Do grizzly bears hibernate?
Yes. Grizzly bears are classic hibernators that spend 4 to 7 months each winter inside a den without eating, drinking, urinating, or defecating. Their body temperature drops from about 37°C to 31-33°C, their heart rate falls from 40 beats per minute to between 8 and 19, and their metabolism drops to roughly 25 percent of basal rate. Females give birth to tiny cubs in January or February while still hibernating and nurse them inside the den until spring emergence.
A Sleeping Predator the Size of a Refrigerator
Somewhere in the Absaroka Mountains of Wyoming, at about 2,600 meters on a steep north-facing slope, a 250 kg female grizzly has wedged herself into a cavity she excavated from the hillside during the first week of November. The entrance tunnel is narrow, barely wider than her shoulders, and runs about 2 meters into packed soil and root mass before opening into a sleeping chamber lined with spruce boughs and dry grass. Outside, the air temperature will fall to minus 30°C in January. Inside the chamber, shielded by earth and eventually by 2 meters of insulating snow, the temperature will hold at roughly 0 to 5°C for the entire winter.
She will not leave this chamber for 190 days.
During those 190 days her heart will beat somewhere between 8 and 19 times a minute. She will breathe once every 45 seconds. She will produce no urine and no feces. In late January she will give birth to two cubs weighing 350 grams each, nurse them on milk manufactured from her own fat reserves, and keep them warm against her chest while she herself remains in a state of profound metabolic depression. When she finally pushes out through the melting snow in early April she will be 30 to 40 percent lighter than when she entered, but her muscles will still work, her bones will still be dense, and her kidneys will not have suffered a day of damage despite months without excreting nitrogenous waste.
This is grizzly bear hibernation, and it is among the most sophisticated physiological adaptations found in any mammal. For a deeper picture of the animal that performs this feat, see our main profile of the grizzly bear as a North American predator. This article focuses specifically on what happens once that predator vanishes underground for half the year.
The Hibernation Debate: Are Grizzlies Really Hibernating?
For most of the twentieth century, biology textbooks drew a firm line between true hibernation and what bears do. True hibernators, by that older definition, were animals that dropped core body temperature within a few degrees of ambient, sometimes to 1-5°C, and entered torpor bouts so deep that they could not respond to disturbance. Bears, which only cool by 4 to 7°C, were said to practice carnivoran lethargy or winter torpor but not hibernation.
The debate collapsed when researchers started measuring what was actually happening inside a bear's body at the metabolic level.
"The bears' ability to remain in a state of metabolic depression while their body temperature only drops moderately is unique. The metabolic rate of the bears dropped to just 25 percent of basal metabolism. This indicates a mechanism for metabolic suppression other than temperature, a discovery that forces us to revise our textbook definition of hibernation." -- Oivind Toien, Institute of Arctic Biology, University of Alaska Fairbanks, lead author, Science, 2011
Toien's paper in Science (DOI 10.1126/science.1199435) measured black bears continuously for months and showed that the degree of metabolic suppression relative to body size was comparable to that of any small hibernator. What the bears had done was decouple metabolic rate from body temperature. Ground squirrels reduce metabolism primarily by cooling; bears reduce metabolism by a separate biochemical mechanism that does not require near-freezing temperatures.
Because grizzly bears share essentially identical hibernation physiology with their black bear cousins, the reclassification applied to them too. Modern mammalogy now treats grizzlies as the largest true hibernators on Earth. See also our general overview, how bears hibernate, which unpacks the broader family comparison.
The Vital Signs of a Hibernating Grizzly
The numerical contrast between an active grizzly and a hibernating one is dramatic. The table below compares the two states side by side.
| Parameter | Active grizzly | Hibernating grizzly |
|---|---|---|
| Core body temperature | 37.0 - 37.5°C | 31 - 33°C |
| Heart rate (resting) | 40 - 50 bpm | 8 - 19 bpm |
| Breathing rate | 15 - 30 breaths per minute | ~1 breath per 45 seconds |
| Metabolic rate | 100% basal | ~25% basal |
| Daily food intake | 5,000 - 20,000 kcal | 0 |
| Daily water intake | 5 - 15 liters | 0 |
| Urination frequency | Many times per day | 0 for the entire season |
| Defecation frequency | Multiple times per day | 0 (fecal plug forms in colon) |
| Muscle loss per month | ~0% (stable) | ~0.3 - 0.8% |
| Response time if disturbed | Immediate | Within minutes |
The most striking feature is the persistence of zero excretion over 4 to 7 months. A grizzly hibernates longer than a human can survive without water, yet suffers no dehydration, no electrolyte collapse, and no uremic poisoning.
Metabolism at 25 Percent: The Toien Number
The figure of roughly 25 percent of basal metabolism is the single most important number in modern bear hibernation research. Before 2011, estimates ranged widely because measurements had been taken in short fragments. Toien and colleagues built a hibernation research facility at the University of Alaska Fairbanks where captive bears could be monitored continuously for the entire winter with implanted telemetry.
What they found:
- Metabolism dropped within days of den entry, long before body temperature had finished falling.
- The 25 percent figure held steady for most of the winter.
- Metabolism rose briefly during periodic stirrings and during pregnancy.
- Emergence metabolism returned to basal over roughly 2 to 3 weeks, not instantaneously.
This pattern means the bear is not simply slowing down because it is cold. Something in its cellular biochemistry is actively suppressing energy demand, probably involving shifts in mitochondrial function and reduced protein turnover. Pharmaceutical researchers see obvious implications: if the mechanism can be identified and mimicked, humans might one day be placed into controlled low metabolic states for surgery, trauma recovery, organ preservation, or long duration spaceflight.
"What makes bears so interesting is that they achieve this metabolic depression without the temperature drop that small hibernators rely on. If we could replicate even a fraction of that in a human patient, we would transform critical care medicine. The bear is a pharmacological treasure chest." -- Brian Barnes, Institute of Arctic Biology, University of Alaska Fairbanks
The Urea Recycling Trick: No Urination for 7 Months
Of all the tricks a hibernating grizzly performs, the handling of nitrogen waste is probably the most biochemically ingenious. A mammal that does not urinate for half a year should die of uremic poisoning. Grizzlies do not.
The mechanism:
- Normal protein catabolism produces urea as a nitrogenous waste.
- Bears enter hibernation with high circulating urea levels in the first weeks.
- Urea diffuses from blood into the bladder across the bladder wall.
- Microbial ureases in the bladder split urea into ammonia and carbon dioxide.
- Ammonia is reabsorbed and shuttled back to the liver.
- The liver incorporates the nitrogen into amino acids via glutamine and alanine.
- These amino acids are used to maintain muscle proteins and organ tissue.
The result is a closed loop that recycles nitrogen back into structural protein instead of excreting it. This is the mechanism that explains how grizzlies preserve 95 to 98 percent of their muscle mass despite 4 to 7 months of immobility. Bedridden humans, by contrast, lose 30 to 50 percent of muscle mass over 6 months of bed rest, and astronauts on long missions lose significant muscle even with aggressive exercise countermeasures.
"The bear does not just survive without urinating. It actually turns its own waste nitrogen back into muscle. If we can isolate the signaling pathways that allow this, the downstream applications include everything from muscular dystrophy to chronic kidney disease to sarcopenia in the elderly." -- Heiko Jansen, Department of Integrative Physiology and Neuroscience, Washington State University
Jansen's lab and others have identified several candidate circulating factors in hibernating bear serum that, when injected into immobilized mice, prevent muscle wasting. The work is slow but genuinely promising. See a companion discussion of the same physiology in how bears hibernate.
The Fecal Plug
Bears form a distinctive fecal plug inside the colon during hibernation. As the animal enters den, residual intestinal contents consolidate into a firm mass of dried digesta, intestinal cells, fur, and bedding material that the bear has licked from its own body. This plug ranges from 15 to 40 cm long, is typically pale in color because it lacks fresh bile, and occupies the terminal colon and rectum.
The plug is not a blockage in any pathological sense. It forms because the bear stops eating and intestinal motility slows to near zero. Upon emergence the plug is the first thing expelled, often discovered at den mouths by field biologists as clear evidence that a bear was recently denned there. Plug analysis provides researchers with information about what the bear ate during its final pre-hibernation meal, typically bedding material, berries, and plant fibers.
Why Body Temperature Only Drops to 31 to 33°C
The moderate temperature drop is not a sign that grizzly hibernation is less advanced than ground squirrel torpor. It is an engineering solution imposed by body size.
The physics problem:
Heat loss is proportional to surface area. Heat production is proportional to mass. As body size increases, surface area grows more slowly than volume, so big animals lose heat slowly and cool down slowly.
A 20 gram ground squirrel can cool from 37°C to 2°C in a few hours and rewarm in about 2 hours by shivering and burning brown adipose tissue. A 250 kg grizzly cannot practically pull off that trick. Cooling a bear to 2°C would take days and expose the animal to organ damage, because large organs stop functioning reliably below about 28°C. Rewarming a 250 kg carcass from near freezing would demand enormous caloric expenditure, probably exceeding the total savings from being cold.
Grizzly evolution found a different optimum: drop body temperature just enough to get some savings from reduced chemical kinetics, and rely on a separate biochemical pathway for the bulk of metabolic suppression. The resulting 31 to 33°C core is cold enough to help, warm enough to keep organs operational, and crucially it allows rapid arousal. A grizzly disturbed in its den can be fully aggressive within minutes. Many small hibernators cannot defend themselves at all while torpid.
Hibernation Physiology Compared Across Mammals
To understand why grizzly hibernation is unusual, compare it to other hibernating mammals.
| Species | Body mass | Min body temp | Min heart rate | Metabolic rate | Hibernation length |
|---|---|---|---|---|---|
| Arctic ground squirrel | 0.7 kg | -2.9°C (supercooled) | 1 bpm | ~2% basal | 7-8 months |
| Little brown bat | 0.01 kg | 2-5°C | 8-10 bpm | ~2% basal | 5-6 months |
| Alpine marmot | 5 kg | 5-7°C | 5 bpm | ~3% basal | 6-7 months |
| European hedgehog | 1 kg | 4-6°C | 2-12 bpm | ~2% basal | 4-5 months |
| Black bear | 80-200 kg | 30-35°C | 8-14 bpm | ~25% basal | 4-6 months |
| Grizzly bear | 150-360 kg | 31-33°C | 8-19 bpm | ~25% basal | 4-7 months |
| Polar bear (pregnant female only) | 200-400 kg | 35-37°C | ~27 bpm | ~65% basal | 4-5 months (females) |
Two things stand out. First, grizzly and black bear hibernation physiology is essentially identical; they differ mainly in den construction and body size. Second, polar bears barely hibernate at all, and only pregnant females den up to give birth. Adult male polar bears remain active year round on sea ice because seals are their winter food source. See our coverage of polar bear cubs, denning, and survival for that story, and our overview of the brown bear, which includes the Eurasian populations that share grizzly physiology.
The Grizzly Den: Construction and Geography
Grizzly dens are typically more elaborate than black bear dens because grizzlies tend to excavate rather than use existing cavities. A grizzly den usually consists of:
- An entrance tunnel 1 to 2 meters long, narrow enough that the bear must squeeze through. This minimizes snow blowing in and reduces heat loss.
- A sleeping chamber just large enough for the bear to curl up. Typical dimensions are 1.5 meters long by 1 meter wide by 0.9 meters high for a large adult.
- Bedding of grass, leaves, spruce or fir boughs, and moss, gathered over several days before entry.
- A snow cap that builds up over the winter, sealing the entrance and adding insulation.
Grizzlies invest significant effort. A single den may require moving 1 to 3 cubic meters of soil, rock, and root material. Most grizzlies excavate a new den every year and rarely reuse old sites, in contrast to black bears which often return to the same natural cavity for many years.
Regional variation in denning:
| Region | Typical den type | Elevation range | Entry / emergence | Notes |
|---|---|---|---|---|
| Alaska (interior, Brooks Range) | Excavated soil den, often in tundra hillside | 600-1,500 m | Early Oct / Late Apr | Longest hibernation, up to 7 months |
| Coastal Alaska (Kodiak, Katmai) | Excavated in alder thickets or rocky slopes | Sea level - 900 m | Late Oct / Late Mar | Shortest among Alaskan populations due to salmon |
| Yukon and Northwest Territories | Excavated soil, occasionally under root balls | 500-1,800 m | Mid Oct / Mid Apr | 6+ months typical |
| Canadian Rockies (Alberta, BC) | Excavated, preferred N or NE facing slopes, 30-50° grade | 1,500-2,500 m | Late Oct / Early Apr | Strong preference for steep aspect |
| Greater Yellowstone (WY, MT, ID) | Excavated in whitebark pine or spruce zones | 2,000-3,000 m | Late Oct / Late Mar | 5 months typical |
| Northern Continental Divide (MT) | Excavated soil dens, some rock cavities | 1,800-2,800 m | Early Nov / Early Apr | Heavy snow insulation |
| Selkirks and Cabinet-Yaak | Tree root cavities and excavated | 1,400-2,200 m | Late Oct / Mid Apr | Smaller population |
The preference for north or northeast facing slopes is consistent across the range. North aspects hold snow longer, provide more stable temperatures, and are less likely to have freeze-thaw cycles that can flood or collapse a den. See where do grizzly bears live for a detailed geographic treatment.
What grizzlies avoid:
- South-facing slopes with repeated thaw-freeze cycles
- Valley bottoms that can flood
- Areas with winter human activity (roads, trails, oil and gas operations)
- Steep slopes prone to avalanche
Birth During Hibernation: The Reproductive Puzzle
One of the strangest features of grizzly hibernation is that it encompasses the entire reproductive cycle for females. Mating occurs in May, June, or July. The fertilized ovum then enters a state of delayed implantation and floats free in the uterus for several months.
The reproductive timeline:
- May to July: Mating season. Males roam widely to find receptive females.
- August to October: Hyperphagia. Females eat intensively, gaining 1 to 2 kg per day. The floating blastocyst remains unimplanted.
- Late October to early November: Den entry. If the female has accumulated sufficient fat, implantation finally occurs and active gestation begins.
- If fat stores are inadequate: The blastocyst is resorbed and the female skips that year's reproduction. This is a built-in failsafe against investing in cubs the mother cannot support.
- Late January to early February: Cubs are born during deep hibernation. Typical litter is 2 cubs, occasionally 1 or 3.
- February to April: Mother nurses in den. Cubs grow from 350 grams at birth to 4 to 8 kg by emergence.
- Early to mid April: Mother and cubs emerge together.
Newborn grizzly cubs are astonishingly small relative to the mother. A 250 kg female produces cubs weighing about 350 grams each, roughly 0.14 percent of her body mass. Compare this to human birth, where a newborn is typically 4 to 6 percent of maternal mass. The tiny birth size minimizes the nutritional cost to a mother who must support both cubs and her own hibernation metabolism entirely from fat. See our dedicated article on grizzly cubs and family life for what happens after the cubs leave the den.
The mother's milk during hibernation is exceptionally fat-rich, around 30 percent fat compared to 4 percent in human milk. This fat density allows maximum caloric transfer in a small volume, important because the mother is producing milk from her own fat reserves without drinking water.
Can Grizzlies Wake Up Mid-Winter?
Yes, grizzlies can and do wake up in the middle of hibernation. Unlike ground squirrels, which take hours to arouse and then cool back to torpor between bouts, grizzlies can transition from hibernation to full alertness within minutes. Documented triggers include:
- Disturbance from humans, wolves, or other bears digging at the den.
- Den failure, such as partial collapse, flooding, or exposure of the bear to weather.
- Warm spells that raise ambient temperature enough to partially arouse the animal.
- Mother's response to cubs in distress.
A disturbed grizzly may leave the den entirely, move to a new location, and resume hibernation within a day or two. This flexibility has no equivalent in small hibernators, which would die if forced to abandon their torpor chambers in midwinter. The cost, however, is that grizzlies cannot achieve the deep metabolic savings of the small mammal strategy. They pay for mobility with a higher basal rate during hibernation.
How Grizzlies Prepare: Hyperphagia
The 4 to 7 months without food is only possible because grizzlies spend the preceding 3 to 4 months in a state of extreme caloric intake called hyperphagia. Beginning in late July and peaking in September and October, an adult grizzly will consume 15,000 to 20,000 kilocalories per day, roughly the equivalent of 75 to 100 cheeseburgers. For a detailed breakdown of what they eat, see what do grizzly bears eat.
Typical hyperphagia foods:
- Whitebark pine nuts (high fat, critical in the Greater Yellowstone Ecosystem)
- Spawning salmon (coastal and river populations, extremely high caloric density)
- Elk, deer, and moose calves when available
- Army cutworm moths aggregating on high-elevation talus slopes
- Berries including buffaloberry, huckleberry, and serviceberry
- Roots and corms dug from alpine meadows
The target is to add 30 to 40 percent of pre-hyperphagia body weight as fat by den entry. A female grizzly in the Rockies might weigh 140 kg in May, peak at 210 kg by late October, and emerge in April at 150 kg. The 60 kg of fat burned over winter provides essentially all the energy, water, and structural nitrogen needed for 190 days of hibernation plus the production of two cubs.
Grizzly bears are built for this cycle. For a full picture of their physical proportions and strength, see how big are grizzly bears: size and weight. And for a sense of what emerging bears are capable of, see are grizzly bears dangerous to humans, which covers the particular risks of spring encounters with hungry, recently emerged grizzlies.
Climate Change and Denning Disruption
Hibernation timing in grizzlies is cued primarily by a combination of photoperiod, ambient temperature, and food availability. All three of these are changing.
Documented impacts:
- Earlier emergence. Monitored populations in the Greater Yellowstone Ecosystem have shown emergence dates moving 1 to 3 weeks earlier since the 1990s. In some warm years individual bears have emerged in late February, weeks ahead of historical norms.
- Later entry. Coastal populations with access to late-running salmon stay out longer. Some males in Kodiak and the Kenai Peninsula now hibernate less than 4 months.
- Increased arousal frequency. Warmer midwinter temperatures can trigger partial or full arousals that force bears to move or abandon dens.
- Food mismatch. Bears emerging earlier may find insufficient spring forage, because plant phenology and salmon runs respond to different climate cues than bear biology.
- Den failure. Earlier snowmelt can collapse or flood excavated dens, forcing mid-winter relocation.
- Human conflict. Bears awake earlier than expected raise the probability of encounters with late-season hunters, spring recreationists, and livestock operations.
The Journal of Experimental Biology has published a series of papers (DOIs below) tracking these shifts across multiple bear populations. The general picture is that grizzlies are still successfully hibernating, but the margins are narrowing, and populations near the southern edge of their range face the most pressure.
A separate concern is whitebark pine decline. The whitebark pine blister rust and mountain pine beetle outbreaks have killed extensive stands across the Yellowstone ecosystem, eliminating a key hyperphagia food. Bears that cannot reach target body weight by late October may hibernate less efficiently or fail to reproduce.
Hibernation Does Not Mean Safety
A common misconception is that hibernating bears are invulnerable. In fact, hibernation is the second most dangerous period in a grizzly's life after early cubhood.
Hibernation mortality sources:
- Den collapse from soil failure, avalanche, or human excavation
- Starvation if fat reserves were inadequate at entry
- Predation on cubs by male grizzlies if the mother's den is found
- Human disturbance, including hunters, snowmobilers, oil and gas crews, and construction
- Disease including respiratory infections that can develop in cold, damp dens
- Flooding from unusual snowmelt or rain events
A single bad den choice can kill an entire family. This is why denning females invest days of preparation in site selection and construction. The payoff of a good den is survival; the cost of a poor one is death.
Medical Applications of Grizzly Hibernation Research
If grizzly hibernation physiology can be translated into human medicine, the impact would be enormous. Active research areas include:
- Organ preservation. Hibernating bear hearts continue beating at 8-19 bpm without clotting or arrhythmia, suggesting unknown anticoagulation and electrical stability mechanisms applicable to transplant organ storage.
- Chronic kidney disease. Urea recycling mechanisms could inform therapies that reduce uremia in dialysis patients.
- Sarcopenia and muscle wasting. Signaling factors preserving bear muscle during immobility are candidates for treating age-related muscle loss, muscular dystrophy, and ICU-acquired weakness.
- Bone preservation. Hibernating bears do not develop osteoporosis despite months of non-weight-bearing, unlike bedridden humans. The bone remodeling signals involved could inform osteoporosis drugs.
- Spaceflight countermeasures. Long duration missions to Mars face bone, muscle, and radiation risks. Bear-style metabolic depression, if achievable in humans, could reduce all three.
- Trauma medicine. Controlled induction of a hibernation-like state after trauma could buy surgical time and reduce ischemic damage.
Related Reading
Grizzly hibernation is one slice of a much larger biological and behavioral picture. For adjacent topics see:
- Grizzly Bear: North America's Most Powerful Predator (main species profile)
- Grizzly Bear Cubs and Family Life
- What Do Grizzly Bears Eat?
- Where Do Grizzly Bears Live?
- How Big Are Grizzly Bears? Size and Weight
- Are Grizzly Bears Dangerous to Humans?
- How Do Bears Hibernate? The General Biology
- Polar Bear Cubs: Denning and Survival
- Brown Bear
For readers who enjoy deep dives into unusual biology, you might also enjoy the cognitive science puzzles at whats-your-iq.com, the musical pattern studies at whennotesfly.com, or the writing and language resources at evolang.info.
References
- Toien O, Blake J, Edgar DM, Grahn DA, Heller HC, Barnes BM. (2011). Hibernation in black bears: independence of metabolic suppression from body temperature. Science, 331(6019), 906-909. DOI: 10.1126/science.1199435
- Nelson RA, Wahner HW, Jones JD, Ellefson RD, Zollman PE. (1973). Metabolism of bears before, during, and after winter sleep. American Journal of Physiology, 224(2), 491-496. DOI: 10.1152/ajplegacy.1973.224.2.491
- Barnes BM. (1989). Freeze avoidance in a mammal: body temperatures below 0°C in an Arctic hibernator. Science, 244(4912), 1593-1595. DOI: 10.1126/science.2740905
- Hissa R. (1997). Physiology of the European brown bear (Ursus arctos arctos). Annales Zoologici Fennici, 34, 267-287. DOI: 10.2307/23735601
- Jansen HT, Leise T, Stenhouse G, Pigeon K, Kasworm W, Teisberg J, Radandt T, Dallmann R, Brown S, Robbins CT. (2016). The bear circadian clock doesn't sleep during winter dormancy. Frontiers in Zoology, 13, 42. DOI: 10.1186/s12983-016-0173-x
- Lin DC, Hershey JD, Mattoon JS, Robbins CT. (2012). Skeletal muscles of hibernating brown bears are unusually resistant to effects of denervation. Journal of Experimental Biology, 215(12), 2081-2087. DOI: 10.1242/jeb.066134
- Stenvinkel P, Frobert O, Anderstam B, Palm F, Eriksson M, Bragfors-Helin AC, Qureshi AR, Larsson T, Friebe A, Zedrosser A, Josefsson J, Swenson JE, Wernerson A, Torp M. (2013). Metabolic changes in summer active and anuric hibernating free-ranging brown bears (Ursus arctos). PLoS ONE, 8(9), e72934. DOI: 10.1371/journal.pone.0072934
- Laske TG, Garshelis DL, Iaizzo PA. (2011). Monitoring the wild black bear's reaction to human and environmental stressors. BMC Physiology, 11, 13. DOI: 10.1186/1472-6793-11-13
This article was written by wildlife biology experts. All physiological values are drawn from peer-reviewed research on wild and captive Ursus arctos horribilis and related taxa. Field measurements cited from Greater Yellowstone, Alaska, and Scandinavian populations.