The longest a human being has ever willingly held their breath without dying or suffering brain damage is just over twenty minutes. Some species of deep-diving whales can manage a little over two hours. But a small, unremarkable-looking amphibian called the wood frog can hold its breath all winter.
Frogs, like all amphibians, are cold-blooded, or ectothermic. This means they do not generate their own body heat, instead requiring heat from their surrounding environment to warm them up. The perfect place for an amphibian would therefore be somewhere both warm and very moist, such as the tropical rainforest. Alaska probably wouldn’t be at the top of many amphibian’s must-visit places, yet the wood frog, the most northerly amphibian in the world, spends its entire life there.
During the winter, the temperature in Alaska can reach -20°C or even lower. The wood frog must go into hibernation if it is to survive, but even then it cannot escape the frost. Rather than fighting it, though, the wood frog surrenders to the cold. Its blood freezes and 65% of its internal fluid turns to ice. Its organs, now deprived of the oxygen-carrying blood, stop working. As its lungs falter, it stops breathing. Even its brain and eyeballs freeze. If you bent one of its legs now, it would easily snap. To almost all intents and purposes, the wood frog is dead.
But there is still a tiny shred of life in the amphibian yet. Freezing temperatures are usually lethal to animals because they cause ice crystals to form in living cells, but the wood frog’s liver converted glycogen into glucose just before it stopped working. This acts as a natural antifreeze, preventing some of the animal’s vital cells from freezing up from the inside. Urea accumulates in the tissue to limit the amount of ice that forms and to reduce osmotic shrinkage.
At its northernmost limit, the wood frog can spend up to seven months out of every year in this frozen state. Only when the weather begins to warm up again in spring does the frog stir from its deepfreeze. First, the frog’s heart starts beating again and it pumps fresh blood around its body. This contains a clotting protein that stops bleeding from wounds caused by jagged ice crystals. Slowly but surely, its other organs come back online too.
The wood frog may not have breathed, eaten, or so much as moved a muscle all winter, but in less than a day the frozen, lifeless amphibian has thawed out completely and is going back to its daily business. As Queen Elsa might say, the cold never bothered it anyway.