Author: tthier

Wildlife

Bear Hibernation Part III

tthier

Black bear dens in northwest Montana come in a variety of types. The most common den consists of where a large tree toppled over years ago, bringing up a large amount of soil with the roots. Black bears will hollow out an area under the fallen tree where the trunk meets the roots. Usually, these cavities are not that much larger than the bear itself. There may or may not be nesting material, which usually consists of rotted wood, chewed off bushes, or conifer branches. I found a couple of dens in the Yaak where bears had spent considerable time stripping adjacent cedar trees of their bark to make some very cozy nests.

While working in the Yaak on my M.S. Degree, it was not uncommon to find where black bears used large, old cedars for denning purposes. These dens could either be in the base of a hollowed-out tree, or some considerable distance up the tree. One den that I found was 70-80’ above the ground! Old cedars typically will have several tops and with a hollow interior. If one of the tops breaks away, it provides bears access to the hollow interior. One den tree I found was a “condominium”, with one den at the base of the tree and a second 20-30’ above ground. Another den was inside a large, old cedar that had snapped off completely about 15-20’ above ground and was similar to a chimney. We were able to climb up and look down upon the denning female below. She apparently was satisfied with dealing with falling snow from above, as long as she was protected from the wind and predators on all sides.

Other black bear dens I have seen were in small caves, rock crevices, hollow logs, slash piles and rock overhangs. Probably the most unique den I have ever seen, was where a bear denned in an abandoned wolf den! Unfortunately for the bear, the wolves also returned to den nearby. As soon as the wolves left in early July, we hurried to the site to determine the bear’s fate. Bear hair and bones were found about 100 yards away, clearly indicating the outcome.

While black bear dens are almost always found at low to mid elevations, grizzly dens are almost always found on a steep, remote slope above 6,000’. Grizzlies have long claws that they use for digging, and they are quite adept at digging dens straight into a mountainside that might be 8’ or more from the den entrance to the back of the chamber. They may or may not dig under a tree or large rock that helps to stabilize the material above them. Dens might be in forested areas, or right out in the open. Their dens are almost always lined with beargrass, which is probably how this plant got its name. They also like to add the chewed off branches of Subalpine Fir and Menzesia, a high elevation shrub that gives their beds some additional “springiness”.

Twice I have seen where grizzlies denned in limestone caves. In both instances, a large amount of “soil” derived from decomposed organic material from earlier winter beds indicated the caves had likely been used by grizzlies for centuries.

One of the more unique examples of a grizzly bear denning, didn’t involve a den at all. About 5 years ago we had an instrumented female grizzly that decided to den in the upper Pinkham Creek area. Twice during the winter, the location was pinpointed from the air. When we went to the site to get an idea of what she had used, all we found was a large nest of beargrass against a fallen log. We are unsure if she dug into an existing snowdrift like a polar bear, or simply let the snow fall on her like an Eskimo’s sled dog, with her body heat eventually forming a cavity. Under what seems like unbelievable conditions, she was also able to successfully produce 2 cubs!

Wildlife

Bear Hibernation Part II

tthier

While I was working on black bears in the Yaak for my Masters degree, I had the pleasure of cooperating with Dr. Ralph Nelson from the Mayo Clinic in Rochester, Minnesota. Dr. Nelson was studying hibernation in bears for a variety of human-related reasons, not the least of which was to assist NASA with sending man to Mars. The theory was that if we could induce hibernation in people, we could therefore select astronauts that were somewhat rotund and put them to sleep for 6 months while we sent them off through space. Not only would this alleviate long periods of boredom, but it would also greatly reduce the amount of necessary resources such as food and fuel.

Dr. Nelson had several black bears in captivity in Minnesota that he was studying and from which he collected blood and urine samples. He wanted to know if various parameters such as proteins and hormone levels were similar in wild bears. Dr. Nelson and one of his assistants joined me in the Yaak in March of 1988, where we entered the dens of a number of radio-collared bears that I was monitoring. We took snowmobiles to wherever the bears were denning, tranquilized them, and drew blood and urine samples for later analysis. To my knowledge, researchers have yet to identify what exactly triggers hibernation, but they have found a number of things that can be hugely beneficial for people.

For one thing, bears are able to recycle their nitrogenous waste (urea) and reconvert it back into protein, or muscle mass. Bears have microbes in their gut that converts urea into a form of nitrogen they can use to make new amino acids, which are the building blocks of protein. Imagine doing nothing for 5 or 6 months and you still have the same amount of muscle as before!  This has huge implications for patients on dialysis, whose kidneys are failing and must have their blood cleansed (nitrogen removed) by a machine.

Another thing they discovered is that bears do not experience osteoporosis, or weakening of the bones. Again, if you or I were bedridden for a couple of months, our bones would begin to weaken due to a loss of calcium. This is also a problem with space travel where astronauts do not put pressure on their bones due to a lack of gravity. Researchers have discovered that calcium lost from the bones of bears during hibernation is recycled and re-deposited, resulting in no net loss or strength!

In order for bears to successfully hibernate and reproduce, they must put on very large stores of fat, which are then lost. This process is repeated yearly. Obese people with excess fat often exhibit problems with diabetes and heart disease. Again, bears exhibit no such issues. For patients exhibiting problems with gall stones, scientists have isolated a compound called Ursodiol, which is found in bear bile and is produced by the gall bladder. This compound has since been reproduced synthetically and is currently used by doctors to dissolve cholesterol-laden gall stones and to treat certain liver diseases.

As time goes on, it is likely even more fascinating aspects of bear physiology will be discovered. It will be interesting to see what’s next!

Wildlife

Bear Hibernation (Part I)

tthier

People are fascinated by bears for a variety of reasons, not the least of which is their ability to hibernate. Imagine an animal much like people in many ways, being able to crawl into a den for 5-6 months and remain there, without eating, drinking, urinating or defecating, the entire time. As if that wasn’t enough, females give birth to their young in January and nurse and care for them until they eventually leave the den in April or May.


I began working on bears in 1976 in the North Fork of the Flathead under the guidance of Dr. Charles Jonkel and the Border Grizzly Project. In the 1980’s, I did my Master’s Degree on the effects of hunting on black bears in the Yaak. Part of my research involved entering the dens of hibernating black bears, tranquilizing them, and replacing their aging radio collars with fresh ones and gathering biological data. It was an aspect of my life I will never forget.


For starters, when it comes to hibernation, it is important to first understand bear reproduction. Bears exhibit an interesting reproductive strategy called “delayed implantation”. Breeding occurs from late May to early July. The fertilized eggs of a female will cleave several times and form a blastocyst, but unlike most other mammals, the blastocyst remains free-floating rather than implanting in the wall of the uterus and developing further. The blastocysts remain free-floating until the female bear enters the den in the fall, usually late September or early October. At that time, the female bear’s body somehow senses if she has acquired sufficient fat reserves to not only sustain the mother for 6 months, but also allow the young to develop and then nurse them after they are born. If it has been a good year with lots of berries and the female has acquired a thick layer of fat, the blastocysts will then implant in the uterus and development begins. On the other hand, if it has been a poor berry year and an insufficient amount of fat has been accrued to produce and nurse her young while in the den, the blastocysts are simply resorbed by the female or expelled. This unique reproductive mechanism saves the female from wasting a lot of energy needlessly that might also jeopardize her life.


Hibernation itself is a physiological term that involves much more than just sleeping. Breathing and heart rates slow considerably and body temperatures drop 10-12 degrees F. In “true” hibernators like chipmunks and ground squirrels, heart rates may drop to only a few beats per minute and body temperatures may approach freezing. They are very stiff and comatose and unable to defend themselves, if necessary. Also, they must arouse periodically to urinate and defecate. Because the heart rates and body temperatures of bears drop only moderately, many biologists assert that bears are not true hibernators. However, because they can give birth in the den and defend themselves if necessary, all without eating and drinking for 5-6 months, other biologists refer to them as the “ultimate” hibernator!

-Tim Thier