Amphibian of the Arctic

wood frog on the ground surrounded by plants
Wood frogs like to live near the water, like this one found on the bank of the Kobuk River.

NPS Photo/Emily Mesner

Only one species of amphibian lives in Kobuk Valley National Park – the wood frog (Rana sylvatica) or naaġaayiq in Inupiaq. Frogs, like all amphibians, are cold-blooded, meaning their body temperature is the same as the air around them, so how do these little animals survive a winter where -40˚ is a common temperature?

By being frozen alive, of course.

As the days grow short and the weather grows cold, wood frogs nestle down in the leaf litter blanketing the forest floor. Their internal organs become encased in ice as their abdominal cavity freezes and a layer of ice crystals form between their skin and their muscles. Even their eyes freeze and turn white. They don’t eat or drink, they have no heartbeat and they stop breathing. They’re more like a statue of a frog than a living, breathing frog.

For most animals, this would be fatal. Internal ice crystals can puncture blood vessels and frozen blood no longer delivers oxygen and nutrients throughout the body. Freezing also dehydrates the body’s cells, causing the cell walls to fracture and leading to lethal internal damage. The wood frog, however, avoids all of this by “going sweet.” During the winter, their livers produce large amounts of sugar which prevents their cells from freezing and being damaged. Ice forms around the cells, not inside the cells, allowing the wood frog to wake up completely undamaged after months of being frozen solid.

Most species of frogs survive cold, harsh winters by hibernating deep underwater in ponds, lakes or stream where they’re protected from the subfreezing temperatures of the surface, but wood frogs choose to hibernate above ground. They aren’t protected from the subzero temperatures of the arctic winter, but when the ground begins to thaw in early spring, long before the ice covered lakes and streams melt, the wood frogs wake up and can lay their eggs in small ponds and melt water pools that dry up by midsummer. Their icy adaptation allows the wood frog to thrive in an environment where few animals can survive year-round.

Last updated: September 21, 2023

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