Black bear hibernation is the seminal case study for everything modern science knows about how bears actually hibernate. The 2011 Science paper by Oivind Toien and colleagues at the University of Alaska Fairbanks, the most cited piece of bear physiology research of the last twenty years, used five free-ranging Alaskan black bears outfitted with radiotelemetry and core temperature implants. Their data overturned a century of assumption that bears were not "real" hibernators and showed that the Ursus americanus hibernation phenotype is one of the most efficient energy-saving strategies in mammalian biology.
This article covers what black bear hibernation actually looks like inside the den, how heart rate falls from 40-50 beats per minute to 8-19 bpm while body temperature drops only 4 to 7 degrees Celsius, why duration ranges from 7 months in Alaska to zero in much of Florida, the unusual variety of black bear den types from rock crevices to rotted-out trees thirty metres up to crawl spaces under suburban porches, why females give birth in midwinter while still in the hibernating state, and how climate change is rewriting the denning calendar across the species range. For the full biology of Ursus americanus see the main American black bear species reference.
Do Black Bears Hibernate?
Yes. American black bears are true hibernators, and the evidence is now unambiguous. The twentieth-century debate over whether bears qualified as hibernators or merely entered "winter sleep" turned on the observation that small hibernators like ground squirrels and marmots cool their bodies to within a few degrees of freezing, while bears only drop their core temperature by a handful of degrees. Older textbooks called the bear state torpor or dormancy and reserved the word hibernation for the deep cooling strategy of small mammals.
That distinction collapsed in 2011 when Toien and colleagues published continuous heart rate, breathing, body temperature, and metabolic rate data from black bears denning in undisturbed dens across an entire winter. Their key finding was that metabolic rate falls to roughly 25 percent of basal even though body temperature drops only 4 to 7 degrees Celsius. The energy savings are as deep as those of a ground squirrel once body size is accounted for, and they are achieved by a different mechanism, one that is now intensively studied for biomedical relevance.
"The metabolic rate of hibernating black bears was reduced to 25 percent of basal despite body temperatures that remained high. This decoupling of metabolism from temperature sets mammalian bear hibernation apart from that of small mammals and may have important biomedical implications for treatment of human conditions involving immobility, kidney failure, and bone loss." — Oivind Toien et al., Science, February 2011 (DOI: 10.1126/science.1199435)
What varies across the black bear range is not whether the species hibernates but for how long. That variation tracks the length of the local seasonal food gap. Where snow cover persists for six to seven months and food disappears entirely under it, black bears den for the full winter. Where mild winters and abundant alternative food keep the calorie ledger positive year round, hibernation shortens or in some Florida bears disappears altogether. For the geographic backbone of where these populations sit, see where do black bears live.
How Long Do Black Bears Hibernate by Region?
Hibernation length scales with latitude, elevation, and local food availability. The table below summarises typical denning periods across the major black bear populations, drawn from long-term radiotelemetry studies and field observation programmes.
| Region | Typical hibernation length | Entry window | Emergence window |
|---|---|---|---|
| Interior Alaska, Yukon | 5-7 months | late September - October | April - May |
| Northern Quebec, Labrador | 6-7 months | October | April - early May |
| Minnesota, Wisconsin, Maine | 5-6 months | late October - November | March - April |
| Pennsylvania, New York | 4-5 months | November | March - April |
| Appalachians (NC, TN, VA) | 3-4 months | late November - December | early March - April |
| Rocky Mountains (Colorado, Montana) | 5-6 months | late October - November | April - May |
| Sierra Nevada, California | 3-5 months | November - December | March - April |
| Coastal Pacific Northwest | 3-5 months | late November | March |
| Louisiana, Arkansas (lowlands) | 2-3 months | December | February - March |
| Northern Florida | 0-2 months, many males skip | late December | February |
| Central and southern Florida | 0-1 months, often skipped | January | February |
A few patterns stand out immediately. Interior Alaskan and Yukon black bears den longest, with some collared individuals exceeding 210 days from entry to emergence. The driver is the length of snow cover and the absolute disappearance of every natural food source from October through April. Coastal Alaskan black bears with access to late salmon runs and intertidal foraging den shorter than their interior counterparts at the same latitude.
Florida black bears, Ursus americanus floridanus, show the most extreme reduction in denning behaviour of any North American bear population. In the central peninsula, where average winter temperatures stay above 15 degrees Celsius and saw palmetto, acorns, and human-associated food remain available year round, many adult males do not den at all. They reduce activity for a few weeks but never enter a true hibernation state. Pregnant females always den briefly, often in dense palmetto thickets, to give birth and nurse. This is the southernmost population of true hibernators in North America and the closest thing to a non-hibernating population within the species.
Appalachian and Rocky Mountain populations sit in the middle, with hibernations of 3 to 6 months that are deeply influenced by the autumn mast crop. In years when oak acorns and beech nuts are abundant, bears enter dens late and in heavy condition. In mast failure years, bears enter early and thin, and a fraction of subadults fail to survive the winter.
For the comparison with closely related species see grizzly bear hibernation how it works and brown bear hibernation, and for the cross-species view see how bears hibernate.
Inside the Den: Black Bear Hibernation Physiology
A hibernating black bear is not asleep in the ordinary sense. It is in a state of deep, sustained metabolic suppression that is more like a suspended version of wakefulness than a long nap. One of the most surprising findings of the Toien work, and of decades of follow-up research by Brian Barnes at UAF, is that hibernating bears retain the ability to wake quickly. Touch the bear, make a loud noise, or attempt to extract it from the den, and within minutes it is alert, defensive, and capable of fighting. This is the opposite of the small hibernator pattern, where a ground squirrel disturbed in midwinter takes hours to rewarm and may die from the energy cost of the arousal.
Heart Rate, Breathing, and Body Temperature
The cardiovascular changes are the most dramatic single feature. In an active black bear the resting heart rate is roughly 40 to 50 beats per minute. Inside the den, continuous telemetry shows heart rates falling to 8 to 19 beats per minute, with long pauses of 4 to 6 seconds between beats during the deepest stretches. Toien's recordings included occasional single-beat pauses approaching 20 seconds, an electrical pattern that would be considered immediately fatal in a clinical human cardiac monitor.
Breathing slows in parallel. An active bear takes 15 to 30 breaths per minute. A hibernating bear takes roughly one breath every 30 to 45 seconds, and during the deepest periods one breath every 60 seconds or more. Each breath is a slow, deep cycle that delivers enough oxygen for the suppressed metabolic rate.
Body temperature drops more modestly. Core temperature falls from about 37 degrees Celsius to between 30 and 33 degrees, a drop of only 4 to 7 degrees, fluctuating in a slow daily cycle of roughly 1 degree. This is the key difference from small hibernators, which cool to within a few degrees of freezing and must periodically rewarm every one to three weeks to maintain neurological function. Black bears never leave the functional temperature range for their organs, and they therefore do not need interbout arousals. A single, uninterrupted depression lasts the full duration of the den.
Metabolic Depression Without Deep Cooling
The Toien 2011 Science paper was critical because it separated metabolic depression from body temperature and showed they are not the same thing. Bears achieve a metabolic rate of roughly 25 percent of basal with a temperature drop of only 4 to 7 degrees. That is far more efficient, per degree of cooling, than any small mammal. The mechanism involves changes in mitochondrial function, in the gain of cellular respiration, and in circulating metabolic regulators that slow the rate of ATP turnover without requiring deep cold.
"When we recorded the data continuously through an entire winter from bears in undisturbed natural dens, we saw something nobody had documented before. The metabolic rate fell far more than the body temperature. The bear was running its engine at a quarter speed without ever turning the thermostat down." — Brian Barnes, University of Alaska Fairbanks, Institute of Arctic Biology
The biomedical implications are substantial. Researchers studying organ preservation, long-duration spaceflight, and trauma medicine are working to identify the molecular signals bears use to suppress metabolism without lowering temperature. If those signals can be reproduced pharmacologically, the prize is large.
Urea Recycling and Nitrogen Balance
Over a full winter hibernation a black bear does not urinate or defecate. The kidneys continue to produce urine at a reduced rate, but that urine is reabsorbed across the bladder wall, and the urea it contains is broken down by gut bacteria back into ammonia, which the liver then re-uses to build amino acids. This urea recycling is the reason bears do not poison themselves with nitrogenous waste during a half-year fast, and it is a process that pharmaceutical researchers are studying intensively for applications in chronic kidney disease.
Muscle and Bone Preservation
A human immobilised in bed for six months loses 30 to 50 percent of muscle mass and suffers severe osteoporosis. A hibernating black bear loses only 2 to 5 percent of muscle mass across the same period, and bone density loss is minimal and fully recovered within weeks of emergence. Several genes involved in muscle protein turnover are dramatically upregulated inside the den, and specific circulating factors in hibernating bear blood have been shown, in cell culture experiments, to protect human muscle cells from atrophy.
"The bear hibernation phenotype combines metabolic suppression, urea recycling, prevention of muscle atrophy, prevention of osteoporosis, and the maintenance of pregnancy and lactation in a single coordinated package. Comparative physiology has nothing else like it in a large mammal." — Journal of Experimental Biology, review summary of ursid hibernation research, 2019
Hibernation Physiology Compared With Other Mammals
The strategy black bears use is one of several mammalian solutions to long food gaps. The table below compares the core physiological parameters with small hibernators, polar bears, and humans, to put the black bear phenotype in context.
| Parameter | Black bear (hibernating) | Arctic ground squirrel (torpor) | Polar bear (maternal den) | Human (bedridden) |
|---|---|---|---|---|
| Duration per cycle | 3-7 months continuous | 2-3 weeks between arousals | 4-6 months, pregnant females only | N/A |
| Core body temperature | 30-33 C | -2 to +3 C | ~35 C | 37 C |
| Heart rate | 8-19 bpm | 3-5 bpm | 25-30 bpm | 60-80 bpm |
| Breathing rate | 1 per 30-45 s | 1 per 60+ s | 1 per 10-20 s | 12-16 per min |
| Metabolic rate (% basal) | ~25% | ~2-5% | ~50-70% | 90-100% |
| Eats, drinks, urinates, defecates | No | Yes during arousals | No | Yes |
| Muscle loss over period | 2-5% | variable | 5-10% | 30-50% |
| Gives birth during dormancy | Yes | No | Yes | N/A |
| Fully arousable in minutes | Yes | No (hours) | Yes | N/A |
Two things jump out. First, only bears give birth during dormancy. A ground squirrel wakes in spring and breeds in April; a black bear gives birth to blind, 250 to 350 gram cubs in late January or early February, inside the den, while still in the hibernating state, and begins nursing immediately. Second, black bear arousability is unusual. The bear maintains body temperature high enough that the brain and skeletal muscles can resume full function within minutes. Toien's group documented bears reacting to den-side activity, repositioning, and in one case successfully defending the den against predators all without exiting the broader hibernation state. This blend of deep metabolic suppression and rapid arousability is unique to ursids.
For more on the polar bear maternal denning case, which is similar in form but shallower in physiology, see polar bears (cross reference) and the broader hibernation comparison at how bears hibernate.
Black Bear Den Types
Black bears use a wider range of den types than any other bear species. This is partly because the species range covers boreal forest, swamp, mountain, desert edge, and increasingly suburban interface, and partly because black bears are smaller and more flexible than brown bears, which lets them fit into structures a grizzly never could.
| Den type | Typical regions | Approximate frequency | Notes |
|---|---|---|---|
| Excavated slope burrow | Rocky Mountains, Appalachians, interior Alaska | 25-40% | 1-3 m deep, beneath root masses or boulders |
| Rock crevice or small cave | Karst regions, rocky slopes, BC, WV | 15-25% | Energy-cheap, often reused for years |
| Hollow standing tree | Southeastern US, Louisiana, Florida | 10-30% locally, up to 60% in cypress swamps | Bears climb up to 30 m to den; iconic black bear pattern |
| Hollow log or fallen tree base | Pacific Northwest, New England | 10-15% | Often combined with brush cover |
| Brush pile, blowdown, root wad | Mid-Atlantic, southern Appalachians | 10-20% | Low effort, mild climates |
| Ground nest in dense vegetation | Florida, Louisiana, Texas, palmetto thickets | 5-15% locally | No structural cavity, just thick cover |
| Under building (porch, deck, shed, crawl space) | Suburban edge in NC, NJ, CA, NV, CO | 1-5%, increasing | Frequent in human-habituated populations |
| Open ground bed | Florida males, mild climates | <5% | Reduces to a long rest rather than true hibernation |
Excavated Slope Dens
Across the Rocky Mountains, the northern Appalachians, interior Alaska, and the Canadian boreal, the typical black bear den is an excavated chamber dug into a hillside. The bear chooses a south or east-facing slope, usually at mid elevation where snow accumulates reliably. Excavation takes several days. The bear digs a short downward tunnel, 40 to 70 cm in diameter, opening into a chamber barely larger than its curled body. Total depth is commonly 1 to 3 metres, and the bear lines the chamber with grass, leaves, conifer boughs, and bark.
Hollow Tree Dens
The most distinctive black bear den type is the elevated hollow tree. In the southeastern US, particularly in cypress, tupelo, and old cottonwood swamps, black bears regularly climb 10 to 30 metres up the inside of a rotted-out standing tree to den. The classic Louisiana and east Texas black bear den is a hollow bald cypress over swamp water, with the bear curled in a chamber high enough that flooding is not a concern. Lynn Rogers, who has studied wild black bears in Minnesota for over forty years, has documented dozens of these elevated dens and noted that bears appear to prefer them where available. Tree dens are dry, well concealed, and largely inaccessible to wolves or other large predators.
"The first time I climbed a hollow cypress and found a black bear sow with newborn cubs forty feet up, I understood that this species has options most other large carnivores do not. A bear that can den at ground level, in a cave, or thirty metres up a hollow tree is a bear that can persist almost anywhere humans have not actively eliminated it." — Lynn Rogers, Wildlife Research Institute, Minnesota
Rock Crevices and Caves
In karst landscapes and rocky slopes, black bears bypass excavation entirely and use natural cavities. This is common across West Virginia, parts of Pennsylvania, the southern Appalachians, the Sierra Nevada, and rocky coastal British Columbia. Cave dens can be reused for decades, and individual caves sometimes host different bears in successive years. The energy savings of skipping excavation matter most for thin bears entering late after a poor mast year.
Brush Piles, Blowdowns, and Ground Nests
In milder climates, black bears often den at or near ground level with minimal structural shelter. A blowdown root wad, a brush pile created by logging activity, or a dense thicket of mountain laurel can serve as a den. In Florida, Louisiana, and other lowland southern populations, dens are sometimes nothing more than a hollowed-out spot under thick palmetto or briar cover.
Suburban and Built-Structure Dens
Black bears denning under human structures have been documented for decades but have increased markedly with suburban range expansion. Under porches, decks, sheds, and low crawl spaces beneath houses are now well-known black bear den sites in parts of New Jersey, North Carolina, Nevada, Colorado, California, and the Tahoe basin. A pregnant sow may give birth in such a den, with the homeowner sometimes unaware until cub vocalisations or movement are heard underfoot. Wildlife agencies typically advise leaving the family undisturbed until natural emergence in spring, which usually goes smoothly. The risk of habituation is real, however, and chronic-conflict bears that den under houses are sometimes relocated.
For the broader picture of how black bear range overlaps with human development, see where do black bears live, and for the question of whether suburban-edge bears are dangerous, see are black bears dangerous to humans.
Birth in the Den: Cubs Born in Midwinter
One of the most remarkable aspects of black bear hibernation is that females give birth while still hibernating. The pregnancy is hidden inside the broader reproductive strategy of delayed implantation. Black bears mate from May through July. The fertilised egg develops only to a blastocyst stage and pauses, floating free in the uterus for roughly five months. If the female has accumulated enough fat by autumn, the blastocyst implants in late October or November, just before or after denning. Active gestation then lasts about eight weeks.
Cubs are born in late January or early February, deep in the hibernation period. They are tiny, 250 to 350 grams, blind, nearly hairless, and totally dependent on the mother. She nurses them while remaining in the hibernating state, her metabolism still suppressed, her heart rate still around 15 to 20 bpm. The cubs climb through her long fur to reach the nipples and stay pressed against her body for warmth.
By the time the mother emerges from the den in spring, the cubs are 2 to 4 kilograms, furred, able to walk clumsily, and ready to follow her outside. She has lost 25 to 40 percent of her autumn body mass over the winter, most of it to lactation rather than to basal maintenance. The mother fasts continuously for 5 to 7 months, including the pre-pregnancy implantation period, the active gestation, and the lactation period inside the den. The energetic cost is one of the reasons pregnant females need the highest pre-denning fat reserves of any black bear class. Thin females often reabsorb the blastocyst in autumn rather than implant, skipping reproduction for the year.
For the full maternal story see black bear cubs and mothers, and for the food side of how females reach denning condition see what do black bears eat.
Pre-Denning: Hyperphagia and Fat Accumulation
Hibernation is only possible because of the massive fat reserves accumulated in late summer and autumn. During hyperphagia, a voluntary overeating phase that lasts 6 to 10 weeks, the black bear feeds almost continuously, sometimes 20 hours per day, and may gain up to 1.5 kilograms of fat per day. By the time a healthy adult female enters the den she may carry 30 to 40 percent body fat, and a large adult male over 25 percent.
The fuel is region-dependent. In the eastern US, oak acorns, beech nuts, and hickory nuts are the dominant pre-den food. In the Rocky Mountains, white-bark pine seeds, huckleberries, and chokecherries drive fattening. In the Pacific Northwest and parts of British Columbia, salmon and berries dominate. In Florida, saw palmetto fruit and acorns are key. In Louisiana and east Texas, acorns and agricultural crops including corn and sugarcane fill the autumn calorie budget. For a deep look at the year-round feeding calendar see what do black bears eat.
If the autumn food supply fails, hyperphagia is not enough. Bears may delay denning, search longer, and travel further into human-developed areas for alternative calories. In severe mast-failure years across the southern Appalachians, black bear nuisance complaints triple or quadruple in October and November. This is a well-documented mechanism behind the bumper crop of human-bear conflict in some autumn seasons. Cubs born to thin mothers in such years often weigh below threshold and have low first-year survival.
The unusual coat colour variants of Ursus americanus, including cinnamon, blond, and the rare white Kermode bear, do not appear to alter denning physiology, but coastal populations carrying these variants tend to den shorter than interior bears at the same latitude due to richer year-round food. For the colour story see black bear color phases cinnamon, blond, and Kermode.
Climate Change and the Shrinking Denning Period
Across most of the black bear range, hibernation is becoming shorter. Long-term radiotelemetry studies in Minnesota, Pennsylvania, North Carolina, the Rocky Mountains, and the Sierra Nevada have all documented measurable shifts since the 1990s. The mechanism is not primarily warmer winters, although those play a role, but warmer and earlier springs combined with longer autumns. Bears enter dens later because food remains available, and they emerge earlier because snow melts earlier and vegetation greens up sooner.
The most consistent finding across multiple studies is that emergence dates have advanced by approximately 1 to 2 weeks since the early 1990s in temperate-latitude populations. Entry dates have shifted later by a similar amount in many populations, although with more variation tied to autumn mast crop. The total compression of the denning period therefore runs from a few days in some populations to nearly a month in others.
"Across the southern Appalachian populations we have monitored since the late 1980s, black bear emergence dates have moved earlier by roughly 10 to 14 days. Entry dates are also slipping later in mast years. The denning calendar that defined this species at this latitude is no longer the calendar of the 1980s, and it is changing fast enough that we expect further shifts well within the next two decades." — Lynn Rogers, Wildlife Research Institute, on long-term Minnesota and Appalachian field data
Consequences of shorter hibernation are mixed. A bear that is active for more of the year needs more food, which can increase suburban conflict, raid garbage incidents, and beehive losses. Spring emergence ahead of reliable food availability raises mortality of thin bears, especially subadults. Cubs born slightly earlier in the season may emerge from dens when late-winter weather still poses risks. Human-bear encounter season expands at both ends of the year. In suburban interfaces where bears already den under porches, the risk of a sow and cubs emerging during human activity windows is rising.
At the same time, longer active seasons can raise annual energy intake, improve body condition, and increase reproductive success in populations not already limited by food or by human-caused mortality. The net effect varies population by population.
Black Bear vs Grizzly vs Polar Bear Denning
All three of the large North American bear species den, but the patterns differ in ways that reflect their ecology.
Black bears across North America hibernate 0 to 7 months depending on latitude and food. Both sexes den in northern populations; in southern populations adult males may skip denning entirely. Pregnant females always den briefly to give birth, even in Florida.
Grizzly bears, Ursus arctos, in North America hibernate 5 to 7 months in interior populations and 3 to 5 months in coastal Alaska where salmon runs extend the feeding season. The physiology is broadly the same as black bears, but grizzlies tend to use larger, more strongly excavated dens at higher elevation. For the full story see grizzly bear hibernation how it works. For the Eurasian side of the same species see brown bear hibernation.
Polar bears, Ursus maritimus do not hibernate as a species. Males and non-pregnant females remain active on the sea ice throughout the winter, hunting seals at breathing holes. Only pregnant females enter a maternal den, usually in a snow drift on land or on stable sea ice, and stay there from November until March or April. Their metabolic depression is real but shallower than the black bear pattern, with body temperature staying closer to normal.
The evolutionary picture is that all three species retain the ursid hibernation machinery inherited from a common ancestor. Black bears and brown bears use it fully every year, polar bears use it only during maternal denning, and grizzlies and brown bears occupy the middle in terms of typical depth and duration. For more on the species-level comparisons see american black bear.
Waking Up: Emergence and Post-Den Behaviour
Emergence from hibernation is not a sudden event. Over several days to two weeks before leaving the den, the bear's heart rate, breathing, and body temperature climb back to normal levels. The bear stretches inside the den, shifts position, and passes small amounts of urine as kidney function resumes. Females with cubs typically emerge last, often several weeks after adult males, and they stay close to the den for another 1 to 3 weeks after emergence, allowing cubs to build strength before long movements begin.
The first weeks after emergence are nutritionally difficult. Bears typically lose additional weight for several weeks after leaving the den, because early spring vegetation is not yet productive and digestive function needs to ramp back up. This period is sometimes called walking hibernation or post-den depression, and it is when bears are most likely to scavenge winter-killed deer and other ungulates. For many Appalachian and Rocky Mountain bears, winter-killed deer carcasses are the single most important spring food.
Young adult males may seek new territories immediately on emergence, which drives the spring dispersal peak and is the period when most accidental human-bear encounters in suburban areas occur. Family groups stay closer to the den site and move slowly as cubs gain coordination for sustained travel. The emergence window is the riskiest part of the year for newly independent yearlings, who must locate food, avoid larger conspecifics, and establish a home range without maternal protection.
Further Reading and External Resources
For the main species reference on Ursus americanus see the American black bear guide, which covers size, range, diet, behaviour, conflicts, and conservation. For the maternal biology side of denning see black bear cubs and mothers. For what fuels the autumn fattening see what do black bears eat. For the geographic backbone see where do black bears live. For the colour variants discussion see black bear color phases cinnamon, blond, and Kermode. For human-conflict context see are black bears dangerous to humans.
For comparable hibernation accounts in related species see grizzly bear hibernation how it works, brown bear hibernation, and the cross-species summary at how bears hibernate.
Readers interested in other kinds of knowledge and skill may enjoy the logical puzzles and cognitive challenges at whats-your-iq.com, the music and piano training resources at whennotesfly.com, and the technical writing platform at evolang.info.
References
- Toien, O., Blake, J., Edgar, D. M., Grahn, D. A., Heller, H. C., and Barnes, B. M. (2011). Hibernation in black bears: independence of metabolic suppression from body temperature. Science, 331(6019), 906-909. DOI: 10.1126/science.1199435
- Laske, T. G., Garshelis, D. L., and Iaizzo, P. A. (2011). Monitoring the wild black bear's reaction to human and environmental stressors. BMC Physiology, 11(1), 13. DOI: 10.1186/1472-6793-11-13
- Nelson, O. L. and Robbins, C. T. (2015). Cardiovascular function in large to small hibernators: bears to ground squirrels. Journal of Comparative Physiology B, 185(3), 265-279. DOI: 10.1007/s00360-014-0881-5
- Johnson, H. E., Lewis, D. L., Verzuh, T. L., Wallace, C. F., Much, R. M., Willmarth, L. K., and Breck, S. W. (2018). Human development and climate affect hibernation in a large carnivore with implications for human-carnivore conflicts. Journal of Applied Ecology, 55(2), 663-672. DOI: 10.1111/1365-2664.13021
- Tinker, D. B., Harlow, H. J., and Beck, T. D. I. (1998). Protein use and muscle-fiber changes in free-ranging, hibernating black bears. Physiological Zoology, 71(4), 414-424. DOI: 10.1086/515429
- Hellgren, E. C. (1998). Physiology of hibernation in bears. Ursus, 10, 467-477. JSTOR stable URL: https://www.jstor.org/stable/3873159
- Stenvinkel, P., Jani, A. H., and Johnson, R. J. (2013). Hibernating bears (Ursidae): metabolic magicians of definite interest for the nephrologist. Kidney International, 83(2), 207-212. DOI: 10.1038/ki.2012.396
- Friebe, A., Swenson, J. E., and Sandegren, F. (2001). Denning chronology of female brown bears in central Sweden. Ursus, 12, 37-45. (Comparative reference) DOI: 10.2307/3873224