernation like a hedgehog or a bat. Instead, a bear’s hibernation is “torpor”, a long period of rest without a fully dormant state, where the animal can be woken within minutes if suddenly disturbed.
Hibernating bears aren’t motionless statues: they commonly curl up to preserve heat and shift position in their dens. Pregnant mother bears can wake up to give birth and fall back to sleep within minutes. Protecting her cubs is her top priority, and if the mother bear subconsciously detects danger then she can wake up again, whereas hedgehogs have no need to because the species doesn’t breed in its den.
For example, a news story did the rounds in 2012 when a 12 year old Finnish boy had the terrible luck to ski into a brown bear’s den by accident. This was January, but the bear was alert enough to claw his back and thighs, before fleeing and leaving her cubs helpless.
A hedgehog’s body temperature falls to 3 degrees during hibernation, but a bear only falls to 33C, 5 to 6 degrees below its core body temperature. Bears also reduce their metabolism far further than squirrels and hedgehogs. Their massive bulk and thick rug of fur means that they need to generate far less body heat themselves.
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