This Frog Freezes Solid Every Winter and It Could Change How Doctors Preserve Human Organs

When frogs freeze solid, medicine thaws a radical new idea.

On a winter walk through Alaska’s forests, you might step over what looks like a dead frog, locked stiff beneath the leaves. Its eyes are glazed with ice, its heart doesn’t beat, and its lungs do nothing.

And yet, this frog is alive.

Come spring, it will thaw, wake up, and hop away as if winter never happened. For scientists struggling to keep human organs alive outside the body, that trick has a lot of them thinking.

A Frog That Breaks the Rules of Life and Death

Wood frogs — Lithobates sylvaticus, also known as Rana sylvatica — live farther north than any other amphibian, even beyond the Arctic Circle. Unlike most frogs, they don’t overwinter underwater. They tuck themselves beneath leaf litter and surrender to the cold.

As temperatures drop below freezing, up to 65–70 percent of the water in their bodies turns to ice. There is no more breathing, nor nerve activity.

“For all intents and purposes, this animal is clinically dead,” Shannon Tessier of Massachusetts General Hospital, whose research draws lessons from freeze-tolerant animals, told The Scientist.

“If you hold a frozen frog, it’s literally like a rock,” added Rasha Al-Attar, a biologist in Tessier’s lab, per The Scientist.

Then, when temperatures rise, the frog thaws and resumes its life. “And start going about [their] day as though nothing had happened,” Al-Attar said.

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This ability has evolved as a carefully timed cascade of changes that protect cells from the violence of freezing. As you might imagine, this ability sounds mighty useful for a lot of things we need; some of which could save tens of thousands of lives each year.

Homemade Antifreeze: Sugar, Urea, and Survival

Freezing usually kills cells by shredding them from the inside. Ice crystals form, poking cell membranes and causing the delicate structures to collapse. Humans cannot survive this process, which is why cryonics remains science fiction. For now.

Wood frogs, however, do something different. They force ice to form outside their cells rather than inside. At the same time, their livers flood the body with glucose. Their kidneys stop excreting urine, allowing urea to accumulate in the blood. Together, sugar and urea act as cryoprotectants — literally natural antifreeze.

This biochemical cocktail prevents cells from shrinking or bursting during freezing and thawing. The process doesn’t happen once, either. In the wild, frogs experience repeated freeze–thaw cycles throughout winter.

“We hypothesize that it is the pattern of freezing under natural conditions, which includes multiple freezing and thawing cycles, that causes the high concentrations of glucose that accumulate in tissues of Alaskan wood frogs,” wrote Don Larson of the Institute of Arctic Biology at the University of Alaska Fairbanks and colleagues in a 2014 study. Larson’s study was among the first to examine the ecological physiology and behavior of freeze-tolerant wood frogs overwintering under natural conditions.

“Our results demonstrate that Alaskan wood frogs can survive being frozen for up to 7 months with minimum temperatures below -18°C,” Larson’s team reported.

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Only Siberian salamanders (Salamandrella keyserlingii) rival them. These reptiles also thaw and reanimate when spring arrives, showing remarkable resilience against temperatures as low as -50°C (-58°F). 

Not All Frogs Freeze the Same Way

Wood frogs are not alone in this icy strategy. Cope’s gray tree frogs (Hyla chrysoscelis) also survive freezing and subsequent thawing cycles, but they rely on a different chemistry. Instead of glucose, they accumulate glycerol.

“What the glycerol does really is [it] cryoprotects the cells from the physical insult of freezing and thawing,” said Carissa Krane of the University of Dayton in an interview with The Scientist.

Glycerol spreads evenly across cell membranes, preventing dangerous water imbalances when the frog thaws. Krane’s team found that cold-acclimated frogs’ cells are far less likely to burst under stress than those from warmer frogs.

“One of the reasons why this organism is very interesting to us is that this glycerol mechanism has been shown in other systems to work as a very good cryoprotectant,” Krane said.

Scientists already use glycerol to preserve microbes and sperm. What frogs may bring to the table is a method for deploying it safely, repeatedly, and at scale inside a living body.

From Leaf Litter to Operating Rooms

For transplant surgeons, time is the enemy. Once removed from a donor, most organs remain viable for only hours.

“You put the organ in a bag of ice and run to the recipient. You have in the order of hours,” said Korkut Uygun of Massachusetts General Hospital.

That short window shapes the entire transplant system. Organs cannot be stockpiled or shipped flexibly. In the U.S., about 13 people die every day waiting for a transplant.

But Alaskan frogs suggest this limitation is not inevitable.

Toner, Uygun, Tessier, and their colleagues have begun translating frog strategies into medicine. They are not trying to turn people into amphibians. Instead, they are now closely studying these animals for clues about how this fantastic ability can be repurposed for organ storage.

One lesson is ice control. Frogs allow ice to form in predictable places. Inspired by this, researchers identified bacterial products that help trigger ice formation outside cells, reducing damage during freezing.

Another lesson comes from glucose. Wood frogs use massive sugar doses that would poison human cells. So Toner’s team developed a modified glucose analog that cells cannot metabolize.

The workaround worked. Using the analog alongside controlled cooling, scientists preserved rat livers for up to four days. They later transplanted a pig kidney that had been frozen for more than a week. More recently, they pushed that boundary to ten days.

Slowing Life to Save It

Cryoprotectants are only part of the story. Frogs also shut themselves down, as their metabolism drops to bare survival.

Inspired by this, Toner’s team treated mammalian cells with drugs that slow biosynthesis before freezing. Cells treated this way survived thawing better than untreated ones.

Many animals — from hibernating bears to dormant seeds — survive harsh conditions by dialing life down rather than fighting the environment head-on.

“There’s no magic molecule,” Uygun said. “It needs to be orchestrated because that’s really how nature does [it]. There’s some coordination, some choreography.”

A Future Built on Frozen Time

Wood frogs do not think about transplant waiting lists. They freeze because winter demands it. But research about their survival strategy arrives at a moment when medicine is ready to listen.

Researchers envision a future where organs move through a true supply chain. Surgeons could schedule operations days or weeks in advance. Imagine having donated organs that travel across continents. Matching donors and recipients would become more precise and less frantic.

Back in the forest, the frog thaws first in its brain and heart. Within hours, life resumes. What looks like resurrection is really preparation — a reminder that evolution has already solved many problems humans are just beginning to ask.

In some cases, the future of medicine lies frozen under our feet, waiting for spring.