Frogs That Freeze and Thaw

The wood frog, a small amphibian found across North America, including regions as far north as the Arctic Circle, possesses a truly astonishing survival strategy for winter. Instead of burrowing deep underground or seeking unfrozen waters like many other frogs, it allows up to 60 to 70 percent of its body to freeze solid. During this remarkable process, the frog's heart stops beating, breathing ceases, and brain activity halts, rendering it metabolically inactive and appearing lifeless for months.

This extraordinary feat is possible due to a complex biological mechanism involving natural cryoprotectants. As temperatures drop, ice begins to form on the frog's skin, initiating a controlled freezing process where ice crystals grow in the extracellular spaces, between cells, while the cells themselves remain unfrozen. Simultaneously, the frog's liver converts stored glycogen into massive amounts of glucose, essentially flooding its bloodstream and cells with this sugar. This glucose, along with urea accumulated in its tissues, acts as a natural antifreeze, stabilizing proteins, preventing cellular shrinkage, and minimizing damage to vital organs. Some species also produce specialized antifreeze proteins that bind to ice crystals, inhibiting their growth and preventing them from piercing cell membranes.

When spring arrives and temperatures rise, the frog thaws, often from the inside out (Review), and its bodily functions gradually resume. Within hours to a few days, its heart restarts, breathing begins again, and the frog is ready to hop away. This incredible adaptation has fascinated scientists for decades and continues to be a subject of intense research. Understanding how these frogs can withstand such extreme conditions and revive without cellular damage holds immense potential for medical advancements, particularly in the long-term preservation of human organs for transplantation.