The Wood Frog Spends Winter as a Block of Ice

Inside the leaf litter of an Ohio woodlot in January, a wood frog is doing what would kill any mammal: about two-thirds of its body water is solid ice. Its eyes are frosted white, its heart has stopped, its lungs aren't moving. By every measurable sign it is dead.

It isn't. As soon as the first ice crystals form on the skin in autumn, the frog's liver starts converting stored glycogen into glucose and dumping it into the bloodstream. Cells flood with sugar. Water moves out of them and into the spaces between, where it freezes harmlessly around organs that have shrunk into a kind of dehydrated jerky. Special nucleating proteins make sure the ice forms outside the cells, never inside.

The trick has limits set by latitude. Wood frogs collected in the eastern United States survive only a few weeks of freezing, and no colder than about –7 °C. In 2014 Don Larson, then a graduate student at the University of Alaska Fairbanks, monitored frogs in their natural hibernacula near Fairbanks. They sat at –18 °C for 218 straight days, with 100 percent survival — the coldest body temperature ever recorded in a vertebrate that lived to tell about it. The Alaskan animals get there by going through repeated short freeze-thaw cycles in early autumn, each one provoking the liver into stockpiling more glucose for the long winter.

Spring is faster than autumn. According to Kenneth Storey at Carleton University, who has been working on the species since the 1980s, the ice melts in about twenty minutes; the heart finds a beat twenty or thirty minutes after that. Within an hour the frog is breathing, then gulping, then hopping. Storey's lab is studying the molecular details with one eye on organ transplantation, where keeping a heart alive on ice is a problem the frog has been quietly solving every winter for a long time.