Freaky Frogs

Wood Frog

With Christmas just a week away, we decided that for our final Freaky Frog article of the year, we would take a look at a frog that is certainly no stranger to the cold. Amphibians require heat from their surrounding environment to warm up, but, thanks to their thin, permeable skin, they risk fatally drying out if exposed to intense sunlight. Therefore, they are often found in warm but humid places such as the tropics. The wood frog, however, breaks that mould. This North American amphibian is one of the most northerly distributed in the world, with populations living in Canada and even Alaska – well into the Arctic Circle.

During the winter, temperatures in Alaska can fall to -20°C or lower. If it is to survive, the wood frog must go into hibernation – but even then, it cannot escape the frost. Rather than fighting it, the wood frog surrenders to the cold. Up to 65% of its internal fluid, including its blood, turns to ice. Its organs, now deprived of 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 off. For almost all intents and purposes, the wood frog is dead.

But there is still a tiny shred of life left in this amphibian. Freezing temperatures are usually lethal to animals because ice crystals form within their cells, puncturing them and causing irreparable damage. The frog avoids this fate by way of a process that first sucks most of the water out of its cells and then fills each with a sugary syrup, which acts as a natural antifreeze. In this way, blood and tissue can effectively turn to ice without fatally damaging the frog itself.

At its northernmost limit, the wood frog can spend up to seven months out of every year in this cryogenic state. Only when the weather begins to warm up again does the amphibian stir from its deep freeze. First, the frog’s heart starts beating again, pumping fresh blood around its body. This contains a clotting protein that stops bleeding from any wounds caused by jagged ice crystals. Then its other organs slowly come back online too. It may not have breathed, eaten, or so much as moved a muscle all winter, but in less than a day, almost magically, this frozen, lifeless frog has thawed out completely and is going back to its daily business.

The next Freaky Frogs article (and The Nature Nook’s first article of 2021) will be a special one, looking at five different fantastic frogs and their amazing adaptations.