Spring Peepers, Wood Frogs, and a Spring Serenade

How Wood Frogs Freeze Their Bodies

Every year, the wood frog comes back from the dead. Not in the sense that it appears again after a long absence, but in the most literal one: an inanimate, mostly frozen object, which has ceased to function, is suddenly resuscitated into being.

How does a wood frog make it through a frigid northern winter? No fur like the bear, no feathers like the owl, no warm blood of the squirrel, cozy in its nest. No burrow deep in the mud, where the box turtle’s pulse slows to one beat every few minutes. And no shivering like the man on his spring log, listening to the wood frogs wake up. The frogs haven’t shivered all winter: their defense against the cold is having no defenses.

Wood frogs (Lithobates sylvaticus) are native to much of the northeastern U.S. and most of Canada, west to Alaska. They endure sub-zero temperatures by going sub-zero themselves, shutting down almost all bodily functions.

More than half of the frog’s blood and other liquids may turn to solid ice for weeks at a time. The frog’s urea acts as a kind of antifreeze to prevent permanent damage to body structures. By raising levels of glucose more than a hundredfold, individual cells keep themselves from breaking down internally, each one a kind of pilot light within a cold stove.

As the weather warms, the stove warms, too. Just a couple weeks after the thawing of the ponds, the edges get loud with a chorus of newly awakened voices. The sound of many wood frogs singing at once is like small-town gossip passed through a synthesizer: all the tones of articulate expression are there, but without the words. Thoreau called the wood frog “the very voice of the weather”:

The weather, what is it but the temperament of the earth? and he is wholly of the earth, sensitive as its skin in which he lives and of which he is a part.
(March 24, 1859)

Listen to the Wood Frog Calling!

Credit: Michigan Wildlife

The Sound of Spring Peepers

Spring peepers (Pseudacris crucifer) are one of the first species to begin calling in the spring, and among the earliest amphibians to arrive in the woodland ponds to breed.

They spent the winter buried under soil—a natural antifreeze in their blood keeping them thawed.

This small, slender frog can appear pink, gray, tan, or brown, with a dark ‘X’ on its back. Only one-inch long, they breed in ephemeral swampy ponds and pools in woodlands.

Males fertilize eggs as females lay them. Eggs attach to vegetation in shallow water, and hatch in three-to-four days. Tadpoles metamorphosize two months later.

By contrast to the wood frog’s, a peeper’s voice is a shrill, piercing alarm clock that you can’t set back to snooze.

Males can produce a clear note of well over 100 decibels—at close range, not too different in sound or intensity from a fire alarm going off in your kitchen! Multiply this by the hundreds or even thousands of individuals calling in a single marsh, and lo and behold, your woods have become about as soothing as the inside of a jet engine. The air is shredded. It breaks on all sides with the force of a riled-up ocean wave.

Confronted with such a sound, we can keep our distance, or put our hands over our ears. But how does a peeper, right in the thick of it, avoid blowing out its eardrums? The tympanum, the membrane that transmits sound to the frog’s inner ear, is thin enough to rupture, but it’s actually anchored by tissue to the animal’s lungs. Scientists have suggested that the deafening vibrations of the marsh are diffused throughout the frog’s body, the way a building funnels the force of a strong wind into its foundations. At 3.5 grams, a frog is fragile architecture, but it holds up. Its whole body is quaking. The air inside its lungs is already turbulent, even before it gets pumped out into sound. Each frog’s call is ashiver with the riot of his neighbor’s.

Listen to the Song of the Spring Peeper!

Credit: Kelsey Frey

The Frog Chorus

The term “frog chorus” has ancient, literal origins. The playwright Aristophanes won first prize at the festival of Dionysus in 405 B.C., for his Frogs, a comedy in which Dionysus himself appears on stage. The god is trying to make his way across Lake Acheron to pay a visit to the Underworld. His purpose is to bring back from the dead the playwright Euripides, who, as the best of the tragic playwrights, will bring glory to him, patron god of the theater. As Dionysus goes, he is mocked by a raucous army of frogs, which sing:

Brekekekéx-koáx-koáx
Brekekekéx-koáx-koáx

Who knows something about death? Who knows how to come back and tell us about it? Who knows how to fill their lungs with the temperament of the earth and express it? Stop rowing—look no further. After just a few verses, Dionysus finds himself singing along:

Brekekekéx-koáx-koáx
Brekekekéx-koáx-koáx

Listen to a Frog Chorus! 

Spring and Summer in 22 seconds! In this animation, eight common species of frogs (including a toad!) give their calls through the season. Wood frog, boreal chorus frog, spring peeper, northern leopard frog, American toad, gray treefrog, green frog, and bullfrog. See if you can identify each species as it joins the chorus.

Credit: Cable Natural History Museum

Do you enjoy listening to calls of frogs in the spring? Please share your experiences or questions below!

And if you enjoy the natural world, take a listen to our bird sounds!