COLUMBUS, Ohio -- An earless species of Panamanian frog hears by picking up sound waves through its lungs, according to a new Ohio State University study.
The finding may shed light on the evolution of hearing systems in early vertebrates.
The Panamanian golden frog hasn't come under study much before now, because it lives only along remote mountain streams. The species is endangered, partly because it's very choosy about its habitat. But a bigger threat comes from frog collectors, who prize the species for its brilliant yellow color.
The frog lacks both a middle and external ear, but it does possess a typical amphibian inner ear. Despite being earless, the frogs call out to each other and respond to sound, a fact that has puzzled zoologists.
To gauge the frog's hearing ability, Ohio State researchers set up speakers in the wild, and played recordings of frog calls. The frogs turned to face the speakers and called out in reply.
"Not only can they hear, but they can also localize sounds, and all this without a middle ear," said Thomas Hetherington, professor of zoology, who is collaborating with Erik Lindquist, a doctoral student, on this study.
The researchers think the frog's lungs have taken on a second role, as ear substitutes. The lungs lie very close to the surface, just beneath the frog's skin. When sound waves hit the frog's side, the lungs vibrate.
"In a sense, the frog's lungs act as eardrums," said Hetherington. "But we still don't know how the vibrations get from the lungs to the inner ear. That's the rest of the story."
In tests, the frogs responded to sound waves that bounced off their lungs, but didn't hear as well when the researchers set up barriers that blocked sound waves from reaching that area.
The study, part of which appeared in a recent issue of the Journal of Herpetology, also found that when taking an aggressive stance, the frogs supplement audio communication with visual -- they wave their forearms at foes in a kind of semaphore signal.
The researchers found that the males of the species will wave at their own reflections in a mirror, as well as at a miniature flag that flashes the frog's characteristic yellow color. Even during the speaker experiments, some male frogs waved at the speakers as if the sound indicated a rival's presence.
This study proved that the frogs employ the wave to relay a specific message. They gesture with whichever forearm is closest to the recipient, in a move that resembles the beginnings of a step forward in attack. "The frogs wave in the same way a bull makes a fake charge. It seems to be an aggressive signal that says 'stay away,'" said Hetherington.
The signal comes in handy. Solitary male golden frogs claim territory along a mountain stream, then wait for fertile females to arrive and lay eggs in the water. In the meantime, the males ward off potential rivals with the wave.
When two males come face-to-face, they call out and wave to each other. If one doesn't back down, they meet in a ritualistic battle of intimidation. The frogs don't hurt each other, but rather attempt to assert their dominance in a waving contest.
One frog jumps on top of the other and starts waving, and the subordinate frog waves back. Only when one frog stops waving and admits defeat does the fight end.
"Most people tend to think that frogs are pretty simplistic," said Hetherington. "But when we looked at them closely, we found them engaged in a long series of very subtle behaviors."
For example, males will wave away females that don't appear ready to mate, but will permit fertile females to enter their territory unhindered.
Fertile females turn the tables, however, by waving aggressively at the males. If a male approaches despite her warning, the female may mate with him. If he appears discouraged by the wave, she won't. "The female may be testing the resolve of the male," said Hetherington.
How did this behavior evolve? Only a few other frog species use hand signals, and they live as the Panamanian golden frogs do, along torrential mountain streams where the sound of flowing water drowns out nearly everything else. "It may be that in noisy habitats, visual signaling is just more reliable," said Hetherington.
Hetherington thinks the frog's method of communication is nothing new. "Perhaps the oldest way of hearing in vertebrates was using the lung as a sound transducing organ," he said.
In fact, many fishes today use an air bladder, a lung-like organ specialized for sound reception. Under water, sound readily penetrates to the air bladder, and passes into a fish's inner ear.
"These frogs may be using the very primitive system that their distant fish ancestors used, only on land," said Hetherington.
Hetherington and his colleagues want to return to Panama and continue their studies of the frog, to discover exactly how the lungs send signals to the inner ear. They also want to find out whether the calls or the hand-waves are more important to the frog's system of communication.
Contact: Thomas Hetherington, (614) 292-0832; Hetherington.1@osu.edu
Written by Pam Frost, (614) 292-9475; Frost.18@osu.edu
Editor's Note: Photos and video of the Panamanian golden frog are available on request by bona fide news media by contacting University Communications at (614) 292-9475.