The best studied of all freeze tolerant vertebrates, wood frogs (Rana sylvatica) have a broad distribution across the boreal forests of North American from the Southern Appalachian mountains up to the tree line above the Arctic Circle. Wood frogs breed very early in the spring in melt-water forest ponds and their eggs and tadpoles develop quickly before the ponds dry out. They live their lives on the forest floor and when autumn comes they seek shelter under the leaf litter that is later blanketed with snow. However, when freezing temperatures penetrate into their insulated sites, the frogs employ an extreme mode of survival: they freeze solid. When touched by external ice, the frog’s body is seeded and ice penetrates through all body cavities until ultimately as much as 65-70% of the frog’s body water is frozen in extracellular ice masses and all vital signs cease (no breathing, no heartbeat, no movement, no measurable brain activity).
As frogs freeze, they assume a crouched position with limbs drawn in close to the body, digits tucked underneath, and head lowered. This is the "water-holding" position used by all frogs when under dehydration stress and for frozen frogs this helps to reduce evaporative water loss from the body over what could be months in a continuous frozen. Frozen frogs have no heartbeat, no blood circulation, no breathing, no detectable brain activity, and cannot move yet miraculously all vital functions return within a few hours of thawing. The main ecological advantages of freeze tolerance are (a) wood frogs can sense the rewarming of their environment in the spring and initiate breeding long before other animals that winter underground or underwater become active, and (b) by breeding in ephemeral snow-melt ponds on the forest floor, their eggs and tadpoles are safe from fish and other aquatic predators present in permanent bodies of water.