Some animals use a puzzling strategy to survive winter, finding a den and entering a state of minimal activity marked by low metabolism and body temperature, slow breathing and depressed heart rate. As black bears exit hibernation, let’s examine the evidence connecting our species with this mysterious biological phenomenon.

Months after taking the long nap, these animals come out of this catatonic state a little dizzy and starved, though fully aware and conscious, ready to eat for the first time in over half a year.

We assume that hibernation, a state of dormancy that lasts from days to several months, depending on the species, is a process foreign to our species. Or is it? Could our organism adapt to hibernation and come out of it undamaged? And did some of our ancestors who adapted to frigid environments ever hibernate?

Into a hibernating black bear’s den

Episode 812 of Ira Glass’ radio and podcast program This American Life, called The Bear at the End of the Tunnel, explains an encounter that could have been conceived as a short story by Edgar Allan Poe: Jeff, the naïve brother of a wildlife biologist, Wes, decides to help his brother by putting new collars on bears that had broken into Utah’s campsites to track their behavior, post signs and, perhaps, relocate facilities:

“And it turns out the easiest way to do that is to visit the bears when they’re hibernating in their dens. In most bear dens, that job isn’t actually too difficult or scary. The dens are small, like a cavity in some rock or under a tree, Wes says,” contextualizes Ira Glass.

This American Life, The Bear at the End of the Tunnel, radio/podcast Episode #812 (October 20, 2023)

So Jeff decides to help his brother by entering a narrow, tucked-away den with a massive black bear inside a round, cozy chamber at the end (who, supposedly, is in the midst of its dormancy state).

Given the right weather conditions, bears can hibernate for 5 to 7 months a year, and in that condition, devoid of alertness and energy, suppressing metabolic activity to 25% and decreasing the body’s temperature between 30 to 36 degrees Celsius. In other words, the bear—they assume—is safely groggy.

Following GPS coordinates, Wes and Jeff climb 1,000 feet up a steep, snow-covered hillside and locate the narrow entry. It’s a dark, very narrow grotto, and the presence of a big, hibernating mammal at the end is about to spark off a sense of fear and alertness that our remote ancestors may have felt when competing with other animals for natural shelters in wintertime.

Unwanted visit

Wes, the biologist, goes first, followed by Jeff. The brothers will soon find out that the black bear seems alert enough to hear the brother’s clumsy, noisy visit:

“Wes: And I just see these two green eyes at the very end of this tunnel, staring back at me. And the thing about these hibernating bears is they– bears don’t hibernate very deeply. So if a bear hears you approaching its den, it can come out of that hibernation, and generally, within 5 to 10 minutes, they can be mobile.”

“Jeff: The light starts to catch these two eyeballs at the very end of the den that are just, like, gleaming back at us. So, at that point, I’m, like, terrified. And I start kind of freaking out. I start saying, Wes, this is crazy, this is crazy. Like, I don’t know about this.”

This American Life, The Bear at the End of the Tunnel, radio/podcast Episode #812 (October 20, 2023)

Wes has a small action camera rolling when he sees that the bear isn’t only awake but is making the distinctive noise of a potential charge against what they may consider a threat. Then the brothers decide to use a 7-foot pole with a syringe full of sedative at its end, which they administer to the bear, though its size (350 pounds) calls for more sedative. So one of the brothers heads out of the den to get a bigger dose of the sedative to put the bear to sleep.

Before they can do so, the bear, weak from hibernation and now heavily sedated, decides to make its way out of the den, which represents a potential danger for the animal, so Wes tries to stop it wrestling with the massive, drunk-like animal. By that time, the bear gets a second dose of sedative and Wes installs the collar. However, the bear is still awake.

In and out of the long nap

The bear is getting dangerously close to the steep slope. Luckily, a tree prevents the advance, and the sedative’s effect finally puts the animal to sleep. Ira Glass goes on with his narration:

“Looking back, what Wes and Jeff both talk about is the terror that they felt in that tunnel. Wes says it was a whole new level of fear he didn’t know existed, like the kind of primal fear that, long ago, humans must have had all the time facing off with animals that wanted to kill them and eat them.”

This American Life, The Bear at the End of the Tunnel, radio/podcast Episode #812 (October 20, 2023)

This scene is surely not the only encounter of our species with a huge mammal slowly coming back from hibernation. But did any of our ancestors, or any of the hominins related to modern humans, ever hibernate? And, if so, which metabolic purpose had hibernation to our remote ancestors? Did hibernation propel metabolic processes that may contribute, according to some inconclusive studies, to healthier outcomes —or even the potential of life extension?

Before researchers even considered it as a possibility, human hibernation has also been explored by science fiction as a solution to overcome the constraints of space exploration or even as a futuristic life extension alternative to “cryonics,” the low-temperature freezing and storage of human remains: Instead of storing dead people, could science preserve humans in hibernation?

As we enter the field of the “long nap,” researchers are aware of the revolutionizing consequences of making hibernation work in humans, from medicine to space travel. Big animals that hibernate can transform their metabolism in mind-bending ways. When hibernating, bears recycle their proteins and urine. Despite not eating protein or urinating for months, bears don’t develop muscle atrophy and maintain their bone mass, avoiding osteoporosis.

Bears produce their own water by metabolizing fat despite not eating or drinking for months. And hibernating female bears are even capable of producing breast milk to feed their cubs during hibernation.

Can we warm ourselves with our mind?

A new study published in eLife shows the molecular mechanisms behind hibernation. According to the study, myosin—a type of motor protein that controls muscle contraction—plays a role during hibernation, allowing for the generation of bodily heat with no need for a nervous reaction to produce heat: the muscle reaction of shivering.

Researchers at the Colombani Andersen lab in the section of Cell & Neurobiology, Department of Biology of the University of Copenhagen, have studied the mechanisms of “nutrient-dependent gut resizing”: the small intestine can adapt its size to the amount of nutrient intake. Gut plasticity also appears in animals exposed to prolonged periods of fasting. In humans, gut resizing allows pregnant women to increase the uptake of nutrients to support the fetus’ growth.

Speculation aside, studies have explored and noticed the metabolic parallelisms between advanced meditation and hibernation in mammals and other vertebrates. Meditating yogi monks experience a rapid decrease in whole blood and cell glycolysis rate, which appears similar to the glycolytic changes in animals going into and coming out of hibernation.

Judging by the engagement they obtain in social media streams, images of yogi monks meditating for long periods, even in the freezing temperatures of the Himalayas (video):

“One yogi went into a state of deep bodily rest and lowered metabolism and was able to remain in an airtight box with no ill effects and no sign of tachycardia or hyperpnea for 10 h. In a different study done in a more naturalistic setting on a different adept, Yogi Satyamurti (70 yr of age) remained confined in a small underground pit, sealed from the top, for 8 days. He was physically restricted by recording wires, during which time electrocardiogram (ECG) results showed his heart rate to be below the measurable sensitivity of the recording instruments.”

“Meditation as a Voluntary Hypometabolic State of Biological Estivation,” NIPS, Volume 13, Issue 3, June 1998

Meditating in the snow

Watching a yogi monk stay for hours and sometimes days on top of a mountain covered in snow, as freezing temperatures cover his long hair and beard, is a graphical expression of our cognitive power to slow our bodily activity down to residual levels without fatal consequences, contradicts basic common sense, knowing the effects of frozen limbs at high altitude.

But in the same way we can swim in a lake with its surface frozen or take an ice bath for a limited amount of time, the idea of therapeutic cooling (already considered by ancient Greeks) hints at the use—and potential benefits—of controlled hypothermia for medical purposes. Could hibernation be considered a state of hypothermia adapted to long periods?

Unlike other animals, mammals hibernating today barely experience a temperature drop when in such a state and can be easily awakened, as the brothers from the mentioned episode of This American Life learned the hard way. Other winter dormancy states are more extreme. Fishes, amphibians, and reptiles that resort to hibernation enter near-freezing body temperatures.

More concerning is the ability of ancient moss, seeds, viruses, and bacteria to come back to life once the ice they got trapped thousands of years ago melts. Scientists have resuscitated anammox bacteria after over 10,000 years of dormancy. Dormant threats could awaken with rising temperatures and permafrost melting down at record rates.

If anything, our complex organisms could aspire to emulate the expertise of bears and other mammals, capable of slowing down their life-sustaining processes to a minimum so they avoid going to the bathroom and stay alive by feeding their slowed-down metabolism off body reserves:

“Hibernation is a state in which the metabolic activity of the body is reduced. The drop can be as high as 98%. It is a solution used by some mammals to survive periods of scarcity of resources. Obligate hibernators are those mammals that seasonally decrease their metabolism and body temperature regardless of the surrounding environment. Facultative hibernators enter hibernation only when experiencing a negative energy balance. Hibernation is not necessarily linked to a decrease in body temperature, but always comprises a reduced metabolism (Heldmaier et al., 2004).”

“Hibernation for space travel: Impact on radioprotection,” Life Sciences in Space Research, Volume 11, November 2016, Pages 1-9

Hibernation in primates

Why don’t humans hibernate? The simplest and most established hypothesis considers the evolution of primates and our branch within them, hominins: our ancestors evolved in a tropical to warm environment, in a changing ecosystem between forest edges and the savannah. Food and game were abundant all year round, so one survival strategy based in resting during the coldest months of the year had no sense near the African equator.

More than a lack of biological capabilities, humans wouldn’t have acquired a senseless strategy within their earliest and most decisive environment. As hominins and, later on, modern humans migrated out of the African continent to cooler (and sometimes freezing) climates, they had acquired advanced techniques to withstand harshness almost anywhere below permanent ice sheets: the cognition and ability to plan ahead and use fire, shelter, and warm clothing out of animal skins and weaved grasses and plants.

But there’s evidence that supports the primates’ ability to hibernate. Today, at least one remote species from our main lineage, the small fat-tailed dwarf lemur from Madagascar, hibernates in tree holes for seven months a year. Madagascar is far from freezing in winter, with temperatures rising above 30 degrees Celsius, which proves that hibernation is more than an adaptation to extremely low winter temperatures and is more related to a combination of stressors. Interestingly, fat-tailed dwarf lemurs won’t hibernate when the tree hole is insulated enough to provide an environment that allows the animal to maintain low body temperatures.

We are far apart from lemurs, however. The human lineage diverged from apes around 7 million years ago, but modern estimates set the time even further back. Fossils prove that our remote ancestors started in Africa, and their spine configuration, well suited for tree climbing though also adapted to walk at ground level, hints at an environment halfway between open pastures and thick forests.

Did some Neanderthals hibernate?

The genus homo, from which we descend, emerged around 3 million years ago. Evidence suggests a big diversity at the dawn of homo genus. As some homo species left Africa, homo heidelbergensis, the ancestor of Neanderthals and Denisovans, settled along Eurasia and nearby islands. Peter Frankopan’s essay The Earth Transformed: An Untold Story argues that humans began “terraforming” our planet from the very beginning, and the idea of our ancestors being neutral stewards of Mother Nature is somehow a myth.

As several hominin species, including our earliest ancestors, learned to use and control fire, make rudimentary tools, and protect themselves from the elements and predators, populations expanded and, according to one of several hypotheses, rising population levels are correlated with the increase in brain size.

In Western Eurasia, climatic stress caused by several cold episodes sparked a dramatic divergence within different Neanderthal populations and between other hominin groups living in Europe and Asia. The second chapter of Frankopan’s book, dedicated to the origins of our species, gets interesting when he mentions challenging climatic events around 450,000 years ago, when ice-cap and permafrost advanced so much to the south that made most of the continent uninhabitable for a long period. Several researchers suggest that such a harsh environment caused several adaptations, and according to the interpretation of some evidence, early Neanderthals could have possessed the ability to hibernate.

“It has even been suggested that surprisingly high levels of serum parathyroid hormone in skeletal finds in Atapuerca, northern Spain, provide indications of bodies entering a hypometabolic state. Some of our ancestors, in other words, appear to have hibernated during cold winters. This hypothesis is highly speculative; but if the findings are accurate, it may be that adults were able to do so successfully on occasion, while adolescents appear to have found the process more difficult.”

“The Earth Transformed: An Untold Story,” Peter Frankopan 2023, p. 45

The health reversal ability of hibernators

A hypometabolic state, like hibernation, when an organism’s metabolism is decreased to residual activity levels to the point of suppressing normal activities like eating and excreting, is an evolutionary strategy to increase the chance of survival during stressful times.

Perhaps the evidence of hibernation in primates, if still testimonial, opens the door to discovering the links between states of low body activity like profound meditation and the deep seasonal sleep of obligate hibernators, or animals that must hibernate annually regardless of ambient temperature and food constraints.

Unlocking the health secrets of hibernation could open new pathways to speed recovery from heart attacks, avoid muscle degradation and atrophy, and bypass all sorts of bodily stress, explains Ashley Zehnder, a veterinarian scientist, one of the founders of the biotech company Fauna Bio based in Emeryville, California:

“If we can understand how animals have adapted to hibernation, then we can make the link to specific human diseases.”

According to Monique Brouillette from Popular Mechanics, Zehnder got interested in the potential of hibernation when she found out about its effects on the brain. During the state of dormancy, the neurons retract and lose connection, whereas the brain fills with special proteins called “tau tangles,” which is a phenomenon experienced by Alzheimer’s patients. But, unlike them, hibernating animals’ brains return to their optimal healthy and intact state after waking up.

Exiting the disease circle

Animals like rodents undergo processes of potential deterioration coming out of torpor, but they can reverse these effects. According to Zehnder:

“What happens in humans is that they sort of get stuck in a disease process. But these animals have figured out how to turn them off, and it is those turning-off points that we use a lot for our disease programs.”

Understanding the bodily mechanisms of hibernation could allow scientists to define the boundaries between adaptation and disease: some animals suffer processes that their metabolism has learned to revert.

Comparing genes of different hibernators, they hope to find the gene expression of such processes that avoid damage after temporary deterioration across species of hibernators. By testing these findings in several human diseases, researchers hope to develop molecules that could reverse the effects of strokes, lung fibrosis, or Alzheimer’s disease.

Brian Barnes, director of the Arctic Institute at the University of Alaska in Fairbanks, tells Popular Mechanics that inducing hibernation in humans could generate metabolic reactions capable of fencing off heart attack, stroke, or hemorrhagic shock.

Scientists believe that the ability to reduce the metabolic rate also offers clues to longevity. In the future, hibernation could be a human affair. Or was it already in the remote past, at least among some related species, as findings in Atapuerca suggest?

As we understand more about hibernation induction and maintaining processes in big mammals, major advances could arrive sooner than some expect.