Maybe you can relate: Winter isn't always the ideal time to be an animal.
The sun is low, temperatures flirt with freezing (or worse) and fresh food is scarce. Some of us migrate in search of brighter skies until spring rolls around. But a subset of creatures who tough it out are equipped with a biological game-changer: the ability to hibernate.
The term "hibernation" is derived from Latin, and it refers to "spending the winter in a lethargic state," said Brian M. Barnes, professor of zoophysiology in the Institute of Arctic Biology at the University of Alaska Fairbanks. (But hibernating isn't exclusive to cold climates — there's a lemur in Madagascar that does it, too.)
Animals enter hibernation by slowing down their metabolic rates to a near — or, in some cases, complete — standstill. With those bodily processes no longer fully in action, in most cases they don't need to take in any nutrients, nor do they excrete any waste. Instead, they enter an in-between state that can resemble a kind of living death. When the temperature warms, they emerge from their dormant state and pick up where they left off.
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But tragedy can strike if an animal miscalculates where it chooses to hibernate, or if there's a mismatch between when its internal clock tells it to venture back into the world and the on-the-ground conditions that greet it. That scenario may create more problems for some species as climate change fuels more unpredictable weather patterns.
If you got up this morning and — like a certain groundhog — saw your shadow, or are thinking "sign me up for a weekslong nap," perhaps you won't be surprised that there are ways that humans eventually might use these strategies to our advantage.
Wood frogs freeze solid
The wood frog has an impressive geographic range. This species is found across a large swath of North America, from where Canada meets the Arctic Ocean through parts of the Appalachian Mountains. When autumn comes to a close, adult wood frogs hunker down fairly close to the shores of ponds where they plan to breed the following spring.
Plant matter like leaves and moss on the ground serve as their shelter for the cold months ahead. When they find a spot that suits them, Barnes said, these frogs spin to the left, then spin to the right, and repeat that process until they've sunk down about 2 or 3 inches and are all covered up.
As the cold sets in, frost creeps across the leaves and reaches the immobile frogs, who then begin the process of releasing a cryoprotectant that keeps their bodies from drying out as they freeze. Making sure ice doesn't form in their cells is a top priority.
This sped-up video illustrates the freezing and thawing process for hibernating wood frogs. Video credit: Roger Topp/University of Alaska Museum of the North; Don Larson/University of Alaska Fairbanks Department of Biology and Wildlife; Brian Barnes/University of Alaska Fairbanks Institute of Arctic Biology.
"It's glucose — blood sugar — that acts as a cryoprotectant, and they become extremely, extremely sweet as they freeze," Barnes said. He noted that they produce that glucose using a supply of glycogen that's stored in their livers.
But this process doesn't happen overnight. It can take a week or so for the frogs' new shelter to completely freeze for the rest of the winter. Over that time period, the wood frogs engage in a cycle of freezing and thawing.
During the cold night, the glucose flows into their bloodstream, tissues and cells. During the day, the frogs thaw alongside their leafy homes. The sugary cryoprotectant builds up inside them the whole time, Barnes said, adding that a new glucose production event is triggered each time ice touches their skin.
"They're building up glucose levels in their tissues like a stair step — higher and higher and higher levels," he said. "Until [one] morning it doesn't thaw, and then they're good to go."
Months later, when the spring sun melts the ice and signals that it's time to wake up, the wood frogs slowly begin to thaw. They start to breathe again, Barnes said, and then their hearts start to beat. Over the course of around half a day, the frogs take their time coming back to life before they're ready to hop on over to their local pond and kick off the breeding season.
The ability to tolerate a deep freeze is an advantage for wood frogs, said Ken Storey, a professor and researcher in the department of biology at Carleton University. But needing their habitats to hit freezing temperatures could become an issue in some places due to climate change. He noted that in a lab setting, wood frogs that are kept cold but never freeze use up their glycogen stores too fast and die.
"They need to be frozen. If they're not frozen, they're not going to live," Storey said. "So as soon as winter retreats [for good] — as soon as winter goes away — they're more or less toast."
Barnes noted that wood frogs can also live in places where temperatures aren't as likely to freeze regularly. But he said that those that freeze probably fare better than those who don't.
Arctic ground squirrels get super cold
Some animals are able to harness the power of supercooling, which occurs when liquids resist becoming a solid below their freezing points, to stay alive even as their body temperatures drop below freezing. Scientists only know one mammal — the Arctic ground squirrel — that can do this.
Arctic ground squirrels spend the winter in underground burrows situated above the icy permafrost that coats the Arctic. When it's time to hibernate, Barnes said, they get comfortable and – though we don't exactly know how – rid their bodies of particles that the water in their bodies needs in order to freeze. That process allows them to reach a temperature as low as around 26 degrees Fahrenheit without freezing.
Researchers know that some type of physiological change occurs because they've assessed how cold blood samples taken from Arctic ground squirrels throughout the year have to get before they freeze.
Hibernating Arctic ground squirrels can stay alive even as their body temperature plummets. Photo credit: Brian Barnes
"If you draw the blood just prior to and during hibernation, [the temperature] goes much lower than if you do that experiment on blood drawn in the summer. So they changed it," Barnes said. "What we surmise is that they got rid of something" that would otherwise cause the water in their bodies to freeze.
When Arctic ground squirrels enter torpor — that's the official term for the kind of hibernation they experience — their metabolic rate drops to just 2 percent of normal, Storey said. By contrast, he added, humans' metabolic rate can only drop to about 98 percent or so of normal.
In their torpid state, Arctic ground squirrels have no measurable brain waves and their bodies cannot produce any proteins, Barnes explained. They can only remain in torpor for about three weeks at a time before they have to warm their bodies up again for around 15 hours, when they might groom themselves or rebuild their nests. Then they supercool right back down.
While they're essentially in shutdown mode, torpor is not the same as rest. So when they rouse themselves for those 15 hours, Arctic ground squirrels spend the majority of time in a deep sleep, Barnes said. Researchers generally believe that's because they've built up a serious sleep debt.
"Instead of having to sleep every day like they do in the summer — like we do all the time — they can go three weeks without sleeping," he explained. "But then the need for sleep is so great that they pay for it, they use energy to warm right back up to normal, sleep it off, and then, rested, they go back into torpor."
What hibernation research can teach us
Researchers say studying hibernation is important not just because it's a fascinating example of nature's complex handiwork, but because we could potentially use the same mechanisms to benefit humans and animals alike – if we could only crack its code.
Take the Northern Star coral, for example. This temperate species is found off U.S. coastlines from Florida from Massachusetts at a fairly wide range of depth, and it can tolerate cold waters. It enters quiescence, or a dormant state, during the cold winter months, and emerges in the early spring.
In the warmer months, this species feeds from the water column using its tentacles, said Anya Brown, an assistant professor in the department of evolution and ecology and at the Bodega Marine Lab at the University of California, Davis. But when these corals are dormant, she noted, those tentacles retreat and no longer respond to disturbances like touch.
Brown was inspired by previous research efforts to investigate how the microbiomes of Northern Star coral shift before, during and after they go dormant. She analyzed what types of bacteria and archaea were present in the coral over that time and found the ones most abundant during dormancy might be a clue as to how the coral get nutrients even as they stop feeding during the coldest months.
On the left, dormant Northern Star corals are pictured. On the right, active Northern Star corals are pictured under a microscope. Image credit: Tommy DeMarco/Roger Williams University; Alicia Schickle/Roger Williams University
For this species, Brown said, the chilly temperatures that trigger dormancy are a "predictable stressor" that they're equipped to endure. Better understanding its microbiome could help researchers come up with new techniques for conserving tropical coral species, which she noted are affected by temperature stress associated with heat instead of cold. That could potentially look like giving them probiotics designed to help them tolerate conditions they're not used to.
"What I hope is to better understand recovery dynamics and the microbes using these temperate corals so we can apply that to tropical corals and their response to stressful temperature," Brown said.
Hacking hibernation would have major implications for us, too. Though there are several different ways to achieve dormancy, all hibernating animals have to depress their metabolic rates at a molecular level to remain dormant during the winter, Storey said. That's the mystery he and his colleagues are focused on solving.
"We study the off-switches and try to then move those off-switches from animals that do have them to animals that don't have them, like humans," he said.
Understanding those off-switches could have major implications for medicine, like extending the shelf life of organs that are designated for transplantees. Cooling is already used in some medical contexts, like when the temperatures of hospitalized stroke or heart attack patients are slightly reduced as a protective measure so that their bodies require less oxygen and energy, Barnes noted.
Depending on where hibernation research takes us, it's possible that one day we could have "a cocktail of drugs" that first responders would "have in their kit to be able to produce this protected state in injured people," he added.
Hibernation science also interests big players in the world of space exploration, Storey said, noting that the European Space Agency funds some of this research. That's because putting astronauts in a dormant state on their long journeys to Mars and beyond could be a huge advantage. Though this sci-fi sounding effort is far from being achievable, the potential is there.
"Give me an elevator ride with Elon Musk and I'll convince him to fund hibernation research, because that's how we can get to Mars and get to even destinations outside our solar system someday," Barnes said.