Organisms that are best suited to their environment (either physically or behaviorally) survive and reproduce with the greatest success. Any organisms that are not well suited to their environment soon die out. As a result of this natural selection that occurs over the course of many generations, species evolve with particular traits—biological adaptations—that allow them to better survive in their habitats. One such adaptation is freeze tolerance, which allows certain organisms—such as some species of frogs, snakes, lizards, and turtles—to survive in very cold habitats

Most organisms suffer injury when exposed to freezing temperatures: ice crystals form in their bodies, damaging cells and tissues. When ice crystals form in the area just outside of cells, they deform the cells and puncture cell membranes. In addition, ice is made of pure water, so as it crystallizes, it leaves behind solutes (substances that were dissolved in the water). As the solutes become more concentrated outside the cells, water inside the cells diffuses across the cell membranes through osmosis in an effort to equalize the concentrations of solutes inside and outside the cells. As a result, the cells become dehydrated from the loss of water and shrink.

The wood frog, found in the northern United States and Canada, has adaptations that help it survive the freezing temperatures that are typical of its habitat. It is able to prevent cell and tissue damage by circulating large amounts of cryoprotectants (such as glucose) throughout its body before it freezes. The cryoprotectants act as a natural form of antifreeze that prevents the insides of the cells from freezing. High levels of cryoprotectants inside cells also minimize the amount of water that diffuses out of the cells, which reduces both the amount of ice that forms around the cells and cell shrinkage. In addition, the wood frog's body also prepares for freezing by moving some water to areas where it would do the least harm, such as between layers of skin and muscle. Through these mechanisms, the wood frog can survive at temperatures several degrees below freezing, with 65-70 percent of its body water in the form of ice. While frozen, the wood frog has no heartbeat and does not breathe or move. However, because it is possible for its body functions to resume, the frog is not dead, but rather preserved in this frozen state. When the temperature warms up, its body thaws and resumes normal activity.

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