How Can a Frog Freeze Until Its Heart Stops and Survive?

A wood frog can spend part of winter with ice throughout its body, no heartbeat, and no breathing, then thaw and become active again.

The frog shelters beneath leaves rather than escaping deep underwater. When ice begins forming around it, the animal’s body responds rapidly. Its liver releases large quantities of glucose into the bloodstream. Urea also accumulates. These substances enter tissues and help protect cells from dehydration and damage.

The frog does not allow every part of itself to freeze. Much of the water outside its cells becomes ice, while protective chemicals help keep the interiors of cells liquid. Water is drawn outward as ice forms, shrinking the cells but reducing the risk that sharp crystals will grow inside and rupture them.

During this state, circulation stops and ordinary signs of life nearly disappear. Metabolism falls to an extremely low level. When temperatures rise, the ice melts, the heart resumes beating, and normal breathing and movement gradually return.

Freezing allows wood frogs to overwinter close to the forest surface. They can awaken early and reach temporary spring pools before frogs that remain beneath ice covered lakes. Those shallow pools often contain fewer fish that might eat their eggs.

The frog is not brought back after being completely dead, nor is every molecule frozen solid. Its achievement is more controlled: it permits ice in normally fatal amounts while chemically protecting the living structures that must restart in spring.