The spring chorus of frogs is a familiar sound in many parts of the United States. Frogs prefer warmer climates, and the variety decreases as you head north. There are 30 species of frogs in Mississippi, 14 species of frogs in Oregon, and just two in Alaska. Alaska is home to the Columbia spotted frog, found only in a few parts of Southeast, and the wood frog, found from Southeast north to the Arctic.
The wood frog is the northernmost frog in the world. It is one of the most freeze tolerant species on Earth; it has the amazing ability to freeze solid and thaw out as temperatures warm in the spring. Wood frogs hibernate in shallow bowl-shaped depressions under a layer of dead vegetation (duff), with snow cover providing additional insulation.
At the onset of freezing temperatures, wood frogs begin pumping much of the water out their cells and organs and into extracellular spaces and body cavities. At the same time, they pump large amounts of glucose - a sugar created in the liver - into their cells. The syrupy glucose solution inside the cells serves as a cryoprotectant - essentially an antifreeze - protecting the cells from freezing and from drying out. As the temperature drops, ice fills the abdominal cavity and encases all the internal organs. Flat ice crystals form between layers of skin and muscle, and the eyes turn white because the lens and fluids freeze. Nearly 70% of the frog's total body water is converted to ice. The blood freezes, the heart stops beating, and all breathing and muscle movements cease.
In April and May, as ice begins to melt along the shores of ponds or lakes, the wood frogs thaw and re-animate.