The wood frog survives winter by allowing ice crystals to form in much of its body. Unlike most vertebrates, it can endure repeated freezing and thawing while its heartbeat, breathing, blood circulation, and detectable brain activity temporarily stop.
As temperatures fall, the frog accumulates glucose and urea in its tissues. These compounds act as cryoprotectants: they reduce cellular dehydration and help prevent ice from forming inside cells, where crystals would rupture delicate structures. Ice instead forms mainly in spaces between cells. Northern populations can tolerate freezing of roughly 65–70% of their total body water.
Wood frogs overwinter in shallow soil, leaf litter, or the upper organic layers of forest floors, rather than retreating far below the frost line. In Alaska, some remain frozen for about six months and can reach exceptionally low body temperatures. When spring arrives, they thaw and are among the first frogs to migrate to temporary breeding pools.
A common mix-up is assuming all frogs use the same strategy. Some avoid freezing by sheltering underwater or below the frost line, while the wood frog is famous for true freeze tolerance.