Feb 28, 2011

Quacking Frogs Get a Jump on Spring

I've always been an avid frog watcher. Even before the Spring Peepers’ chorus heralded the arrival of spring, I’d pull on my boots and go to still-icy pools to listen for the sound of Lithobates sylvaticus, the Wood Frog, kicking off the annual frog-breeding cycle.

Back then, this frog's scientific name was Rana sylvatica, but scientists recently decided figured out that it really belonged to the Lithobates genus. Of course, I had no clue about scientific names back then, so they were just Wood Frogs to me.
A young Wood Frog (Lithobates sylvaticus

Photo by MichaelZahniser

L. sylvaticus, a forest dweller, is the widest-ranging frog species in North America and appears the farthest north—all the way to the Alaskan arctic. Further south, it prefers higher, cooler elevations. In Virginia, it ranges from the upper Piedmont throughout the Mountain region, but also occurs in the northern part of the Coastal Plain.

During most of the winter, Wood Frogs hibernate under rocks, logs, or detritus in wooded areas. Their emergence from hibernation is triggered by warm spells that, in Virginia, can occur as early as January. At that time theys mass in shallow, short-lived pools for a frenzy of breeding that lasts only a few days. These vernal pools are formed from rains and thaws in winter and spring and dry up as temperatures rise and rains disappear.

How does the Wood Frog manage to survive breeding in the cold of winter? As author Elizabeth Colburn explains in Vernal Pool: Natural History and Conservation, “Within five minutes of the start of freezing, Wood Frogs accumulate high levels of glucose in the liver and leg muscles, subsequently releasing the glucose into the blood and other tissues, where it functions as an antifreeze.”
While other frog species can pull off the antifreeze trick in anticipation of winter, only L. sylvaticus can adjust to changes in the immediate temperature conditions. It can survive freezing for up for four weeks and is back to normal within hours of thawing out.

Medium-sized (1.5 to 3.25 inches), the Wood Frog is easy to distinguish from other Virgina frogs because of its black mask, which runs from each eye to above the foreleg. The rest of its body color ranges from pink to black.

The Wood Frog’s distinctive clacking call—likened to the “feeding call of the mallard” on the Minnesota Herpetological Society’s website—has earned it the nickname of “quacking frog.” The Handbook of Frogs and Toads of the United States and Canada notes that Wood Frogs give two, four, or six short notes in rapid succession that are “high and grating in character.” (Click here to hear the call.)
A pair of Wood Frogs mating
(pulled out of the water for the photo)
Photo by Cephas
Male Wood Frogs are so frenzied in their breeding that they actually have a secondary call to let males who have trapped them in a lusty embrace know that the object of their desire isn’t a female. The clacking can go on day and night when temperatures are right but doesn’t carry far.

In northeastern forests, wood frogs are numerous, with almost 5,000 recorded at just one vernal pool in Massachusetts, according to Colburn. During mating sessions, they can turn breeding pools black in an orgy of procreation. 

Each Wood Frog is extremely loyal to its natal pool (where it was born), with up to 85 percent returning to their natal pool to breed, bypassing other pools along the way. The remaining percentage disperses to nonnatal pools, ensuring genetic diversity. While the dispersing males may plop down in any shallow pool they find, females are pickier, refusing to breed in any pool with fish, the main predator of frog eggs.

A Wood Frog tadpole morphing into an adult 
Photo by Brian Gratwicke

Each female produces 1,000 to 3,000 eggs. Barring water that is overly acidic, dries up. or refreezes, or predation, the eggs’ rate of success in hatching out is high—80 to 96 percent, according to Colburn. Tadpoles don’t fare so well, with less than 4 percent usually surviving to adulthood. Wood Frogs prefer to breed in pools that appear very early and disappear fast, so they have to grow fast, becoming adults within 50 days.

Once the breeding is over, the frogs go silent and return to their small terrestrial home ranges (500 to 700 square feet) on the moist forest floor 1,000 or more feet uphill from their breeding areas. The breeding migration can be dangerous, especially across areas that have been cleared of forest—roads in particular.

Wood Frogs are voracious eaters, especially in the far north, where insects are around only briefly in the short summer. They chow down on ground beetles, crickets, bugs, caterpillars, other small insects, earthworms, snails, and spiders, and in turn are eaten by hawks, wading birds, snakes, and turtles, among other predators.

On a recent warm day after a rain and thaw, I donned my boots and headed for the wetlands along the Rappahannock River. I was disappointed by the silence there, but the winter has been unusually dry and cold, and no one else in my conservation network had reported hearing the distinctive clacking yet this year, either. However, with the recent warm spell and rain, I'm now hearing from my contacts that the quacking frog is back. With the late start and the short time they breed, there has to be a real frenzy of the little guys getting their groove on now.

Want to Help Our Frogs?
Amphibians of all kinds are under siege from changing climate and environmental degradation. A fun way to help monitor their health is to volunteer for FrogWatch USA. You just need an area with frogs that you can visit at least a couple of times a week from now through August. It only takes a few minutes to do the monitoring by ear and then enter the results into the Frogwatch online database. With only a few species in the Piedmont and western Virginia, it’s easy to learn all the calls.

The Virginia Herpetological Society’s website has sound recordings of native frog calls and lots of other good information, and the Virginia Department of Game and Inland Fisheries website has a breeding chart showing where and when each species breeds. VDGIF also offers a CD, Calls of Virginia Frogs and Toads, for $5.

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