Most of my discussion will be on Alaska's brown bears since that's what my customers are interested in seeing, but I've included information on all bears, even in my home state of Arizona for those more interested.
When do the bears go into hibernate?
That depends on the sex, but generally late October or early November. Pregnant females are the first to go in, followed by females with older cubs and younger males. The larger males are last and may not hibernate until late November or early December. Occasionally a bear will skip hibernation but that is rare in colder areas like Alaska. A male, or even a barren female may rise briefly for a few days during the winter, and some will even leave one den and dig another, but this is not the norm. Most bears stay in the same den and rarely rise unless bothered.
When they leave the den is the opposite with respect to order. Larger boars are first and may leave in April. The last to leave are sows with new cubs. They will be awake by mid May but she will probably stay in very close proximity to her den until as late as mid June. The rest of the bears will leave between the 2 extremes.
You may find it interesting that bears all over the world, including black bears as far south as Mexico will go into and exit their dens about the same time as brown bears in Alaska or Kamchatka. This certainly indicates a genetic component to it.
Where do bears den in Alaska, and what do dens look like?
Generally, coastal brown bears will den in the higher elevations on steep slopes just off the coast. They will dig a fairly substantial hole with somewhat of a tunnel and a "main sleeping room". The primary concern with den placement for a bear is to keep moisture out. If water does get in, they may have to leave and dig another.
In their dens, they build a nest. Studies done in Alaska found dens lined with alder and willow branches along with dried grass. You may think a bear den as a smelly musty area with all kinds of bones over in the corner, but it's quite the opposite. Since bears don't urinate or defecate when they hibernate they have a nice nest with dried vegetation and it can actually be quite nice. Although no one has been crazy enough to crawl into a brown bear den while occupied, it's frequently done by black bear biologists to check reproduction. In the 30 odd black bear dens I entered during my research I was always amazed how sweet they smelled and how clean they were.
Do they reuse the same den?
No, surprisingly not given the amount of work that goes into it. They do however have a strong fidelity to den in the same area each year and the new den will not be far from last year's.
Why do bears hibernate?
The fact that bears spend the winter hibernating is known by all school kids. But "skipping" the winter is not unique to bears. In fact all animals that are dependent on a seasonal food source, be it vegetation covered by snow, salmon that are no longer available, or insects that die must do something to survive a season of no food. Birds and some large mammals migrate to find their food. Bears don't have that option; there is nothing to eat during winter so they hibernate. They don't hibernate to skip the cold, although it probably helps. They have advanced hair that keeps them warm and a good layer of fat (blubber) that maintains their temperature. In good food seasons male polar bears are known not to hibernate, and in wet years black bears in Mexico skip hibernation because berries are available all year.
What happens to the bear physiologically when they hibernate?
Well, if you were to ask a hibernation specialist they would tell you that bears don't actually hibernate. So what you thought you knew from grade school is not entirely true. A specialist would tell you that bears go into long term torpor. In contrast, the arctic ground squirrel, a neighbor and food source for brown bears, is what they would consider a true hibernator. The difference being that bears don't allow their body temperature to drop but 6 to 8 degrees below normal (100 F, 38 C). Bears slow their heart rate from an average of 40 beats per minute (bpm) to 8 to 10, and there is a general slowing of all other the body functions. But the definition of a true hibernator is one that allows it's body temperature to drop to the temperature in the den. So in an arctic ground squirrel den, if the temperature drops to 34 degrees F, then that's the body temperature of the animal. They are" totally out of it", so much so that you can pick a true hibernator up, hold it, twirl it, anything you want as long as its humane and the animal is never aware of it. A torpid animal however does not allow its body temperature to drop that low, and one of the advantages, specifically for that of a bear, is that it can wake up quicker when it finds itself in danger and protect itself. When I visited those 30 odd occupied black bear dens I would have liked the bear to be totally out, they never were. In fact each and every one of them was looking right at me and was probably as aware as I was that there was nothing but 4 feet of solid air in between us. Yes they could move, and fast if they wanted to.
Another reason to maintain temperature is that pregnant bears have to keep their body temperature warmer. Remember, that the cubs don't start growing beyond the 300 cell stage until the female enters the den. If the female allowed her body temperature to drop to the just above freezing temperature in her den, then the young 1) would not be able to develop fast enough, and 2) they would likely freeze to death after birth since the mother's warmth and fur keeps them insulated against the cold temperature in the den.
Why would bears need to be aware when hibernating?
What the heck is going to crawl into the den, especially of North America's largest carnivore? Well, it does happen, much rarer today than probably historically, but more on that slight chance later (Can you guess what animal [not humans] might crawl into a brown bear den, kill it and eat it?) But behaviors we see today are a product of the species evolution, in other words their past. So from 500,000 to just 20,000 years ago brown (and black) shared their habitat with other large carnivores, larger wolves, large hyena like animals, a stronger cave bear, and large cats such as the sabre toothed tiger. These formidable creatures probably had the ability to take a brown bear while it was not hibernating, and certainly would have been able to when it was incapacitated. A bears ability to waken quickly and flee or try and defend itself would have an advantage over those that didn’t. And that's how natural selection works, those with traits that let them live longer and reproduce successfully have traits that become more and more common and end up "fixed" in the population. How often bears were killed in their dens historically is difficult to determine, but some bone remains found in caves do indicate it happened. And as humans became more advanced with better weapons (> 10,000 ya), indigenous people would take advantage of bears while hibernating as that was the easiest time to kill them, even with their ability to recover from torpor quickly. So today, bears do have an advantage by going "torpid" instead of hibernating as well as historically. OK, so what did you guess that is a current brown bear predator in its' circumpolar distribution from northern NA through Asia, all the way to Europe? As I said, it doesn't happen often because this predator is endangered, but in a recent brown bear study in Kamchatka, 3 radio collared female bears were killed and eaten in their dens by Siberian tigers. A 700 lb cat is just unfair! No animal has a chance against that.
Are you wondering why arctic ground squirrels don't do what bears do, keep their body temperature higher "in case of emergency". Then simple answer is they can't. Only animals as large as a bear can carry enough fat for the energy to stay at a warmer temperature all winter, a small ground squirrel can't. A bear can lose 25% of it's weight over the winter, a pregnant female may lose 40%, a true hibernator can't or won't. So ground squirrels have the advantage of not having to put so much weight on, but the disadvantage is if a weasel finds a way into their den, they are dinner.
What are some of the mysteries scientist may be able to solve by knowing more about bears hibernation?
Medical Doctors, endocrinologists, biomedical engineers, and others are all studying bears in hopes of learning more of their physiological adaptations. Understanding how bears survive and thrive in hibernation could have huge benefits for our species. They are particularly intrigued by how bears maintain their muscular and bone strength while immobile; the processes their kidneys, liver, and endocrine glands utilize to actually recycle waste products, and what substance or compound causes the bear to start the less dramatic form of hibernation.
Bear bone density does not decline while in the den, when in humans or most animals periods of disuse result in the lack of strength commonly known as osteoporosis. The recovery period needed in humans for the bone to regain strength is often 2 to 3 times the length of disuse. So spinal injuries, disease, or even space travel can result in reduced bone strength and increase in fractures, particularly in the large femur (leg) or humerus (arm). Bear bones do not lose their strength. Researchers know that during hibernation that the Parathyroid Hormone is higher, and that is an important part of bone rebuilding in mammals. They also know that one of the benefits of not urinating is that calcium is not lost, which is the principle building block of bones. Humans, or other mammals of course continue to urinate when bed ridden, and the elevated calcium ions due to bone disuse leave the body that way. Somehow the bear has a trigger that recycles the calcium, and triggers bone "rebuilding" just as if it were active.
Another key difference is that bears don't lose the muscle strength that you or I would when they are immobile. If you don't use a muscle, for a series of reasons it begins to break down and is known as muscle atrophy. If you or I were bed ridden for 6 months, the average human would lose 80% of their strength. Both brown and black bears have been tested for strength both pre and post hibernation and at most they lose an average of 10%. Imagine the benefits to us, or other animals if researchers could find how they retain strength during long bouts of inactivity.
At present they know 2 things are going on that reduce muscle atrophy. It does not matter if an animal is active or resting, the proteins in the muscle are slowly breaking down and need to be replaced, and movement helps the formation of the key muscle proteins. A byproduct of protein break down is ammonia, a toxic substance, which mammals quickly break down to urea. Urea is one of the key constituents of a mammal's urine because even though it's much less toxic than ammonia, it still can create problems if levels get to high in the blood. Since bears are not urinating, scientists were first interested in how high the levels of urea were and how much they could endure. From blood samples taken from hibernating bears, it was actually found that urea levels were slightly less than those of active bears which had researchers totally perplexed. It has since been hypothesized that bears use the waste urea and to build back proteins in the muscles. So as the muscle breaks down, it is rebuilt from a normal waste product that is not urinated. How exactly bears do this is not known, and the mechanism is of real interest to medical researchers.
A more recent discovery was made with miniature temperature sensors that were surgically implanted in wild bears before they hibernated. Sensors planted near the central body cavity showed the temperature dropped the initial 8 degrees in early hibernation, but never varied by more than one degree the rest of the hibernation period. Sensors planted near the skin however showed that temperatures varied from near freezing (the temperature of the den) but then would frequently rise again to the central body temperature. Although this may sound strange, you need to realize that not all parts of your body are 98.6. When you are outside and it's cold, your hands and feet and even muscles directly below the exposed skin are much cooler. This is normal until they get too cold and then the pain/numbness will force you to try and do something about it from putting on gloves to going inside to shivering. Hopefully that's possible for you, but for sleeping bear it's not. Researchers wanted to see how cool bear extremities (feet, exposed skin) could cool. From further study is was determined that on average of 4 times a day hibernating bears go through a shivering period where the muscles near the skin would move so much that they would produce enough heat for the body surface to warm back up to the body core temperature. Then they would stop shivering, cool down, and then warm up again. These bouts of shivering serve 3 purposes in a bear. They help the bear maintain it's whole body temperature from getting to cool, they keep the outer surface of the bear warm enough that there is no tissue damage during extremely cold periods, and they work the muscles enough that the atrophy is discontinued and the movement helps the rebuilding of key muscle proteins with waste urea. So by frequent shivering that does not cause them to wake and mystically using a waste product to rebuild muscles bears are able to maintain strength while hibernating.
The search for the compound that circulates throughout the bears body to slow it down, yet not completely, is an important and ongoing research project as well. In their search, researchers have dubbed the still unknown compound as Hibernation Induction Trigger, known as HIT. Their hopes are that if they can find this substance, it could be crucial in slowing down the decay of organs used in transplants. At present, an organ lasts about 16 hours before tissue deterioration makes them unusable. It is hoped that HIT might increase the life of an organ threefold, so we could all benefit from the physiological secrets of a hibernating bear.
In my Animal Physiology class, I have several Pre - Med students that at first just can't seem to understand why we have not learned more from the bears. Why with such possible human benefits has bear hibernation been more well studied, causes identified, and cures for humans developed? The answer is relatively simple, bears just don't make good laboratory subjects and are not easy to get to in the field. A wild bear will not allow you to visit several times to take the samples that are necessary to solve such complex questions. They have tried with captive bears as well, but they are still large and powerful, and not really tame in the sense of a dog. Plus captive bears often change their physiological status due to continued interruptions, and captive bears often go through a much lighter torpid or hibernation phase, or if well fed, don't go into hibernation. As more sophisticated micro electronic sensors that can be implanted in animals are developed, researchers may be able to learn much more about how bears can do things that other animals, including humans, can't.
So there is literally a wealth of information to be gleaned from even a torpid or hibernating bear.