Rana sylvatica is a species of wood frog whose special abilities boggle the mind. A study recently published in The Journal of Experimental Biology explains how they manage to be frozen, but not to death.
National Geographic reports, "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 frogs have cryoprotectants, or solutes in their blood that dramatically lower their freezing temperature. It protects the body's cells and limits the amount of ice that can form in the body. By having a higher concentration of these cryoprotectants in its blood, this species of wood frog can survive being frozen even multiple times in a single winter.