The wood frog..

Wood frogs tolerate the freezing of their blood and other tissues. As the temperature drops below freezing each winter, the wood frog drifts into a deep hibernation, its breathing and heartbeat grind to a halt, and as much as 65% of the water in its body gradually crystallizes into ice.Urea is accumulated in tissues in preparation for overwintering, and liver glycogen is converted in large quantities to glucose in response to internal ice formation. Both urea and glucose act as "cryoprotectants" to limit the amount of ice that forms and to reduce osmotic shrinkage of cells. It spends two or three months of each winter frozen.When spring finally arrives, the ice melts, heartbeat and breathing return, and the frog continues its life.