This Frog Freezes. Then It Wakes Up.
Wood frog in Acadia National Park. National Park Service, public domain, via Wikimedia Commons. Image source.
Winter arrives, and a wood frog settles into the fallen leaves. Ice forms inside its body. Its heart stops beating, and it stops breathing.
This sounds like the end of the frog’s story. But when spring arrives, the same animal can thaw and head off to a breeding pond.
How does a frog survive something that destroys the cells of most animals?
Frozen does not mean every cell freezes
The distinction is where the ice forms. Wood frogs tolerate ice outside their cells while protecting the living material inside them. Their livers release large amounts of glucose, a sugar that helps limit freezing injury and the dehydration that ice can cause.
That makes the familiar phrase “frozen solid” a little misleading. The frog becomes stiff and icy, but its survival depends on preserving unfrozen conditions within its cells. It is a carefully managed physiological state, not an ordinary frog that happens to recover from a deep freeze.
Why spend winter on land?
Many frogs escape freezing conditions by overwintering underwater. Wood frogs instead shelter in leaf litter. Leaves and snow provide insulation, although the animal can still encounter temperatures below freezing.
The land strategy brings a spring advantage. Ground can thaw while nearby lakes remain covered in ice. A wood frog that becomes active early can reach shallow meltwater pools, breed, and give its offspring a start before those temporary pools disappear.
The winter arrangement and the spring breeding schedule belong to the same story. Tolerating ice helps the animal use places that would otherwise be difficult to exploit.
An extraordinary adaptation with limits
This is a specialized ability of the wood frog, not a rule for frogs generally. Nor does it mean the animal is indifferent to temperature, exposure, or its surroundings. Its sheltered winter habitat and its protective chemistry both matter.
Researchers are interested in freeze tolerance partly because preserving living tissue is difficult. The frog does not supply a ready-made method for freezing people or transplant organs. It supplies a remarkable example of how an animal can control damage that would normally be lethal.
Come spring, what looked like an icy little statue becomes a frog on its way somewhere.
Sources and image credits
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