Tag Archives: Brown Bear

Bear Hibernation (Part Two)

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Last week I posted about the mechanics of bear hibernation, but how does a large mammal manage to curl up in a ball in a cave and sleep for five months? Humans and other mammals would die if they tried to hibernate. During hibernation, bears do not eat, urinate or defecate. What physiological adaptations allow them to do this?

While in hibernation, a brown bear’s breathing drops from 6 to 10 breaths per minute to one breath every 45 seconds. His heart rate drops from forty to fifty beats per minute to nineteen beats per minute, but his body temperature decreases only a few degrees and does not drop below 88° F (31°C), which is within 12°F (6°C) of his normal body temperature. Some scientists consider bears to be “super hibernators.” Because they have thick fur and also a lower surface area to mass ratio than do smaller hibernators such as rodents, bears lose body heat slowly, which allows them to cut their metabolic rate by 50-60%. Physiologist Øivind Tøien at the University of Alaska Fairbanks has discovered that while a black bear’s body temperature only drops an average of 9.9° F (5.5° C), the bear’s metabolism plunges to 25% of the average summer rate. Furthermore, his studies indicate that when a black bear comes out of hibernation in the spring, it takes several weeks for the bear’s metabolism to return to normal.

The amazing physiological adaptations of bears during hibernation are of much interest to human medical researchers. If a human must endure prolonged bed rest due to paralysis or illness, if a broken limb is immobilized, or if an astronaut spends several months in space; the human body faces such risks as blood clots, heart failure, significant loss of muscle mass, a breakdown of carbohydrates, proteins, and fats, as well as life-threatening bedsores. A hibernating bear has none of these risks, including no loss of muscle function. Scientists are interested in determining what specific changes in metabolites, proteins, and hormones allow bears these physiological adaptations during several months of inactivity. Humans, as well as all other mammals who maintain non-weight-bearing positions for an extended period, suffer from osteoporosis, but bears do not lose bone mass during hibernation. When the secret to how bears accomplish this feat is discovered, it may help people with weak bones, patients who become bedridden for a prolonged period, people who suffer paralysis, and astronauts on long space missions.

While bears are hibernating and metabolizing body fat, their cholesterol levels are twice as high as respective cholesterol levels in humans. Bears, however, do not suffer from arteriosclerosis or gallstones, conditions which plague humans with high cholesterol. Furthermore, a bear’s liver secretes a substance that dissolves gallstones in humans. Insight into how bears recycle urea during hibernation could lead to advances in treatments for kidney failure and dialysis in humans. Also, bears gain a great deal of weight in the fall before going into hibernation, but unlike many obese humans, they remain insulin-sensitive. Conversely, they become insulin-resistant once they are in hibernation, so their fat does not break down too quickly, but when they wake in the spring, they once again respond to insulin. In other words, bears can put themselves into a diabetic state while in hibernation and then reverse out of it in the spring. Understanding what allows bears to do this could lead to breakthrough medical advances in the treatment of diabetes and obesity in humans.

There’s so much more going on with hibernation than simply curling up for a long winter’s nap. I look forward to reading new scientific studies on bear hibernation.

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Kodiak Bear Hibernation (Part One)

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Hibernation is one of the most amazing physiological adaptations in nature, and the more I learn about bear hibernation, the more the process fascinates me. Most bears have recently left their dens here on Kodiak, and they are slowly restarting their digestive systems as they prepare for the summer feeding season. Over the next two weeks, I’ll post about denning and hibernation for Kodiak bears. I wrote a post a year ago about den emergence on Kodiak, but this time, I will discuss hibernation in more detail.

Bears hibernate in the winter to conserve energy when weather conditions are harsh, and food is scarce. While hibernating bears experience a degree of dormancy, it is nowhere near as extreme as it is in many other species such as bats, squirrels, and rodents. On Kodiak where winters are relatively mild, bears often go into and out of hibernation and some bears (mostly boars) do not hibernate at all. It is common to see bear tracks in the snow all winter on Kodiak.

Bears typically enter their dens in the order of pregnant females followed by lone females and then females with cubs. Males are the last to enter their dens, and large, old boars in particular may not hibernate at all. Denning conditions vary from year to year depending on the weather conditions and the availability of food.

The time of den emergence in the spring is also dependent upon temperature and weather conditions and varies by sex and age. Males typically leave their dens first, followed by single females and then sows with cubs. Sows with newborn cubs are the last to emerge   Males usually spend three to five months in hibernation while pregnant females may hibernate as long as seven months.

While there is an instinctual aspect to denning, it also appears to be a learned behavior that sows teach their cubs. Perhaps the most important information the mother bear relays to her cubs is how to choose a den site. On Kodiak, there are few natural rock caves, so bears must dig their dens into the sides of the mountains or the sides of snow banks. Lawrence Van Daele, Victor Barnes Jr., and Roger Smith studied and compared denning behaviors on the northern part of the island to those on southwestern Kodiak. On the northern portion of Kodiak where the mountains are taller, they determined that the bears in their study group denned at an average elevation of 2180 ft. (665m). The bears in this region preferred steep slopes in alpine habitat for their dens, probably because the dens were high enough that the soil remained frozen throughout the winter, and the den structure remained stable. On southwestern Kodiak Island with its gentler topography, bears denned at an average elevation of 1499 ft. (457 m) and preferred midslope habitats near alder thickets. The researchers believe that the alder roots help stabilize these dens that were dug in loose, unfrozen soil. It is important that a bear chooses an area for his den that will remain stable throughout the winter and won’t collapse, but dens often do collapse in the spring and summer after the bears have emerged, and the bears are forced to dig new dens the following fall.

Before a bear digs and enters his den, he eats a large amount of food to build his fat layer. Berries provide natural sugars, and a bear may gain as many as 20,000 calories per day from eating berries in the late summer and fall. Bears also drink large quantities of water and consume foods high in protein, such as salmon. Brown bears add six to eight inches (15.24 to 20.32 cm) of fat before hibernation. They stop eating shortly before they enter their dens.

During hibernation, bears do not eat, urinate, or defecate. They curl up to conserve heat, but they may change position in the den. They are sensitive to their surroundings, and hey may awaken and move about or even temporarily leave the den. Pregnant sows give birth while in hibernation, and they can lactate to nurse their cubs. They can also lick and groom their cubs.DSC_0208

Bears emerge slowly from their dens in the spring. A bear may go in and out of his den many times over the course of several days before leaving his den for good. This is especially true of sows with newborn cubs who may stay near the den for one to two weeks, slowly allowing the small cubs to adapt to the outside world. Upon emergence, bears are groggy and sluggish, and it takes time for their bodily functions to return to normal. Soon after leaving the den, a bear will pass a fecal plug that may be as long as two feet in length (61 cm). This plug consists of digested food that has accumulated in the lower intestine as a result of not defecating all winter. After leaving the den, bears drink large quantities of water and slowly begin to eat new plant shoots.

Next week I will write about the amazing physiological adaptations bears employ during hibernation and how and why these adaptations are being studied by human medical researchers.

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Tragic Bear Tale

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My tragic bear tale occurred in the mid-1980s during our summer bear-viewing trips. My husband, Mike, and I were walking down the beach with five guests. We were finished bear viewing for the day, and since there were no bears in sight, we were talking quietly. Mike looked up on the hill above the beach and saw a sow watching us. Mike knew immediately that she was agitated. She popped her teeth, and foam frothed from her mouth. Mike yelled at us to get back, and although I had never before been frightened around bears, the sound of his voice made my legs tremble. I repeated his orders to our guests, who were trying to understand the situation. Mike yelled at the sow again and then pumped a shell into the chamber of the .375 H&H rifle he always carries on our bear-viewing trips. Normally, the loud, metallic sound of a shell being injected into the chamber of the rifle is enough to deter curious bears, but it had no effect on this bear. She stomped her front feet on the bank and lunged from side to side, while she continued to foam at the mouth. Mike fired once into the dirt in front of her, a maneuver sure to make her flee. She stood still for only a moment and then flew down the cliff straight toward Mike. He shot again, and she dropped six feet from him.

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At the time, I didn’t realize what an impact those few seconds would have on the rest of my life. All I felt then was grief and sympathy for her two yearling cubs. Mike was so distraught over the experience that he considered never taking another bear viewer into the woods, but he knew brown bears rarely charge humans, and this probably never would happen to us again. The following day, Mike skinned the bear and turned the hide over to the Alaska Department of Fish and Game (ADF&G). A biologist determined that the sow had been 23 years old, and both the biologist and Mike believed that at her advanced age, her senses may have been impaired. She’d probably been asleep, and when she awoke and heard us walking down the beach, she considered us an immediate threat to her cubs and didn’t hesitate to charge. The biologist believed the cubs had approximately a 50% chance of survival through the winter. Not only would they have to avoid being killed by larger bears, but they’d need to build up their fat reserves, find or dig a den, and survive hibernation without the aid of their mother.

For many years after the sow charged us, I was terrified every time we took a group of guests bear viewing, and I was especially wary of sows and cubs. Looking back, I now believe I suffered from a form of post-traumatic-stress disorder, and it took a long time to overcome the trauma of that sunny, July afternoon. The experience heightened my respect for the speed and power of Kodiak bears, and it was also a crash course in understanding the differences between a bluff charge, often seen with sub-adult bears, and the real thing.

Kodiak Bear Sow and Cub

I no longer dread getting close to brown bears. On the contrary, I love sitting on a riverbank watching bears chase salmon, and seeing a sow interact with her cubs is a special treat, but after that July encounter so many years ago, I will never be complacent around brown bears.