AUSTIN—For wood frogs in Alaska's interior, Arctic weather is more than a temporary, headline-grabbing phenomenon. Frigid temperatures are the norm for months at a time. But these tiny amphibians cope quite well and represent an extreme in freeze-tolerant organisms, researchers reported here Monday at the annual meeting of the Society for Integrative and Comparative Biology. For two seasons, they followed wood frogs to where they dug in for the winter and put temperature sensors next the frogs' skin and in the surrounding leaf litters, discovering that 100% of the 18 animals coped well despite being frozen solid for 7 months, with temperatures sometimes dropping to -18°C. That was much longer than frogs survived in the lab under cold conditions. Experiments proved that this remarkable survival rate stems from the gradual buildup of the sugar glucose in their cells. In the lab, researchers had at first gradually cooled the frogs, thinking that slow cooling gives the animals time to make glucose, which helps cells retain water that would otherwise seep out and freeze. But in nature, during October, frogs undergo more than a dozen nightly freezes and daily thaws (see video). Each cycle boosts glucose several-fold, increasing its winter concentration up to fivefold compared with frogs frozen without this cycling and giving them an extra edge in the cold. The 7 months represent a record for frozen vertebrates, and while Siberian salamanders have lived through -30°C temperatures for shorter durations, those salamanders did not have as good a survival rate as the wood frogs.
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