Each fall, wood frogs across North America do something that would kill almost any other vertebrate: they let themselves freeze. Ice forms throughout their body cavities, their heart stops beating, and for weeks or months at a stretch they show no breathing, no blood flow, and no detectable brain activity. Then, as the ground thaws in spring, the frogs thaw with it, their hearts restart on their own, and within a day or two they are hopping through the leaf litter as if nothing happened.
Ice Everywhere Except Inside the Cells
The wood frog’s trick is not resisting freezing but controlling exactly where the ice forms. Specialized ice-nucleating proteins in the frog’s blood trigger freezing to start in the spaces between cells rather than inside them, where growing ice crystals would otherwise shred delicate cell membranes. Studies of overwintering wood frogs have found that up to 65 to 70 percent of the animal’s total body water can end up locked in extracellular ice, while the cells themselves are largely spared from freezing directly.
Biochemist Kenneth Storey at Carleton University in Canada, who has studied wood frog freeze tolerance for decades, has described the process as a finely tuned sequence rather than a passive shutdown. Within minutes of ice first contacting the frog’s skin, a cascade of hormonal and metabolic signals begins reorganizing the body for survival, well before the animal’s core has actually reached freezing temperature itself.
The Liver Floods the Body With Glucose
As soon as ice begins to form, the frog’s liver responds by breaking down its glycogen reserves and releasing enormous quantities of glucose into the bloodstream. That glucose travels to every organ, where it acts as a cryoprotectant, holding water inside the cells and keeping them from shrinking to a lethal degree as ice draws water out of the surrounding tissue. Research on wood frogs archived by the National Institutes of Health has measured glucose concentrations in the blood and major organs reaching 100 to 250 millimoles per liter during a freeze, many times the level found in an unfrozen frog.
A wood frog does not necessarily freeze only once each winter. In milder parts of its range, a frog sheltering under leaf litter or shallow soil can experience several separate freeze-and-thaw cycles across a single season as air temperatures fluctuate above and below freezing, and each new freeze triggers another round of glucose mobilization from the liver.
A Body With No Measurable Vital Signs
Once freezing is underway, the frog’s heart stops beating entirely, and researchers monitoring frozen individuals detect no breathing, no circulating blood, and no measurable brain activity. Metabolic rate drops to a small fraction of normal as the animal’s cells shift into a kind of biochemical standby mode, with gene and protein activity reorganized to limit damage and conserve what little energy remains available. By most conventional measures a fully frozen wood frog would be classified as clinically dead, yet the state is fully reversible as long as ice never forms inside the frog’s individual cells.
Unlike a hibernating mammal, which continues to burn energy steadily through the winter at a reduced but ongoing rate, a frozen wood frog is not simply running slow. Large parts of its normal cellular machinery are switched off entirely for the duration of the freeze, a state some researchers compare to a form of suspended animation rather than ordinary hibernation.
How Cold, and How Far North, Wood Frogs Can Go
Wood frogs range farther north than almost any other North American amphibian, with populations found from the southeastern United States all the way to the Canadian Arctic and interior Alaska, one of the broadest ranges of any amphibian on the continent. Research published in the Journal of Experimental Biology found that frogs hibernating in natural shelters in interior Alaska remained frozen for as long as 193 consecutive days, enduring average temperatures around minus 6 degrees Celsius and brief dips as low as minus 18 degrees Celsius, with full survival. That is a far greater cold tolerance than wood frogs from the upper Midwest or southern Canada display, which typically survive freezing only down to about minus 3 to minus 6 degrees Celsius, suggesting Alaska’s population has evolved extra biochemical safeguards for its harsher winters.
What Thawing Looks Like
When spring temperatures rise enough to melt the extracellular ice, the frog’s heart resumes beating within hours, typically starting at the body’s core before circulation reaches the extremities. Breathing and reflexes return over the following day, and within a day or two of full thawing the frog is able to resume normal activity, including calling and breeding in the vernal pools that form from the same snowmelt that thawed it.
Why Medical Researchers Study Frozen Frogs
The wood frog’s freeze-thaw cycle has drawn interest well beyond amphibian biology because the same underlying problem, protecting cells and organs from ice damage, is central to organ preservation for human transplants. Scientists studying the frog’s cryoprotectant chemistry hope that understanding how its cells tolerate such extreme dehydration and cold could eventually help extend how long donated human organs can be safely stored before transplant, one of the persistent bottlenecks in transplant medicine.
This article was produced with the assistance of AI and reviewed by Morning Overview editors.
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