A chorus of frogs fills the air on a summer night in the Pacific Northwest. When fall comes and this lake in the Cascade Mountains cools, the frogs will burrow into the leaves and forest litter and hibernate below the frost line. Insulated by soil and a layer of snow, their bodies cool to temperatures just above freezing, but they can't freeze. If they do freeze, they die.
In Alaska, being cool is not good enough. Here, the ground freezes and so do the frogs - but they don't die. Alaska's frog, the wood frog, has a different strategy. When ice crystals begin to form on a wood frog's skin as the fall days cool below freezing, it triggers an adrenalin response, much like the fight or flight response in mammals. The wood frog responds by flooding its bloodstream with glucose - blood sugar. This blood sugar acts as an antifreeze that protects the cells, even though most of the water in the frogs body outside the cells does freeze. In a person, a blood sugar level of 90 micrograms per 100 milliliters of blood is normal. In freezing wood frogs, the blood sugar level is 450 times higher, which would kill a person many times over. The frogs survive in part because their metabolism has essentially shut down.