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‘Quacks’ in the spring woods? Think frogs

By Ben Moyer 5 min read
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A juvenile wood frog displays the black mask that identifies the species. Adult wood frogs can reach three inches in length. Listen for their “quacking” song in early spring as they congregate at temporary pools to breed and lay eggs.

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Wood frogs and other forest amphibians need temporary puddles—called vernal ponds—to successfully reproduce.

The songs of robins, bluebirds and wrens are sure signs of spring. But you’d be missing an earlier spring-indicator if you ignored the annual mating song of the wood frog. That’s right; a frog.

Not to be confused with the more widely known spring peeper frog, which also heralds spring with its mating song, the slightly larger wood frog is generally found in wilder surroundings, deeper in the forest or in temporary pools along the flooded banks of mountain streams.

Casual observers would seldom find wood frog colonies if it weren’t for the calling males. At breeding pools, males inflate air sacs at the sides of the head to emit a call that most people who have heard it say sounds more like quacking ducks than the croaking we associate with frogs.

I remember hiking as a kid along the lower reaches of Dunbar Creek, just above its confluence with the Youghiogheny near Connellsville very early in spring. We heard what we thought was a flock of ducks just over a slight rise in the ground. We crept up to flush the ducks and surprised instead thousands of wood frogs gathered in the pool for their annual singing, mating and egg-laying.

From a global perspective, wood frogs in this region are “southerners.” The species lives in tundra and in spruce and hardwood forests, from north of the Arctic Circle in Alaska, across interior Canada to the Great Lakes and New England. Only the southern extreme of their range probes along the Appalachians from Pennsylvania to Georgia.

Unlike most frogs, when not congregated at breeding pools, wood frogs are at home on dry land. Their narrow, athletic trunk and powerful hind legs enable them to hunt flies, crickets, beetles and other insects in upland forests through the summer and early fall. But like all amphibians, wood frogs must return to water to breed and lay eggs.

“It is amazing how quickly large numbers of frogs are able to migrate to the breeding pool,” said Keith Berven, associate professor of Biology at Oakland University in Rochester MI, who has studied wood frogs from Maryland to Michigan every spring for 27 years. “It’s important that they get started mating early, so the eggs can hatch and the embryos complete their metamorphosis before the pool dries up.”

Berven says that if you want to see a wood frog, you had better find a spring breeding pool.

“The only time you will see them is during that brief period when they reproduce,” he said. “The rest of the time, this frog is out in the woods and undetectable. In my studies, I’ve had as many as 100,000 juveniles leave the pond within a couple of weeks. You would think you would have trouble avoiding stepping on them but they assimilate to the forest so well, you just do not see them, ever.”

Paying attention to conditions and being stealthy are the keys to finding wood frogs, according to Lindsey Noele Swierk, a doctoral candidate studying wood frog reproduction at Penn State University. “You have to be conscious of temperature and precipitation,” Swierk explained. “Right around the time of the first warm rain of the year, when the night temperatures are above freezing, you’ll want to take a walk in the woods where you’ve seen vernal ponds. Stop and listen for the wood frog chorus. But if you get too close, they’ll stop chorusing.”

Seen at the breeding pools, wood frogs are handsome amphibians. Gracefully built, each one sports a rakish dark stripe across the sides of the head through each eye. Their body color can range from rose-pink to nearly black, but most are generally tan-khaki to olive. The slightly larger females can exceed three inches in body length.

Despite their odd call and visual appeal, what scientists find most intriguing about wood frogs is their way of surviving winter. Wood frogs are one of only a very few vertebrates which are “freeze tolerant.” From Alaska to Georgia, when the first frost nips they wriggle into shallow solitary burrows and, in layman’s terms, “freeze.”

“They don’t exactly freeze solid, but about half of the water in their bodies can be frozen, and they’ll still survive,” Swierk said. “Freezing and thawing is highly damaging to most vertebrates because if living tissue freezes, cells can be ripped open by the ice crystals that form. Wood frogs use glucose as a ‘cryoprotectant’ that prevents tissue damage. They will also dehydrate their hearts and livers to limit damage by ice crystals. This explains how they can live so far north, where no other frogs survive, as long as their other needs are met.”

Temporary woodland pools, where uninformed ATV riders too often “play in the mud,” are vital to wood frogs and other forest amphibians.

“Riding motorized vehicles in the breeding habitat is clearly detrimental to the eggs and larvae,” Berven said.

“Because vernal ponds typically dry up over the summer, predators like fish do not live in them. Wood frogs evolved the use of these temporary pools to minimize the types of predators that could eat their offspring,” Swierk said. “Vernal ponds play a critical role in the maintenance of wood frog populations.”

Hearing a colony of wood frogs is a good sign of health of the local environment, Berven observed.

“They are a species that is associated with forested habitat,” he said. “If wood frogs are present, they are reflective of good woodland habitat.”

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