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A pre-Saint Patrick’s Day musical treat: bring on the wood frogs

March 21, 2012

Photos by Bruce Fellman

A single wood frog.

The first wood frogs—black-masked amphibians whose calls sound more like the mutterings of annoyed mallards than anything Kermit might utter—came out of suspended animation (we’ll talk about that a bit later) on Saturday, March 3, at three in the afternoon. This was actually pretty early for the members of the species, for, according to a tradition established by yours truly, wood frogs don’t usually wake up from their winter survival mode—something the casual observer would call, well, death—until St. Patrick’s Day.

Perhaps the frogs are brought back to life by the whine of the fiddle and the thump of the bodhran, as Irish musicians honor the patron saint of the Emerald Isle. Maybe the amphibians have heard the words about Patrick’s supposed heroism in freeing Ireland from all those deadly snakes that ruled the land until he drove them all into oblivion, and decided that the time was right to get down to the business of renewing life—just in case the saint returned with a notion to also do in the frogs. (And yes, it’s geologically and biologically true that in Patrick’s time, there were no snakes, deadly or benign, in Ireland, but a legend is a legend, so, in the spirit of “you never know,” a frog just might have cause for concern.) It’s even possible, in that same spirit, that the real wakeup call comes from pints of Green Guinness, which are dutifully poured by legions of meandering leprechauns into every vernal pool on the ridge and elsewhere.

Possible, to be sure, but not very likely.

What actually wakes the wood frogs is a combination of a long enough stretch of temperatures in the 40s and 50s and a good soaking rain. A strong thunderstorm does an even better alarm clock job.

Forget the legendary miracles wrought by St. Paddy—wood frogs perform real miracles. These amphibians have a huge geographic range, from the southern Appalachians all the way to the northern edge of the boreal forests of Canada and Alaska. And since they don’t have the means to migrate away from winter, they’ve had to evolve a way to cope with the cold.

Some amphibian species simply lay low at the bottom of ponds. Others dig or adopt someone else’s lair below the frostline. But the wood frog does something even more remarkable—something that, the first time I reported on it in the early 1980s, I found almost unbelievable.

Wood frogs freeze solid. When the cold descends in November, members of the Rana sylvatica crew burrow a few inches under the leaf litter and then, as soon as the ground starts to freeze, the animals stop breathing, stop all brain function, and stop their hearts. The body turns into a frogsicle, an amphibian ice cube, and if you happened to find one during the winter, your response might be, “Oh, the poor thing.”

Don’t weep for the wood frog. Come the right conditions in March, biology’s brand of resurrection occurs. In response to an as-yet unknown signal, the animal’s heart begins to beat, its vital internal organs warm, the frozen tissues thaw, and, within a day, the wood frog is as good as new. (You can watch the entire process at www.youtube.com/watch?v=Fjr3A_kfspM.)

In our area, the proper re-frogging meteorology tends to occur around the middle of March. Hence, the St. Patrick’s Day tie-in. But it varies widely, and over the nearly 30 years I’ve been monitoring this ridge’s vernal pools, I’ve heard wood frogs begin to call as early as the end of February and as late as the second week of April.

While this year’s March 3 chorus start date is certainly noteworthy for earliness, it’s hardly surprising, given the lack of winter cold and snow. During the numerous thaws we had, I more than half-expected the wood frogs to start calling in January. I was happy to learn, from the lack of sounds in the vernal pools, that the amphibians had the good sense to hold to vows of silence until the proper time approached.

“Every year is a new experiment,” my mentor Bob Shoop, who was then a URI herpetologist, told me when I interviewed him for one of my first feature-length science stories. When it comes to words of wisdom about natural history, this may well be my favorite quote. In a handful of words, Bob told me just about everything I needed to know about the way the natural world works and about the challenges that evolution has had to overcome.

The challenge in 2012 is the non-winter and the uncertain approach of spring, which, according to the calendar, arrives on the 20th. The wood frogs decided to experiment by starting the mating season, which is what spring is all about, after all, a couple of weeks early. But no sooner did the experiment begin than the amphibians called a halt to their singing. The temperature dropped, there were a few flakes of snow in the air, and the amphibian chorus ceased, the attempt to impress the females with vocal quality over almost as soon as it began.

A week later, after things grew warmer and wetter, the wood frogs took up singing once more—until the return of the cold. With Jupiter and Venus converging in the western sky and Mars dominating the east, the frogs again became silent. I shivered, watching the heavenly show. Perhaps the frogs watched as well, this time, from under the water, which would protect the amphibians from frostbite.

Fortunately, evolution had prepared them with all the strategies to successfully deal with nature’s constant experimentation. When the warmth stormed back on the 12th, the wood frogs were ready to make the ridge ring, well, mutter, with their calls.

 

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