Freezing solid is supposed to be the end. Ice forming inside a body generally stops the chemistry of life. For almost every animal on Earth, that is exactly how it works. The wood frog did not get the memo.
Each winter, Rana sylvatica lets a large share of the water in its body turn to ice, loses the mains signs of life we measure, and then, months later, restarts. The freezing is the famous part. The thawing is the part that should keep biologists awake.
What “frozen” actually means here
The common assumption is that freezing must mean slowing down, a deep chill that lowers the heart rate and breathing to a crawl. The wood frog does not slow down. It stops. When frozen, the frog shows no heartbeat, no breathing, no blood circulation, no muscle movement, and no detectable brain activity.
The proportion of ice is hard to believe. In a study published in Physiological Reviews, Kenneth and Janet Storey noted that around 65% of water in their bodies can be frozen as extracellular ice, with no physiological vital signs, before returning to normal life within hours of thawing. The ice forms outside the cells, in the spaces between them and around the organs, not inside the cells themselves. That distinction is most of the story.
Janet Storey, a research associate at the Institute of Biochemistry at Carleton University, has described the effect plainly. “They look like they’re totally dead, and then they’re not,” she told the Up Here. The frog looks dead because, by the ordinary measures, it is indistinguishable from dead. It is not.
It is tempting to reach for the phrase “clinically dead,” and the wording almost fits. “Clinical death refers to the medical state involving the complete and irreversible cessation of all body functions,” the cryobiologist Jon Costanzo of Miami University has explained. The word doing the work there is “irreversible.” A frozen wood frog reverses, spontaneously and completely, which is precisely why Costanzo has been careful that the frog only loosely qualifies, and why what its brain is doing during the freeze remains an open question.
How it survives
The trick is sugar. As explained by the folks at National Park Service, glucose keeps the the frogs blood from freezing. As they noted “Hibernating wood frogs can tolerate blood sugar levels 100 times higher than normal without the damage suffered by human diabetics when their blood sugar is only 2 to 10 times above normal”.
Wild frogs even appear to rehearse the whole performance before committing to it. In Alaska, wood frogs go through repeated freeze-thaw cycles in early autumn before settling into the long freeze of winter, and those cycles seem to prime the system.
The reversal
Restarting a stopped heart, rebooting a brain with no recorded activity, and clearing months of accumulated metabolic waste, all without permanent injury, is harder to account for than the freezing itself.
The field evidence is striking. Working with wild Alaskan frogs in their natural winter burrows, Larson and colleagues tracked 18 animals that stayed frozen for months at a stretch. The frogs survived being frozen for up to 218 days at minimum temperatures below minus 18 degrees Celsius, with every frog surviving.
Recovery is fast once it begins. On thawing, the heart and brain restart spontaneously as the soil warms in spring, the contractions resuming on their own after months of silence. Why the restart works at all, after every vital sign has been absent for so long, is the question researchers find hardest to answer.
What the frog is quietly telling us
The wood frog is perhaps a working model for the long-running effort to freeze and bank human organs. A review by Al-Attar and Storey treats the frog’s natural freeze tolerance as a template for cryopreservation and biobanking.
The gap that research is trying to close is large. As of her 2018 comments, Janet Storey noted that “so far there’s nobody that’s been able to freeze an entire organ and get it to survive and function when it comes back.”
The stakes are concrete. Tens of thousands of people sit on the U.S. organ transplant waiting list at any given time, far more than the number of transplants performed each year. A way to bank organs for longer would likely change those numbers.
