A frozen wood frog has no detectable heartbeat, breathing, circulation, or muscle movement, and much of the water in its body has turned to ice. Warm it up, however, and within hours the machinery of life begins switching back on.

If you found a wood frog beneath the winter leaf litter in Alaska, you could be forgiven for pronouncing it dead. It would be stiff. Its heart would not be beating. It would not be breathing, its blood would not be circulating, and there would be no detectable muscle movement. In freeze-tolerant vertebrates such as the wood frog, as much as about 65 percent of the body’s water can freeze as ice outside the cells. Then spring arrives, the frog thaws, its heart starts beating again, and it gets on with being a frog.

The trick is not preventing ice altogether. It is controlling where the ice forms. Ice crystals inside a cell can tear membranes and wreck the machinery required to keep that cell alive. As freezing begins around a wood frog’s tissues, its liver rapidly converts stored glycogen into enormous quantities of glucose. That sugar floods into organs and cells, helping them retain water and protecting delicate structures while much of the water outside the cells freezes. Urea, which the frog accumulates before and during winter, provides additional protection. Alaskan populations push the system particularly far, building unusually large stores of these natural cryoprotectants before the deepest cold arrives.

If that sounds vaguely familiar, you may be remembering our old friend the tardigrade. Tardigrades can also survive freezing, but they generally employ a different strategy called cryptobiosis. Under extreme conditions, many species can reduce their metabolism to barely detectable levels; freezing can trigger a form called cryobiosis. The wood frog does something that feels even more improbable because it is a comparatively large, complex vertebrate with a heart, liver, brain, blood vessels, and all the usual physiological plumbing. Rather than retreating into a microscopic cryptobiotic package, it lets ice spread through much of its body while protecting the living cells between the crystals.

There is a useful distinction here. The wood frog is often said to possess “antifreeze,” but its chemistry does not simply keep the frog liquid the way antifreeze keeps a car radiator from freezing. The frog does freeze. Its glucose, urea, and other physiological defenses allow it to survive the dehydration, oxygen deprivation, and mechanical stresses that accompany that freezing. Researchers have found particularly hardy Alaskan wood frogs surviving experimental freezing at temperatures far below those tolerated by more southerly populations.

We usually think of the heartbeat as one of the dividing lines between life and death. The wood frog makes that line considerably less tidy. For part of the year, an animal can lie beneath dead leaves with no beating heart and much of its body turned to ice—and still be waiting for spring.

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