Some Frogs Can Freeze Solid and Survive

The remarkable ability of the North American wood frog to endure being frozen solid is a masterclass in natural adaptation. As winter approaches, these amphibians don't burrow deep into the soil below the frost line like many other frogs. Instead, they find shelter (Review) in the leaf litter on the forest floor, a location that offers minimal insulation from the plunging temperatures. When ice crystals begin to form on their skin, it triggers a cascade of physiological responses. The frog's liver starts to convert massive amounts of stored glycogen into glucose, which then floods its bloodstream and permeates every cell in its body. This surge of sugar acts as a natural antifreeze, or cryoprotectant, preventing the formation of damaging ice crystals within the cells themselves.

Simultaneously, the frog's body begins to accumulate urea in its tissues, further contributing to the cryoprotectant effect. As the freezing process continues, ice forms in the extracellular spaces, between the cells and surrounding the organs, drawing water out of the cells but leaving the vital internal structures protected by the sugary, syrupy solution. During this time, the frog is in a state of suspended animation; its heart stops beating, it ceases to breathe, and there is no discernible brain activity. To all appearances, it is clinically dead. Yet, when spring arrives and the environment warms, the frog thaws from the inside out (Review), its vital functions resume, and it hops away to begin its breeding season, often well before other hibernating animals have emerged.

This incredible survival mechanism has captured the attention of scientists, particularly those in the field of cryopreservation. The wood frog's ability to protect its organs and tissues from the damage typically caused by freezing and thawing offers valuable insights that could one day be applied to the long-term storage of human organs for transplantation. By studying the precise biochemical processes that allow this small amphibian to cheat death, researchers hope to unlock new methods for preserving life.