Just call them Mr. and Mrs. Freeze. Wood frogs can survive colder conditions than practically any other North American amphibian or reptile. They’re able to freeze nearly solid. They stop breathing, and their hearts stop beating. Then—it’s a miracle!—they thaw and come back to life when the temperature warms up. A frog might freeze and thaw several times during one winter, with no damage to any internal organs.
What’s the secret to this cryopreservation? The decrease in temperature triggers a wood frog’s tablespoon-sized body to become flooded with an enormous amount of glucose, thanks to carbohydrates stored in the liver. The glucose acts as a cryoprotectant, preventing ice crystals from forming inside the frog’s cells. Without all this helpful sugar—enough to raise their blood glucose level to more than 45 times a healthy human’s—their cells could rupture.
Since wood frogs were built for the chill—no surprise—they far prefer the cold to the heat. By summer, when their glucose drops back to practically zero, they’ll hide in the coolest, dampest, shadiest spots they can find.