A photograph purportedly showing a frozen Alaskan tree frog has been circulating on the Internet since at least 2013, frequently accompanied by a brief sentence explaining how this amazing amphibian supposedly survives the harsh arctic winters:
The Alaskan tree frog. Freezes solid in winter, its heart stopping, then thaws in spring and merrily hops off.
While there is a species of frog in Alaska that can survive the area’s harsh winters, that species is not the “Alaskan tree frog.” According to the Alaska Department of Fish and Game, that state is home to two species of frogs: The Columbia spotted frog (Rana luteiventris) and the wood frog (Rana sylvatica). There is no animal known as an Alaskan tree frog.
There is, however, an amphibian that lives in Alaska and has an unusually high tolerance for freezing conditions. In August 2013, a report was published in The Journal of Experimental Biology explaining how the wood frog was able to survive long winters in Alaska:
There are a number of creatures, from reptiles and insects to marine life, that possess some level of freeze tolerance, but few can perform the trick quite like Rana sylvatica. The tiny amphibians can survive for weeks with an incredible two-thirds of their body water completely frozen — to the point where they are essentially solid frogsicles.
Even more incredible is the fact that the wood frogs stop breathing and their hearts stop beating entirely for days to weeks at a time. In fact, during its period of frozen winter hibernation, the frogs’ physical processes — from metabolic activity to waste production — grind to a near halt. What’s more, the frogs are likely to endure multiple freeze/ thaw episodes over the course of a winter.
The way wood frogs avoid freezing to death is due to so-called cryoprotectants — solutes that lower the freezing temperature of the animal’s tissues. These include glucose (blood sugar) and urea and have been found in much higher concentrations in the Alaskan wood frogs than in their southern counterparts.
Increased levels of cryoprotectants help the frogs’ cells survive. In most animals, prolonged exposure to subzero temperatures causes cellular shrinkage — a process in which the formation of ice in the tissues pulls water from the body’s cells, essentially sucking them dry and eventually killing the cell. (Related: Champions of the Cold.)
But cryoprotectants help the cells resist that shrinkage. “The solutes tend to depress the freezing point [of tissue],” said (Jon Costanzo of the Department of Zoology at Miami University in Ohio). “It limits the amount of ice that actually forms in the body at any part. The more of that cryoprotective solute you can accumulate, the less ice will form and therefore the less stress there is on cells and tissues.”
The viral photograph displayed above does not show an Alaskan tree frog (since no such animal exists), nor does it show a wood frog. This widely-circulated image appears to be simply a garden ornament that has been covered with frost. The video below shows what an actual wood frog looks like as it freezes and thaws during the winter: