Some Frogs Can Freeze Solid

For most living creatures, the formation of ice crystals in the body is a death sentence, as the sharp crystals shred cell membranes from the inside out (Review). The wood frog, however, has evolved a remarkable strategy to survive the harsh winters of North America. Rather than burrowing deep below the frost line, it settles into a shallow depression under leaves and debris on the forest floor, where it allows itself to freeze almost completely solid. For weeks or even months, it exists in a state of suspended animation, appearing more like a lifeless ice sculpture than a living animal.

The secret to this incredible survival feat lies in a biological antifreeze system. As temperatures plummet, the frog’s liver floods its body with massive amounts of glucose, a natural sugar that acts as a cryoprotectant. This syrupy solution saturates the frog’s cells, preventing the water (Review) inside them from forming destructive ice crystals. Instead, ice forms in the spaces outside the cells, in body cavities and just under the skin, until the frog is rigid. The heart stops, breathing ceases, and all metabolic activity comes to a halt.

When spring returns and the frozen (Review) frog begins to thaw, the process reverses with astonishing speed. Its heart is often one of the first organs to function, beginning to beat and circulate the glucose-rich blood. Within a matter of hours, the frog is fully revived and ready to hop to the nearest breeding pond. This incredible biological feat has captured the attention of scientists, who study the wood frog for insights into cryopreservation and the potential for preserving human organs for transplant.