Hibernation Life slows down for grizzly bears in the late autumn. They gradually “phase” into hibernation for about two weeks before entering their dens. A threshold level of fat triggers their thyroid glands to change hormone levels and they head up-slope to 8000 feet in a state of “walking hibernation.”
Not long ago, scientists were not too impressed with the hibernation of bears because they only drop their body temperatures about 10 degrees F. They breathe every 45 seconds and their hearts beat 25 times per minute. They use energy at about half the rate they would in the summer. Compared to marmots, ground squirrels and jumping mice, that’s almost “wide awake”. They even called bear hibernation a “false hibernation”.
Lately, bears have been getting proper credit for the amazing things they do during their winter sleep – things that the “true hibernators” wouldn’t and couldn’t try. Some examples: bears give birth and nurse cubs, they don’t wake up to drink, urinate or defecate. They use only their fat for energy (not protein). They re-process urea from urine back into protein, so they gain muscle in the winter! While asleep, they lose 20 to 40% of their body weight, and while losing all that weight, they produce an acid which dissolves gallstones. Humans who try crash diets have trouble with gallstones. This acid-extract from bears is now synthesized by drug companies.
Grizzly bears don’t become pregnant when they mate. They have a special body function called “delayed implantation”, which keeps embryos on hold and only allows the female to become pregnant when she has enough body fat to make it through the winter. Some evidence suggests that especially fat bears have triplets more often.
Cubs are born in January, probably because female bears are loaded with toxins from recycling everything in their bodies for five months. Mom’s milk is 33% fat (compared to human milk at 3%). The cubs, born without hair and the size of a softball, nurse on a sleeping female who wakes every so often to eat the droppings of the cubs. (It’s a good thing that she is drowsy!) She recycles the cub droppings back into milk and protein!
Cubs stay with the female for two years, so females only mate every other year. The young bears will not reach mating age until they are five or six years old.
When bears wake from hibernation, they are not hungry, as you might expect. They take about 3 weeks to operate normally, gradually returning to eating available food and learning how to urinate and defecate again.
Grizzly cubs learn what to eat, where it is and when to move to the food from their mothers. On the east side of W-GIPP, there seem to be two kinds of grizzlies passing on two sets of habits to their cubs. Grizzly bears usually spend April in the high country, firing up their sleepy metabolisms. In May, most bears move into the low aspen parklands for all the new plant growth, and the chance to find a winter-killed deer or elk. Some stay there all year until hibernation time.
Another group of bears moves back into the high country in the late spring and doesn’t come back down. Lowland females use about a 35 square mile area in the parklands during the summer and fall, and backcountry females range over a larger area, averaging about 50 square miles. Males have a feeding range about twice as large in both areas.
Marmots are deep sleepers. They shut down to 10% of their normal energy use. Their heart and breathing rates are so slow that it is hard to tell they are alive. They do wake up once in a while to use the “outhouse”, but then return to the chamber for another deep nap. They do not have their young during hibernation.
While asleep, marmots have a body temperature almost as cold as the ground around them, around 40 to 50 degrees cooler than normal. When they wake up, either temporarily or permanently in the spring, their body temperature jumps up to near normal for a few minutes and then drops right back down again when the go back to sleep. They can only do this because of a special “brown fat”, which is instantly converted into heat and energy. Most fats take much longer to convert to blood sugars. In most hibernating animals (except for bears), the brown fat is in a thin layer across the animal’s back – perfectly located for instant energy.
Newborn humans have a small layer of brown fat which is used to warm them when they suddenly enter a world 20 to 30 degrees cooler than mom’s tummy. The brown fat is used up in a few days and never comes back.
Last updated: February 24, 2015