Biology
Physiological Mechanisms of Freeze Tolerance in Wood Frogs
Quick fact
Wood frogs (Rana sylvatica) can survive up to 65% of their body water freezing, and their hearts stop for weeks. They do this by flooding their cells with glucose, which acts as a natural antifreeze, and by controlling where ice forms. Ice grows only in extracellular spaces, drawing water out of cells and preventing lethal ice crystals from forming inside them.
Why this is interesting
Imagine a frog that can survive being frozen solid, with no heartbeat or breathing, then hop away after thawing. How is that possible?
Read the full explanation
Understanding Physiological Mechanisms of Freeze Tolerance in Wood Frogs
Wood frogs are champions of extreme survival. When winter arrives, they bury themselves in leaf litter and let themselves freeze. Ice crystals form in their body cavities and between cells, but not inside the cells themselves. The frog produces massive amounts of glucose—a simple sugar—that is pumped into cells. Glucose acts as a cryoprotectant, lowering the freezing point and stabilizing cell membranes. As ice forms outside cells, water is drawn out, so cells shrink and become dehydrated, but they don't freeze internally. The frog's heart stops, breathing ceases, and all metabolic activity nearly halts. For weeks, it is essentially a frog-shaped ice cube. When spring arrives, the ice melts, the heart restarts, and the frog comes back to life without damage.
A deeper explanation
The key to wood frog freeze tolerance is the precise control of ice formation and cellular protection. During hibernation, the liver releases large amounts of glycogen, which is converted to glucose and distributed throughout the body. Glucose concentration rises to levels hundreds of times higher than normal, which helps maintain osmotic balance and prevents cell shrinkage from becoming lethal. Additionally, urea, a waste product, also accumulates and helps stabilize proteins. Ice nucleators, such as proteins in the blood, trigger ice formation in extracellular spaces, but prevent ice from entering the cells. This is crucial because intracellular ice would puncture cell membranes and cause irreparable damage. The frog's ability to tolerate anoxia—lack of oxygen—during freezing is also remarkable: even when metabolism is arrested, mechanisms protect cells from damage caused by reactive oxygen species when oxygen returns. These adaptations are not just a curiosity; they offer insights into medical fields like organ preservation and cryosurgery. Understanding these mechanisms also helps scientists predict how amphibian populations might respond to climate change, as warmer winters could affect the timing and necessity of this survival strategy.