Hibernation

• 1. Introduction

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  Hibernation, at its core, is the art of survival through disappearance. Some animals face winter not by enduring it, but by stepping outside of time itself. An Arctic ground squirrel can drop its abdominal temperature below zero and sustain that for more than three weeks at a stretch. Its heart slows to almost nothing. Its breath becomes nearly imperceptible. And yet, come spring, it wakes up.

  For centuries, scientists argued about what truly counted as hibernation. Bears were excluded from the club because their body temperature only fell by 3 to 5 degrees Celsius, while rodents could plunge by 32 degrees or more. Swallows were once thought to sink into ponds each autumn and rest on the muddy bottom until warmth returned. And until recently, no primate on earth was known to hibernate at all.

  That last assumption collapsed when researchers turned their attention to Madagascar and a small creature called the fat-tailed dwarf lemur. What they found forced a complete rethink of the rules. Who actually hibernates? Why do some animals wake up in the middle of their dormancy, briefly return to full body temperature, and then go cold again? And could humans ever do something similar? The answers reach back more than 250 million years, to an ancestor of every mammal alive today.

• 2. What Counts as Hibernation

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  For most of the history of science, hibernation was defined by a dramatic drop in body temperature. Under that definition, ground squirrels qualified easily. Bears did not. The debate over bears persisted well into the late 20th century, with many researchers arguing that their winter sleep was simply too mild to count.

  That argument was put to rest by two separate studies, one in 2011 on captive black bears and another in 2016 on brown bears. The research showed that bears suppress their metabolism substantially during winter, which is now considered the defining criterion. The term has since been redefined around active metabolic suppression rather than any fixed threshold of temperature decline.

  The boundary between daily torpor and full hibernation also turns out to be blurry. Many experts now see torpor and hibernation as points on a single continuum, sharing overlapping biological mechanisms. A chipmunk, for example, sleeps for long stretches in winter but only enters true hibernation when food runs genuinely scarce. It is what researchers call a facultative hibernator, entering dormancy in response to environmental stressors rather than on a fixed seasonal schedule. A white-tailed prairie dog, by contrast, is an obligate hibernator, entering its dormant state every year regardless of temperature or food availability. Its close relative, the black-tailed prairie dog, is a facultative hibernator.

  Summer has its own equivalent: aestivation, the counterpart to hibernation that some animals use to survive periods of heat and drought.

• 3. The Mystery of Mid-Winter Waking

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  Obligate hibernators do not simply go to sleep and wake up in spring. They cycle. A typical winter for a ground squirrel consists of extended bouts of torpor broken by periodic returns to near-normal body temperature, called euthermic arousals. These interruptions have puzzled researchers for decades.

  Why would an animal expend precious stored energy to briefly warm up, only to cool back down again? One leading hypothesis is that hibernators build up a form of sleep debt during torpor. Their brains are not truly sleeping, and so they may need to periodically warm up enough to actually sleep. Evidence from Arctic ground squirrels supports this idea. Other theories propose that these warming periods let the animal restore available energy sources or activate immune responses that cannot function at low temperatures.

  No single explanation has won the debate. What is clear is that the periodic arousals are not accidents or failures of the hibernation mechanism. They are a structured feature of it. The Arctic ground squirrel holds its head and neck at or above 0 degrees Celsius even when its abdomen dips below freezing, which suggests the brain requires protection that the body does not.

• 4. Bears as Hibernators

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  The 2016 study that finally settled the bear debate was led by Alina L. Evans, a wildlife veterinarian and associate professor at Inland Norway University of Applied Sciences. Evans and her team tracked 14 brown bears across three winters, measuring movement, heart rate, heart rate variability, body temperature, physical activity, ambient temperature, and snow depth.

  What emerged was the first complete chronology of hibernation in bears from start to finish in the wild. Bears began slowing down physiologically several weeks before they actually entered their dens. Their heart rate, physical activity, and body temperature all started declining in advance of the first snowfall. Once inside the den, heart rate variability dropped sharply, a sign the researchers interpreted as evidence of deep metabolic suppression.

  Two months before waking, the bears' body temperatures began rising, driven not by internal physiology but by the warming ambient temperature outside the den. Heart rate variability only increased about three weeks before the bears finally emerged. The bears stayed inside until outside temperatures reached their lower critical temperature. The findings suggest bears are thermoconforming animals: their entry into hibernation is triggered by environmental cues, but their departure is governed by internal physiology.

  Bears also perform feats during hibernation that have no equivalent in other mammals. They stop urinating for months without accumulating toxic waste, recycling proteins and urine instead. They maintain bone mass despite months of immobility, elevating CART hormone levels to inhibit the cells that break down bone and suppressing a protein called sclerotin that normally halts bone formation. A process called urea nitrogen salvage allows bears to synthesize essential amino acids, including valine and leucine, from urea broken down by gut microbes, which also limits muscle wasting.

• 5. Primates and the Lemur Exception

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  The fat-tailed dwarf lemur of Madagascar shattered one of the longest-standing assumptions in hibernation science: that no primate hibernates. This small animal spends seven months of the year dormant inside tree holes in a Malagasy forest.

  What makes the discovery stranger is the temperature at which it hibernates. Malagasy winters can see temperatures rise above 30 degrees Celsius. Hibernation has long been framed as an adaptation to cold, but the fat-tailed dwarf lemur hibernates in what, by global standards, is warmth. The relevant variable turns out to be energy availability, not cold alone.

  Researcher Dausmann found that the quality of the tree hole matters enormously for how the lemur hibernates. A poorly insulated hole causes the lemur's body temperature to swing wildly with the outside air. A well-insulated hole keeps it more stable, and in that case the animal undergoes regular euthermic arousals, much like cold-climate hibernators. The finding led Dausmann to conclude that low body temperature and metabolic suppression are not necessarily coupled. An animal can be metabolically dormant without becoming cold.

• 6. Dormancy Across the Animal Kingdom

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  Ectotherms, animals that cannot internally regulate their body temperature, cannot enter hibernation the way mammals do. Fish, reptiles, and amphibians experience metabolic slowdown as their environment cools, but this is a passive effect of temperature, not an active choice. What they undergo is called brumation.

  Reptiles, however, have evolved a more active relationship with dormancy. The Texas horned lizard uses an internal periodic clock, likely triggered by cooler external temperatures, to seek out colder spots and initiate its own dormancy. One mechanism shared between reptiles and mammals is hypercapnic acidosis, the accumulation of carbon dioxide in the blood. In reptiles, this slows metabolism and reduces oxygen consumption. Finding the same mechanism in both groups is considered an example of convergent evolution: two separate lineages arriving at the same solution to the same problem.

  Insects follow their own path through a state called diapause. A 1977 study by G. Edgar Folk, Jill M. Hunt and Mary A. Folk was one of the first to systematically compare cardiac readings across hibernating mammal species using EKG data, including three species of bears, and found that the reduced QT interval, a measure of heart electrical relaxation, changed in the same direction for bears and small hibernators between summer and winter.

  Some animals survive winter not by slowing down but by freezing solid. Wood frogs, certain fish, and some reptiles have evolved freeze tolerance through antifreeze proteins, allowing them to ice over and revive in spring. The ability to endure freezing is biologically separate from the energy-conservation logic of hibernation.

• 7. Ancient Origins

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  The earliest suggested instance of hibernation in an animal traces back to Thrinaxodon, an ancestor of mammals that lived roughly 252 million years ago. This pushes the origins of dormancy deep into the Permian-Triassic boundary, long before the world looked anything like it does today.

  When the ancestors of birds and mammals moved from the sea onto land, they left behind the relatively stable thermal environment of the ocean. Terrestrial seasons are sharper, and the dormant periods of land animals tend to be longer in regions where seasons are more extreme. The current understanding is that hibernation likely evolved at the same time as endothermy, the ability to generate internal body heat, rather than after it. The earlier hypothesis held that hibernation evolved as a later adaptation to cold climates, but the evidence from Thrinaxodon and related research challenges that sequence.

  Body size shaped the trajectory of hibernation across lineages. As endothermic animals grow larger, the ratio of body surface area to volume shrinks, making it cheaper to maintain a high internal temperature. Hibernation becomes metabolically unnecessary. Bears are one of the few exceptions to this pattern, retaining hibernation at a body size that would normally rule it out.

  There is evidence that hibernation evolved separately in marsupials and placental mammals. Young marsupials from hibernating species develop the capacity to hibernate as soon as they can regulate their own body heat. Placental mammals, by contrast, develop full homeothermy first and only gain the ability to hibernate afterward. This difference points to at least partially distinct evolutionary pathways, though the question is not fully settled.

• 8. Human Hibernation

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  In 1900, the British Medical Journal reported on a practice called 'Lotska' among Russian peasants in the Pskov Government. Families who could not store enough food to last the winter would gather around the stove, sleep for six months, and wake once a day to eat bread and drink water. The report described it as closely akin to hibernation.

  Researchers today are studying the prospect of induced human hibernation from multiple angles. The most immediate motivation is medical: the ability to pause the body's metabolic activity could preserve the lives of critically injured or ill people while treatment is prepared. Space travel to Mars is another proposed application, where the energy and psychological costs of a long voyage might be reduced by putting passengers into a dormant state.

  Anthropologists are also investigating whether early hominid species may have had some capacity for hibernation. The question remains open. What is not open is the scientific interest in hibernation induction trigger proteins, known as HIT proteins, isolated from hibernating mammals. A 1997 study found that delta 2 opioid and HIT proteins could not improve recovery rates of heart tissue during ischemia, but identified their precursors as relevant to preserving veterinary organ function. A 2014 study used recombinant protein technology to manufacture specific hibernation proteins, including HP-20, HP-25, and HP-27, outside of any animal, opening a path toward studying these molecules without requiring the animals themselves.