Hibernation can last days, weeks, or months depending upon the species. Chipmunks reduce their heart rate from the usual beats per minute to an almost undetectable 4 beats per minute during hibernation.
Chipmunks do not sleep through the entire winter however, they wake every few days to feed on stored food and to urinate and defecate. Hibernation is triggered by decreasing day length and hormonal changes in an animal that dictate the need to conserve energy.
Before hibernating, animals generally store fat to help them survive the winter. Many, like the chipmunk, wake up for brief periods, but for the most part, true hibernators remain in this low-energy state through the winter. Waking takes time and uses up an animal's energy reserve. Many animals once thought to hibernate, including bears, really only enter a lighter sleep-state called torpor.
Like hibernation, torpor is a survival tactic used by animals to survive the winter months, and is triggered by colder temperatures and decreased food availability. Torpor also involves decreased breathing and heart rates, and lower metabolic rate. Bears can sleep more than days without eating, drinking, or passing waste!
Bears can actually turn their pee into protein. Unlike hibernation, torpor is not voluntary and often lasts for shorter periods of time. While most mating occurs in June, the implantation of fertilized eggs does not occur until months later, delayed to allow for a successful denning season in preparation for hibernation.
This allows female bears to preserve their energy during hibernation. Additionally, bears have control of their populations in the event that there is not enough food for survival. Fertilized eggs will only attach to the uterine wall if the bears reach a sufficient body fat percentage or weight during the summer and fall.
If these conditions are not met, the fertilized eggs will not attach or progress past blastocysts. Yellowstone National Park visitors may be surprised to see resident grizzly bears up and about during the late stages of hibernation. As temperatures drop into the negatives, they will slowly settle in for their annual hibernation, where their heart rates and breathing rates drop significantly.
Each bear is responsible for digging its own den, and grizzly bears typically dig a new den every year. Grizzly bears prefer digging dens in high elevated slopes with an entrance small enough to squeeze through.
This way, they will insulate them faster. The chamber is almost the size of their bodies to facilitate the retention of heat, creating the ideal conditions to retain heat during hibernation. Experts are taking a closer look at bear hibernation to assist in making medical advancements to treat diabetes and osteoporosis. Studies can also help people experiencing kidney failure and other conditions that impact human organ viability. Pregnant females den at higher elevations than other females and male bears Haroldson et al There are several different types of dens utilized by bears.
Black bears tend to excavate dens, den under windfalls, in hollow trees or caves, and in previously occupied dens Jonkel Grizzly bears tend to excavate dens at the base of large trees often on densely vegetated north-facing slopes. Grizzly bears in YNP usually dig new dens but on occasion, dens especially natural cavities are re-utilized Craighead and Craighead ; Judd et al. Most dens are dug in sandy loam soils with some occurring in clay loam and rocky silt soils Judd et al. Reuse of excavated dens is rare but does occasionally occur.
Usually excavated dens collapse the spring after they are dug due to runoff and are unusable. Some grizzly bears excavate dens long before the onset of hibernation while other bears tend to wait to almost the last minute to construct dens Craighead and Craighead Major den excavation is completed in days during which a bear may move up to a ton of material Brown ; Craighead and Craighead After completion of a den which consists of an entrance, a short tunnel, and a chamber bears will cover the chamber floor with bedding material ranging from spruce boughs to duff.
The bedding material has many air pockets which trap body heat and form a microclimate around the bear helping to keep it warm Craighead and Craighead These bedding materials are related to availability at the den site and not on the bears preference Judd et al. The den entrance is usually just large enough for the bear to squeeze through. This minimal opening size helps prevent heat loss during hibernation since a smaller opening will be covered with snow more quickly than a large opening.
In some dens the tunnel is dug straight into a hillside or at a slightly upward angle, an energy efficient design that reduces heat loss from the den chamber. However, some dens are not energy efficient and have tunnels dug at a downward angle which allows heat to escape through the den entrance. In most dens, the chamber is dug only slightly larger than the bear allowing for efficient heat retention. However in some natural cavities used as dens, the chamber is much larger than the bear.
Males and females with young usually dig the largest dens. Movement to dens is correlated to weather and snow conditions with most movement usually occurring from late October to mid November Judd et al. However, Craighead and Craighead found hibernation onset varied by as much as one month depending on weather conditions.
Latitude also influences den entrance, with bears in northern latitudes denning earlier and longer than bears in southern latitudes Haroldson et al. Bears will remain in the area of their den for a few weeks and enter a state of lethargy during which they eat nothing and sleep frequently Craighead and Craighead According to Craighead and Craighead and Servheen and Klaver , final den entry occurs during severe snowstorms.
In theory the fresh snow will hide any tracks or other evidence of where the bear's den is located. Pregnant females usually enter dens first, followed by females with young, subadults, and lastly, adult males Haroldson et al.
Grizzly and black bears breed from May through July but embryonic implantation does not occur until around December, about one month after solitary females den. The cubs are born in late January or early February and are naked, blind, and helpless Rogers They measure only about 8 inches 20 cm long and weigh from 8 - 12 ounces - g.
The newborn cubs do not hibernate. They sleep next to their mother, nurse, and grow rapidly. When black bear cubs emerge from the den at about three months of age, they weigh about 4 - 8 pounds 1.
At ten weeks of age, grizzly bear cubs weigh 10 - 20 pounds 4. When temperatures warm up and food is available in the form of winter-killed ungulates or early spring vegetation, bears emerge from their dens. Males, subadults, solitary females, and females with yearlings or two-year-olds usually leave the vicinity of their den within a week of emergence while females with new-born cubs remain in the general vicinity of the den for several more weeks Lindzey and Meslow , Haroldson et al.
Several physiological processes bears undergo during hibernation are of interest to medical researchers. When bears are hibernating and metabolizing body fat, their cholesterol levels are twice as high as during the summer and twice as high as the cholesterol levels of most humans Baggett Bears, however, do not suffer from hardening of the arteries arteriosclerosis or gallstones, conditions which result from high levels of cholesterol in humans.
The bear's liver secretes a substance that dissolves gallstones in humans without surgery. Another mystery of hibernation is that bears do not lose bone mass during hibernation. All other mammals which maintain non-weight bearing positions for an extended period of time suffer from osteoporosis, or a weakening of the bones Wickelgren
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