Wood Frogs Freeze Solid Every Winter—And Survive

Every winter, wood frogs across North America do something that would kill almost any other vertebrate: they freeze solid. Their hearts stop beating. Their blood stops flowing. Ice crystals form throughout their bodies. And in spring, they thaw out and hop away like nothing happened.

This isn't a myth or exaggeration—it's one of the most remarkable survival strategies in the animal kingdom, documented by scientists for decades and still being actively researched today.

The Freeze Happens Fast

When temperatures drop below freezing, ice begins forming on a wood frog's skin. This triggers an immediate biological alarm. Within hours, the frog's liver starts breaking down glycogen stores and flooding the bloodstream with glucose—up to 100 times the normal concentration. This glucose acts as a natural antifreeze, but not the kind that prevents freezing entirely.

Instead, the glucose acts as a cryoprotectant, protecting cells from being destroyed as ice forms outside them. About 65-70% of the frog's body water crystallizes into ice, but the glucose-saturated fluid inside cells remains liquid. This is critical—if ice formed inside cells, the sharp crystals would shred cellular structures and cause irreversible damage.

Frozen, But Not Dead

A frozen wood frog shows no signs of life. No heartbeat. No breathing. No brain activity. No blood flow. To any observer, the frog appears dead. In Alaskan populations, frogs have been documented surviving while frozen at temperatures as low as -16°C (3°F) for up to seven to eight months. Midwestern and Canadian populations tolerate slightly less extreme conditions, typically surviving freezing down to -3°C to -6°C.

During this time, the frog enters a state of suspended animation. Its metabolism slows to nearly zero. It's neither alive in the conventional sense nor dead—it exists in a frozen twilight state that scientists are still working to fully understand.

The Thaw: Coming Back to Life

When spring arrives and temperatures rise, the magic happens in reverse. Ice crystals begin melting from the outside in. The heart—one of the first organs to thaw—starts beating again, pumping sluggish, glucose-thick blood through the body. Within hours, the frog's brain reboots. Breathing resumes. And remarkably, the frog suffers no apparent damage from its months-long freeze.

Research published as recently as 2024-2025 continues to reveal new details about this process. Scientists have discovered that wood frogs also accumulate urea (yes, a waste product) in autumn as an additional cryoprotectant. They've found that repeated freeze-thaw cycles actually improve survival in some populations. And they're studying the epigenetic changes—how genes turn on and off—that allow frogs to tolerate conditions that would cause fatal organ damage in humans.

Why This Matters

Wood frogs aren't just a curiosity. Their freeze tolerance has inspired research into:

• Organ preservation: Understanding how frog organs survive freezing could revolutionize transplant medicine
• Cryogenic technology: Biomimetic printing techniques inspired by wood frog biology are being developed
• Space travel: Suspended animation strategies for long-duration missions
• Climate adaptation: How animals might evolve to survive extreme temperature fluctuations

The wood frog's range extends from the southern Appalachians all the way to the Arctic Circle, making it the most widely distributed amphibian in North America. Its ability to freeze solid is why it can survive in environments where no other frog species can—a superpower hiding in plain sight in forests, wetlands, and even suburban backyards across the continent.