Biology
The Physiological Mechanisms of Freeze Tolerance in Wood Frogs
Quick fact
Wood frogs can survive being frozen up to 65% of their total body water, with no heartbeat and no breathing, for weeks at a time, and then thaw out and resume normal life within hours.
Why this is interesting
Imagine being frozen solid for weeks, then waking up just fine. Wood frogs do exactly that every winter—how is that possible?
Read the full explanation
Understanding The Physiological Mechanisms of Freeze Tolerance in Wood Frogs
During winter, wood frogs bury themselves in leaf litter. When temperatures drop, ice begins to form in their body, but it only forms in extracellular spaces—between cells and in body cavities—not inside the cells. As ice forms, it draws water out of cells, causing them to shrink and concentrate solutes. To protect cells from damage, the frog's liver converts glycogen into massive amounts of glucose, which is transported to all tissues. Glucose acts like antifreeze, lowering the freezing point of cell contents and stabilizing cell membranes. At the same time, the frog's heart and breathing stop, and metabolic rate plummets to near zero. When spring arrives, the ice melts, the heart and breathing restart, and the frog resumes life.
A deeper explanation
The mechanism is a well-coordinated cascade. As skin contacts ice, ice nucleates in the body, facilitated by ice-nucleating proteins or bacteria. Ice grows only extracellularly because the cell membrane prevents ice penetration. This extracellular ice creates an osmotic gradient that pulls water out of cells, causing cell shrinkage. The liver responds to the freezing signal by breaking down glycogen into glucose, which is released into the bloodstream and taken up by cells. Glucose acts as a colligative cryoprotectant, reducing ice formation and protecting proteins and membranes. Urea, accumulated from nitrogen metabolism, also contributes to cryoprotection. Blood flow and heart rate cease, and the frog enters a state of oxygen deprivation, relying on anaerobic glycolysis. This tolerance to anoxia is crucial. Upon thawing, circulation and respiration restore, and excess glucose is reconverted to glycogen. This remarkable system allows wood frogs to survive in harsh climates where other amphibians cannot, and the underlying principles have inspired cryopreservation research.