Wild Neighbors—Amphibian Survivors: The Chorus Frogs

By Joe Eaton
Thursday October 02, 2008 - 09:56:00 AM

Contrary to popular belief, not all frogs go “ribbet.” Frogs make an amazing and appalling variety of noises. The eastern green frog sounds like someone smashing a banjo against the wall, hard: “Spunggg!” Some peep like baby chicks, trill like songbirds, bleat like sheep, grunt like pigs, skirl like bagpipes, moan like lost souls in torment. A mixed crowd of a dozen or so species, each carrying on in its own fashion, is an aurally stirring thing. 

The frogs that do say “ribbet” are native Californians: the chorus frogs. These creatures have been subject to a couple of fits of reclassification. All the western forms used to be considered one species, the Pacific chorus frog (Pseudacris regilla), formerly known as the Pacific tree frog. A couple of years ago the taxon was split into northern Pacific, Sierran, and Baja California species, almost identical in voice and appearance. Our local representative is the Sierran chorus frog (P. sierra), which ranges from southeastern Oregon east to Idaho and Montana and south through central California.  

I’ve seen Sierran chorus frogs around backyard swimming pools, in mountain meadows at 9000 feet, and along creeks in the redwoods. There are chorus frogs on Santa Cruz Island, twenty miles across the Santa Barbara Channel from the mainland (how they got there is a good question; maybe hitching a ride in a Chumash canoe) and in Death Valley. 

The Baja California chorus frog (P. hypochondriaca, and no, I don’t know why) is a ubiquitous soundtrack presence; when filmmakers want nocturnal atmosphere, they dub in a chorus of these frogs. It would be instructive to keep track of the countries and continents to which they’ve been cinematically introduced. Extraterrestrial planets, even. I’m pretty sure I’ve heard chorus frogs on one version of Star Trek or another. 

Not everybody hears the call as “ribbet”; one field guide gives it as “kreck-ek”, with a rising inflection on the last syllable. Another book likens it to the sound of a mechanical toy. Not all that impressive as a solo, but what’s remarkable is how a pondful of males performs in synchrony. Marin County naturalist Jules Evens describes “a myriad of voices repeating their creaking sounds together, seeming to encourage one another toward crescendo, then suddenly stopping simultaneously. How does the population perform in such unison? What cue prompts the sudden silence?”  

Chorus frogs are small, about 2 inches from nose to rump. They all have a black stripe through the eye and along the jaw, but beyond that color and pattern are wildly variable: shades of gray, brown, green, even brick red, with assorted of spots and blotches. Individual frogs can change to match their backgrounds, although not as effectively as chameleons. Once in Humboldt County I found a swarm of tiny frogs on a streambank, probably just out of the tadpole stage, all the same dark gray as the river cobbles. You couldn’t pick them out until they moved, one jump ahead of being stepped on. 

Frogs are not in good shape these days. There are alarming reports of worldwide population crashes and local extinctions, diseases and deformities. The California red-legged frog is on the endangered species list, and others may soon join it. So it’s heartening to be able to report that the chorus frogs doing just fine. 

Adaptability may be one of their assets. All they need is someplace suitably wet to gather when winter rains trigger the mating frenzy: a temporary pool, reservoir, stream, garden pond, water hazard or roadside ditch. One East Bay homeowner’s backyard pond was so popular with Sierran chorus frogs that the neighbors complained about the racket.  

Chorus frogs also appear to be resistant to one of the threats implicated in the decline of their relatives: a harmful form of ultraviolet radiation called UV-B. The thinning of the ozone layer has allowed more of this stuff to reach the earth’s surface. UV-B can cause skin cancer and cataracts in humans and limit the growth of plankton in polar seas. And research by Andrew Blaustein of Oregon State University shows that it messes up the DNA of frogs’ eggs. 

Some frogs, that is, but not chorus frogs. In field studies at mountain lakes, Blaustein shielded batches of frog and toad eggs from UV-B radiation while leaving others exposed. Almost half of the unprotected eggs of Cascade frogs and western toads died, but the northern Pacific chorus frog eggs came through unscathed. This reinforced lab results indicating chorus frog cells were better at producing enzymes that repaired radiation damage than those of the other two species.  

With each female laying up to 600 eggs, we could be knee-deep in little frogs if predators didn’t take a heavy toll of the tadpoles. But enough make it to adulthood to keep the chorus going,