Frogs look like delicate animals, but on the contrary they hide astonishing survival skills: some are able to withstand truly extreme conditions. There is a North American frog that can freeze and endure temperatures as low as -20°C in Alaska. This is the Rana sylvatica, also known as the 'wood frog', because it becomes as hard as a log when frost comes. It actually freezes, in the sense that all its organs, including its heart and brain, turn to ice. And how does it do that? While its body becomes covered in ice, the frog's liver secretes large quantities of glucose into its blood. Its blood becomes a thick syrup that acts as an antifreeze for vital organs. The wood frog is not the only animal able to survive the winter in the ice. Other frogs can do this, as well as some insects and certain Arctic fish that do not become as stiff as stockfish due to the antifreeze proteins circulating in their blood.