It should come as no surprise that European explorers were astonished by their first encounters with freeze-tolerant frogs--for although the globe suffers no shortage of cold places, freeze tolerant frogs live only in North America.

The current range of the wood frog (in red) and e xtent of glacial ice 18,000 years ago (light blue.)

Kenneth Storey interprets the narrow geographic distribution of freeze-tolerant frogs to mean that they evolved only very recently, as the last Ice Age drew to a close some 15,000 to 20,000 years ago. As glaciers retreated from their greatest extent (as far south as present-day Indiana), frogs moved north to occupy the newly exposed land.

That ordinary frogs could have evolved the ability to survive freezing in so short a time may sound far-fetched, but Storey believes that frogs are ideally suited for freeze tolerance. He sees freeze tolerance as a natural progression from the ability to withstand severe dehydration, which most frogs already possess.

How does one get from dehydration to freezing solid? Dr. Storey explains .Since frogs spend much of their time in water, their skin is water-permeable; as a result, they are especially prone to dehydration in times of drought. Many frogs survive losing over 50% of their water.

Dehydration tolerance may be an ace in the hole for an enterprising frog looking to become freeze tolerant, but it doesn't count for everything. The spadefoot toad (despite its name, really a frog) can lose 60% of its body water, yet freezing kills it. So what else must a frog accomplish to evolve freeze tolerance ?

Despite their great croaking, these frogs garnered little attention until a 1982 paper published by University of Minnesota biologist William Schmid. Shortly afterward, Storey began to investigate the wood frog's strategy for freeze survival. By 1990, Rubinsky, who was already studying more traditional techniques for organ preservation, took an interest in the wood frog as an alternative route to success.


By the time that Rubinsky began studying the wood frog, Storey already understood the importance of slow cooling to its survival. Slow cooling allowed the frog several hours to respond by producing and saturating its body with prodigious amounts of glucose, which functioned as a cryoprotectant. Glucose concentrations within the frog's central organs typically soared to 100 times their original values. Cryoprotectants like glucose are nothing new to the science of organ preservation. They are known to accumulate within cells, where they bind to water molecules, therefore preventing the cells from undergoing dehydration when freezing occurs in the extracellular spaces.

Yet, many attempts to freeze and revive organs using cryoprotectants like glucose have previously failed. Even for the wood frog glucose isn't always the ticket. A wood frog frozen to -5°C survives without a hitch. But lower the temperature another ten degrees and the wood frog fares no better than any other animal given the same harsh treatment would--inevitably, it dies. Some unknown factor limits just how far the wood frog can be cooled and still survive.

Rubinsky and Storey set out to identify this factor by observing the freezing process on a microscopic scale. They watched through a microscope as sections of liver tissue from the wood frog were cooled to either -7°C (a survivable freeze) or -20°C (an unsurvivable freeze).