It may seem a bit early to be listening for frogs, but wood frogs rush the season. They emerge from beneath forest leaf litter in late winter, and as soon as the ice melts, they sing.
A wood frog’s voice is hardly frog-like. One field guide describes a wood frog chorus as sounding, “like a flock of quacking mallards.”
A terrific new book, The Frogs and Toads of North America (2009, Houghton Mifflin), describes the sound as, “duck-like cackling.” Readers can decide for themselves by listening to the accompanying CD.
Better yet, step outside. If there’s a woodland vernal pond or puddle nearby, you might hear them live.
Wood frogs are active by day and lead solitary lives, except during the brief mating season, which is now underway.
They can grow to a length of three inches, but most are smaller. They weigh about a third of an ounce (that’s less than a chickadee).
The best field mark is a dark mask that widens from nose to shoulder. When my daughter Emma was a little girl, she called them “raccoon frogs.”
Otherwise, a wood frog’s nondescript tan body blends in perfectly with the decomposing leaf litter on the forest floor.
Wood frogs emerge in late winter when rains trigger the breeding season. This is hardly surprising for a species whose range extends north of the Arctic Circle.
In fact, its range extends farther north than any other North American amphibian.
When air temperatures reach 50 degrees, males move to small ponds and even small depressions that collect snow melt and run off.
Here they sing, and within a day or two, curious females arrive. Males are anything but choosy. They clasp any receptive female in a mating embrace herpetologists call amplexus.
Females lay as many as 700 eggs, which the male fertilizes as she releases them. Often these breeding sites are communal so masses of thousands of eggs are possible.
After about a week, the egg mass flattens out, allowing it to rest on the surface of the water. After green algae grows on the jelly around the eggs, the mass appears to be pond scum.
Individual eggs consist of small black spheres (the embryos) surrounded by a clear gelatinous mass. They incubate in the water for up to a month, though hatching time depends on temperature.
At 45 degrees, for example, eggs hatch in about 20 days. A late freeze does not necessarily kill developing embryos. They simply stop growing and wait for warmer temperatures.
Eggs in the center of the mass have an advantage that may explain the rush to breed. The temperature in the middle of an egg mass can be as much as 12 degrees warmer than ambient temperature, so those eggs hatch sooner than those on the perimeter.
Therefore, centrally located eggs hatch first. Furthermore, older larger wood frog tadpoles sometimes eat smaller younger tadpoles. So eggs centrally located in an egg mass enjoy several advantages that promote survival.
Wood frog tadpoles begin life less than a quarter-inch long and can reach a length of two inches over the next three months. Then they transform into adult frogs, which require two years to reach sexual maturity.
In November, shorter days and plunging temperatures signal wood frogs that it’s time to hibernate. They burrow deep into the leaf litter until the end of winter.
Miraculously, wood frogs survive even the coldest winter temperatures. When living tissue freezes, ice crystals form, cells rupture and the organism usually dies. But in the dead of winter, wood frogs freeze solid.
The body is rigid, breathing ceases and the heart stops beating. And yet, when nature calls, they revive.
Wood frogs have perfected the cryogenic freezing process. In winter, almost half of the frog’s body may freeze and turn to ice.
Freezing is made possible by specialized proteins and glucose, which prevent intracellular freezing and dehydration.
But don’t take my word for it. Watch a short video clip and see for yourself at http://www.youtube.com/watch?v=Fjr3A_kfspM.