Focus on Nature

FOCUS NATURE THIS FROGGY GARGLES WHEN IT GOES COURTIN'

Frog sleuth Jim Platz is standing beside a small pond at The Nature Conservancy's Ramsey Canyon Preserve in the Huachuca Mountains near Sierra Vista. Judged on appearance alone, it seems an unlikely place to look for a new species of frog.

Daily, birders from around the globe hike a well-worn path into Ramsey Canyon, passing within a few feet of frogs sunning on the pond's concrete-lined banks.

As bird-watchers stream past, Platz explains why southeast Arizona's isolated wet canyons are best bets to search for new frog species, or new anything.

Platz should know. He teaches herpetology at a college in the Midwest, and, as a graduate student at Arizona State University in the early '70s, he described two new leopard-frog species in Arizona's streams and canyons. Eight inches from head to toe, the specimen Preserve Manager Tom Wood now holds in his fist is number three.

The reason Platz hunts where he does goes back to the Pleistocene epoch. At the end of the last ice age 17,000 years ago, southeast Arizona was a cooler, wetter environment. The San Pedro River probably ran deeper and wider then, good habitat for a variety of amphibians. As the climate warmed and riparian habitat dwindled, animals retreated into wet canyons of the Chiricahuas, Dragoons, and Huachucas.

Time passed. Intermountain zones became desert. The mountains themselves grew increasingly isolated. Blocked by hot deserts from migrating to nearby mountains, animals that were kin branched off genetically.

"That's it in a nutshell," says Platz. "Physical isolation provides a natural theater for evolution of new species. Animals develop differences in behavior, breeding season, or ecological requirements, so that they're just not interested in each other if brought together."

The Ramsey Canyon leopard frog evolved an extraordinary behavior: it croaks underwater.

For Platz, hydrophonic recordings of these calls are a kick. And the sound is astonishing: a long, gravelly snore, huskier in the big frogs and Platz's computergenerated charts graphically differentiate big-frog basses from little-frog tenors.

Platz inferred the submarine frog croaking when he discovered new egg masses one morning but had heard no mating calls the night before. When his speculations were confirmed by sound equipment, he jumped for joy. Then came the questions: why call underwater? What does it mean? How does it contribute to overall fitness of the individuals within the species?

Platz offers three guesses. One: by calling underwater, the Ramsey Canyon frogs trick sharp-eared, night-hunting predators. What you can't hear, you can't catch.

Hypothesis two: back in the late Pleistocene, air temperatures became too nippy after sunset for cold-blooded amphibians to stay active and call into the night. Water, on the other hand, stays relatively warm. What if the frogs called underwater? Well, that would mean earlier breeding, more time for tadpoles to grow to maturity, and a better survival ratio among offspring.

Three: Ramsey Canyon frogs may once have lived in a larger body of water, like the San Pedro, along with many other species of frogs. Imagine all those frogs croaking into the dark at the same time. Confusion reigns! What if you could use another medium, water, say, to broadcast sound? Water's a better sound conductor than air, anyway. With water, you would have a clear channel, no static interference from other croakers. End of confusion!

Plenty of study lies ahead. Different frog calls mean different things, Platz figures, so he'll need to match sound with behavior.

Then there's this puzzle: no bubbles rise to the surface when these frogs croak. Where does the air that produces the sound go? Do they swallow it and reuse it? How long can a male frog croak underwater before coming up for air?Frog sleuthing. Nice work if you can get it.