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The astounding ability of the wood frog (Rana sylvatica) to survive for weeks at a time with two-thirds of its body water completely frozen has caught the attention of researchers. The frogs encounter subzero temperatures as they spend the winters in relatively exposed sites on the forest floor, and they may endure multiple freeze-thaw episodes over the course of a single winter.

In a new study, Jon P. Costanzo (Miami University, Oxford, OH) reports that solutes called cryoprotectants lower the freezing temperature of the frogs' body fluids (J. Exp. Biol. 216, 3461–3473; 2013). Costanzo and his team looked at the blood and tissue concentrations of several metabolites among late-summer, fall and winter Alaskan wood frogs. Compared with late-summer frogs, winter frogs had elevated plasma levels of the two major cryoprotectants, urea and glucose, as well as a greater reserve of glycogen in the liver, which could be converted to glucose during freezing.

The Alaskan wood frogs were also compared with their conspecifics from Ohio, a more temperate part of the species' geographic range. Plasma glucose and urea were found in much higher concentrations in the Alaskan wood frogs than in their southern counterparts. The researchers also detected the presence of an additional mystery solute in the Alaskan frogs, which they have yet to identify. The Ohioan frogs also had a 20% lower concentration of glycogen in their livers than the Alaskan frogs.

Prolonged exposure to subzero temperatures causes the formation of ice in tissues, which dehydrates the body's cells and causes them to die. Cryoprotectants limit the amount of ice that forms in the body. When the freezing responses of the two wood frog populations were compared, the Alaskan frogs contained much less ice upon freezing than Ohioan frogs, owing to their higher plasma concentrations of glucose and urea.

The researchers also compared the freeze tolerances of frogs from the Alaskan population and the Ohio population. While the Ohioan wood frogs could be frozen at −4° to −6° C and revived, the Alaskan wood frogs survived being frozen at temperatures below −16° C. These temperatures are consistent with the annual low temperatures found in the frogs' respective homes.

The ability to freeze and unfreeze living tissues while preserving viability has potentially profound implications for organ transplantation. “There's an obvious parallel between what these frogs are doing to preserve their tissues and our need to be able to cryopreserve human organs for tissue-matching purposes,” Costanzo says.