Wood frogs: farthest-north amphibian cannibals

Their staccato voices can make a muskeg bog as loud as a city street, 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. 

Wood frogs, America's farthest-north amphibians and one of our state's most unlikely residents, are the only species of frog living north of Southeast Alaska.

Rana sylvatica is among only six species of amphibians in Alaska, according to the book "Amphibians and Reptiles in Alaska, the Yukon and Northwest Territories" by Robert Parker Hodge, former curator at the University of Alaska Museum of the North.

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 a few million years ago, Hodge wrote. 

The wood frogs were able to adapt to Alaska’s extremes 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.

Kirton discovered 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 bitter northern air temperatures.

Kirton found frogs endured temperatures of 21 degrees Fahrenheit beneath the snow during hibernation. UAF’s Brian Barnes and his students later found a Fairbanks wood frog that survived a temperature of 10 degrees to revive when warmed. 

Even with the insulation of forest litter and snow that help trap some of the Earth’s warmth, wood frogs' bodies freeze solid in the winter. That includes their heart, brain and eyeballs. As that happens, their livers convert glycogen to sugary glucose, which helps cells resist drying.

A scientist once discovered it's a frog-eat-frog world out there. 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 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 might eat their numerous, highly available sons and daughters to help them survive the winter, Johansen said.

Since the late 1970s, the University of Alaska Fairbanks' Geophysical Institute has provided this column free in cooperation with the UAF research community. Ned Rozell is a science writer for the Geophysical Institute. A version of this story appeared in 1995.