Wood Frogs: The Farthest North Amphibian Cannibals

Their staccato voices can make a wilderness muskeg bog as loud as a city street, even though most are so small they could sit in a coffee cup without scraping their noses.

They surprise hikers, who notice them hopping around in a spruce forest, nowhere near water. The wood frog, America's farthest north amphibian and one of our state's most unlikely residents, is the only species of frog living north of Southeast Alaska.

Rana sylvatica is among only six species of amphibians in Alaska, according to Amphibians and Reptiles in Alaska, the Yukon and Northwest Territories by Robert Parker Hodge, former curator at the University of Alaska Museum. While the rough-skinned newt, the northwestern and long-toed salamanders, the boreal toad and the spotted frog prefer the mild, wet climate of Southeast Alaska, the wood frog thrives throughout the state, even north of the Brooks Range. The wood frog also holds the lonely distinction of being the Yukon Territory's only amphibian.

Since the body temperature of frogs and other ectotherms is largely dependent on their environment, it seems odd they ended up in Alaska. The wood frog probably originated in the equatorial zone of Asia and hopped across the Bering land bridge sometime during the Tertiary period, Hodge wrote. The wood frogs were able to adapt to Alaska's extremes primarily because of their ability to quickly change from tadpole to frog before water freezes in the fall.

How wood frogs, a species that ranges as far south as Georgia, survived Alaska winters was a mystery until Michael Kirton inserted radioactive tags on 27 wood frogs in the fall of 1972 in Fairbanks. For his master's thesis research at the University of Alaska, Kirton followed the frogs with a Geiger counter until they stopped moving in September.

Frogs in warmer climes often hibernate in mud that remains unfrozen on lake bottoms, but Kirton found that his Geiger counter would tick loudest over land. As he probed the ground, he found dormant wood frogs in shallow bowls of compacted forest litter. These nests, only about an inch deep, were insulated by the current year's accumulation of leaves and twigs. Snow cover added a final blanket to shield the frogs from northern temperatures.

Kirton found frogs endured temperatures of 21 degrees during hibernation, suggesting that wood frogs use supercooling to survive the cold here. By supercooling, an animal has the ability to rid its body fluids of impurities that would trigger the formation of ice, thereby allowing it to cool well below 32 degrees without having ice crystals puncture its cell walls.

Another researcher discovered it's a frog-eat-frog world in Alaska forests, tundra and muskeg. In 1961, Kjell Johansen collected 24 Alaska wood frogs and placed them in a tank with water and moss. Two days later, he found only four bloated frogs. He dissected one of the chubby adults and found the remnants of five smaller frogs in its stomach and intestines.

Johansen thought cannibalism might be advantageous for all northern frogs, not just the stressed-out captive ones. To test his theory, he inventoried the stomach contents of wood frogs he caught in the fall, just before hibernation. He discovered assorted frog parts, mixed with the remains of spiders, tiny snails, insects and insect larvae.

Because insects become scarce when the fall chill sets in, wood frogs probably eat their numerous, highly available sons and daughters to help them survive the winter, Johansen said. This trait, not shared with wood frogs in more southerly climes, shows the extremes some species will go to in order to adapt to a land where they spend a majority of their time in the protective slumber of hibernation.