OLYMPIA — As the last robin's call stills in the twilight, and a waxing spring moon rises, this pond, silent moments ago, erupts.

A bold, solo RIB-IT is answered with an equally emphatic WRECK-IT, then another, and another. In seconds, the Pacific Treefrogs are in full chorus.

Also called the Pacific chorus frog, Hyla regilla is Washington state's loudest and smallest frog. At just about 1-1/2 inches long from snout to rump, it would fit on a saltine with room to spare. It weighs about as much as a Hershey's Kiss.

Native to Washington, chorus frogs are also the state's most abundant, thriving on both sides of the Cascades.

Their song, vernal, primal and just plain loud, can drown out traffic. Stand a half-mile away: You can still hear them. Stand close: You practically feel it.

These guys — and it is only the guys that sing — can hit 90 decibels at a half-meter away, a veritable amphibian rock band. Biologists say frog calling is among the most energy-expending activity of any vertebrate. Chorusing is an Olympic performance that goes on hour after hour, night after night, during the spring breeding season.

So consumed are the males with their calling that a female frog must actually bump a male with her snout to let him know: I'm here. Give it a rest. Get on with it. Breeding, that is. And it is always the females that choose.

The males with the equivalent of a red Corvette convertible that get all the girls are those that can really let it rip.

"Loudest is good. That is a sign of how big they are. With some females, it is also how long a male can call without stopping," said Eliot Brenowitz, a professor of psychology and zoology at the University of Washington.

Size indeed matters: Louder frogs tend to be huskier males with tiptop genes the female can pass on to her young. So the females listen up before making their selection.

As soon as a female makes physical contact, the male scrambles on her back — she is about half again his size — and wraps his front arms around her and holds on as she glides off with him to deeper water.

They stay locked together in that position, called amplexus, for hours underwater, breathing through their skins. No one quite knows why.

But in time the female finds some soft vegetation, a tuft of rushes, a bit of grass, perhaps, and extrudes packets of jellied eggs.

As she lays her eggs, the male frog, still with a death grip on her back, deposits his sperm over them, a process that can take hours.

A single female will lay several jellied packets of eggs, with about 25 eggs in each. Nearly all will hatch out to tadpoles within three weeks, but only about 1 percent of the tasty, wiggly polliwogs will survive their many predators to metamorphose into froglets.

Starting today: Meet the bug, rock or plant next door From wild ocean shores to glaciated mountain spires, sagebrush steppe and the undulating hills of the Palouse, Washington state is a feast for the senses, and the spirit. In a series beginning today in The Seattle Times and on seattletimes.com, we'll crisscross the state to deliver a deeper understanding of the denizens and features of our natural environment. With the help of dive boats and dry suits, with ice ax, pack horse, waders and headlamp, we'll profile the birds, bugs, plants, animals and geologic forms that give Washington its unique sense of place. And we'll share surprises we found right in our back yard. The series will explore and explain the arcane, the marvelous, the mysterious and even the ordinary — from why skunk cabbage stinks to the awesome force that created the granite-faced Stewart Range. And you can start your exploration right now by listening to today's subject, the Pacific Treefrog: · MP3 file (192K) · Real Audio file (196K)

Once her eggs are laid, the female leaves the pond, leaving her brood to their fate. The male gets right back to calling, hoping for another mate, even the same night if it's still early.

By June, the fishlike, vegetarian tadpoles change into hopping, carnivorous, terrestrial froglets the size of a pea that can breathe with their mouth, lungs and skin. If very lucky, they might live as long as three years, but that's old for a chorus frog. Two is more likely.

Exquisitely made, with tiny toe pads that allow them to grip and climb and a flashy bandit mask of black across their eyes, they make up in beauty and poetic expression what they lack in stature. Their tiny faces have a certain noble calm about them.

Chorus frogs may be green or brown or mottled, even red or white, depending on their habitat. A single frog may also lighten or darken its color. Some scientists believe they can do this at will, as camouflage to avoid being eaten.

Northwest ponds are loudest with chorus frogs in early spring. But some chorusing can begin in January in the lowlands and continue into June at higher elevations.

Chorus frogs can use just about any still-water wetland for habitat; even stormwater ponds will do. In fact, a pond that dries up by June is preferred: ephemeral pools don't have fish, a deadly predator of chorus frogs.

The most abundant and diverse populations of amphibians are found in ponds closest to intact forestlands. After all, "amphibian" means "dual life." Chorus frogs seek out ponds only at breeding time.

In winter, they hunker down in rock and log crevices and other protected spots, not moving or eating much.

Come spring and summer, they hide in grass, trees and low shrubby growth and stalk beetles, flies, spiders, ants and other small bugs they can nail with their tiny, sticky tongues.

Many a gardener has been startled from a daydream by the loud, languid "krrr-eck" of a chorus frog tucked inside a nearby plant.

That's a different, far more sedate call than the insistent, repeated RIB-IT! or WRECK-IT! called during breeding season. Called the "advertisement call" by biologists, it has but one translation: Come hither.

Ponds will rock on for minutes, then come to a seemingly inexplicable, sudden dead silence. The approach of a predator — or a person — will silence a pond, but not for long. Soon a lone voice will start up again, quickly joined by others.

Some scientists have speculated the chorus is led and silenced by a designated chorus-master. But the rhythms of the frogs' chorus remain a mystery.

Chorusing may begin as early as late afternoon and continue into the wee hours of the morning. Cold temperatures can silence a pond; chorus frogs usually won't call if the mercury sinks below 41 degrees Fahrenheit. The warmer the day, the earlier and more urgent the chorus. A spring rain will also put the guys in top form.

But why bother? Not every species of frog, after all, sings at the top of its lungs for a mate.

Competition, pure and simple, says Brenowitz of the UW. Because the chorus frog breeds communally, the males need a way to distinguish themselves from the next guy.

"You had better be prepared to belt it out," Brenowitz says.

Lynda V. Mapes can be reached at 206-464-2736 or lmapes@seattletimes.com.