A marvel of nature roams the woods, usually silent but for a raucous spring display. Wood frogs, Rana sylvatica, which spent the winter frozen under leaves, have started their annual concert tour of Fairfax County Park Authority forests.
Wood frogs are the only frogs that can live north of the Arctic Circle. They can withstand more than half of their body water freezing without damaging their cells because urea accumulates in their tissues and glucose forms in their livers. This is natural antifreeze that also enables the frog’s heart to cease beating and the lungs to stop breathing. The wood frogs, buried under woodland leaves instead of overwintering on muddy pond bottoms like most other frogs, can thaw and refreeze throughout the winter without harm.
Last month, males began racing to their local hangouts — ponds and vernal pools — emitting a noisy, bizarre call that resembles a laughing cartoon duck to entice the females to join them. Wood frogs are not the first amphibians you’ll hear heralding spring in our area. That honor goes to the smaller spring peeper. However, wood frogs, brown with a black eye mask and about three inches in length, are more easily seen and heard.
Wood frogs commonly choose vernal pools as a breeding area because these temporary bodies of water do not host fish and turtles, which are natural predators of the frog eggs and tadpoles. At Hidden Oaks Nature Center is a pond that is purposely kept free of fish and bullfrogs to support breeding amphibians. Each year we welcome wood frogs, followed by American toads and then spotted salamanders, to breed.
Even with a suitable site, there are difficulties finding love in the pond. During the breeding frenzy, males may mistake a male for a female or may drown the female, especially problematic for the first females to arrive on the scene. Males emit a loud croak to warn off the advances of other confused males. When the females – the silent sex – arrive, the males clasp them with their forearms in an embrace called “amplexus”. The smaller male holds onto the female until she deposits her eggs (over 1,000 of them), which usually attach to submerged plants or other egg masses.
After just a couple of days, breeding is complete and the parent frogs return to the forest to hunt for insects, worms and arachnids. Tucked back into the forest floor, the adults ignore their offspring and leave behind standing pools soon to be filled with hundreds of tadpoles.
Meanwhile, the egg masses develop algae which provide more oxygen for the young. The eggs in the center of the mass, warmed by the other eggs, develop faster. The warmer the water gets, the speedier the development. Within weeks, the tadpoles wriggle out of their jelly-like egg masses and develop rapidly, growing their back legs first. They scrape algae and decaying plants with a beak-like mouth. As they mature, they are omnivorous and will eat other amphibian eggs and larvae, including other wood frog larvae. Overcrowding and low temperatures can be deadly at this stage, and many tadpoles become meals for another frog, a turtle, a salamander, beetles, leeches or even an owl. Ever full of surprises, the wood frog tadpoles actually seem to recognize siblings and congregate with them. Nevertheless, the sisters and brothers are together for only a few weeks.
By June, the froglets leave the pond, unaware that they’re joining their parents in the nearby woods. About one-fifth scatter, unfortunately finding their way onto roads and, without the important skill set developed in the video game Frogger, they get squished, providing food for birds, raccoons and opossums. By the time they settle into their homes in the woods, they are virtually indistinguishable from the leaf litter. They will spend the summer finding food and trying to avoid snakes, raccoons, skunks, coyotes, foxes and birds. Wood frogs in turn hunt for snails, worms, beetles, spiders, slugs and other arthropods. They are adept hunters that can ambush their prey. Staying silent until the end of winter, wood frogs’ cacophonous duck-like chatter will again pierce the March air next year.
You’ll find wood frogs from the southern Appalachians into the boreal forests of Canada and Alaska. In late February and into March, you can see them closer to home in our own park woods and waters.
Meet some of our area’s native amphibians and maybe catch a glimpse of American toad courtship at Hidden Oaks Nature Center as well at the county’s salute to Earth Day and Arbor Day, the April 27, 2013 Springfest at the Workhouse Arts Center in Lorton from 10 a.m. – 4 p.m.
Author Suzanne Holland is the assistant manager at Hidden Oaks Nature Center