The frog who freezes solid for the winter

Posted by | February 18, 2011

On bitterly cold mornings, when dry snow squeaks under boots and mustaches freeze solid, a variety of hardy animals keep the spark of life.

An Appalachian frog holds one such amazing spark. The wood frog (rana sylvatica) overwinters under leaf litter in the forest floor—where it freezes solid. How can this be? By combining three adaptations, the wood frog endures its total-body freeze.

The first adaptation is supercooling. The molecules of a liquid, like H 2O, ricochet off each other in random fashion. Normally, as a liquid cools, its molecules slow their bouncing and become sluggish. At this point, the molecules of the liquid line up in a geometrical pattern, forming a crystal. Once that initial crystal forms, it serves as a seed for the alignment of more molecules, the crystal grows rapidly, and the liquid turns solid.

Stirring a liquid increases the odds that its molecules will randomly conform to the critical angles required to form that first crystal. If, however, a liquid cools slowly and without agitation, its molecules may not strike each other at just those specific angles. Perfectly still and cooled slowly, a liquid’s temperature can drop far below its freezing point without starting the crystallization process—the liquid supercools.

The second adaptation is making and circulating antifreeze, a chemical that lowers the freezing point of water. The wood frog’s antifreeze is glucose. Plus, glucose has other advantages: it is rapidly produced, easily transported, and fuels anaerobic metabolism.

Now let’s put these two ideas together. In the autumn, tucked in its leaf-litter hideout, a wood frog slowly cools to high subfreezing temperatures, like 28oF. Ice crystals start to form in the fluids pooling in the spaces between its cells. Within minutes of ice onset, wood frogs begin pumping glucose to body tissues. To move glucose from the liver, where it is made, requires efficient heart function.

In the first minute of freezing, heart rate nearly doubles to eight beats per minute, slows after one hour of freezing, and then stops at near-complete ice formation at 20 hours. Rapid cooling, in contrast, causes heart failure, which hampers the distribution of glucose throughout the body.

The wood frog is one of the few species of land-hibernating frogs and toads that tolerates ice in its fluids. The frog’s supercooled body water freezes quickly, in less than 30 seconds, after direct contact with external ice crystals. Frozen wood frogs show neither breathing nor heart movements and rely on anaerobic metabolism. Upon dissection, ice chunks can be seen in the abdomen and organs.

In addition to supercooling and distributing glucose, the wood frog’s organs dehydrate, which prevents ice-caused mechanical injury. The organs of slowly cooled frogs retained less water than in rapidly cooled frogs.

The water moves from organs to body spaces, where it then freezes.

Together, the three adaptations of supercooling, producing and transporting glucose, and drawing water from the organs permit wood frogs to survive winter as totally frozen, rock-hard bodies. In the spring, heart beat resumes within an hour after thawing and resurrection is complete in a few hours.

Adapted from “Hollows, Peepers, and Highlanders: an Appalachian Mountain Ecology, 2nd ed.,” by George Constantz, West Virginia University Press, Morgantown WV, 1994

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