When winter temperatures plummet in northern forests, wood frogs do not migrate or dig below the frost line. Instead, they freeze solid in leaf litter. As ice begins to form on their skin, specialized signaling causes their liver to rapidly break down glycogen into glucose, flooding their bloodstream alongside high concentrations of urea.
These substances act as biological cryoprotectants. Urea and glucose lower the freezing point inside the frog's cells, preventing fatal intracellular ice crystal formation that would otherwise rupture cellular membranes. While extracellular water turns to ice, stopping the frog's heartbeat, breathing, and brain activity, the cells themselves remain liquid and protected.
When spring arrives, the frog thaws from the inside out, resumes cardiac function within hours, and hops away completely unharmed.