Some Frogs Can Freeze Solid and Survive

For most living creatures, freezing is a death sentence as sharp ice crystals form and shred cells from the inside. Yet for the North American wood frog, it is a key survival strategy. As winter temperatures plummet, these remarkable amphibians find a shallow burrow in the forest litter, allow themselves to freeze, and enter a state of suspended animation. Their heart stops beating, their breathing ceases, and they become completely unresponsive—clinically dead by any normal standard. This isn't just a surface frost; up to 65% of the water (Review) in the frog's body can turn to solid ice.

The secret to this incredible feat lies in a natural, sugar-based antifreeze. At the first touch of ice on its skin, the frog's liver begins working overtime, converting stored glycogen into massive quantities of glucose. This sugar is rapidly pumped through the bloodstream and floods the frog's individual cells. While the water in the frog's abdominal cavity and between the cells is allowed to freeze solid, this high concentration of glucose acts as a cryoprotectant, preventing lethal ice crystals from forming within the cells themselves and protecting vital organs.

When spring arrives and the ground thaws, the frog's body defrosts from the inside out (Review). Its heart is one of the first organs to thaw and resume beating, pumping blood to the rest of the body. Within a few hours of thawing completely, the wood frog is hopping about as if nothing happened. This unique adaptation allows it to survive in harsh northern climates, including parts of Alaska, and gives it a head start on the breeding season long before other amphibians have emerged from their deeper, frost-free hibernation spots.