As dusk falls over a swampy pond, the chorus begins. First one frog croaks, a little hesitantly. Soon another joins in. Suddenly the pond resonates with the voices of dozens of male frogs, each signaling his species, availability and qualifications as a father. Two populations of the same species, separated geographicallya highway running through a swamp, for exampledevelop dialects, slight differences in their calls. In most species, only males call, though the female midwife toad outshouts her mate. A sudden foreign sound silences them one and all. But soon a single voice starts again, followed by another and another.
The sound of a pond full of frogs can reach deafening levels, far out of proportion to the tiny bodies producing the sound. An air sac on the floor of the frog’s mouth enables it to do two remarkable things. First, when it’s expanded, the sac acts as a resonator, like the hollow body of a violin. Second, by forcing air into the sac from the lungs, then back into the lungs, a frog can croak continuously, even under water. The loudest frogs breed in temporary ponds. When the water’s available, the males muster mates quickly, for all frogs lay eggs only in water, and the tadpoles must grow legs before the temporary ponds dry up.
But for sheer cacophony, the year-round singing of tropical frogs beats all. In the tropical rain forest, one of the richest ecosystems on earth, a swamp houses many frog species. Males would gain nothing by attracting and attempting to mate with females of another species, so they have to make their signals cut through the din.
The male Leptodactylus ocellatus, which lives in South America, calls at about 250 to 500 Hz (roughly an octave, from middle C to C above middle C). His volume is no match, however, for a neighboring species, which overlaps his frequency range and can croak 40 decibels louder. That’s like the difference between a conversational tone and the noise on a factory floor. L. ocellatus, however, has evolved a way to circumvent this potential problemby croaking underwater. Because sound travels well underwater, but doesn’t cross the interface into the air, L. ocellatus avoids sonic competition without changing his preferred octave. In a rich environment, species divide the environment up into tinier and tinier slices, each becoming a specialist.
Singing underwater represents an extreme method of dividing the “airwaves.” Most frogs croak distinctively by varying the pitch of their calls, or by varying the pattern of croaks in a song. Even when they can distinguish the call of males of their own species, though, female frogs must determine the direction from which it comes. This presents a problem.
Humans and other mammals can determine the direction of a sound because their brains detect differences in loudness and time of arrival at each of their two ears. For this system to work, the wavelength of the sound must be much smaller than the distance between the two ears. The longer the wavelength, the harder it is to tell where the sound came from. The crest of a long sound wave may hit the near ear only a tiny fraction of a second before it hits the far ear. A comparatively long time later, the next crest reaches the ears. This is why some stereo systems contain only a single low-frequency woofer, which the listener can place anywhere in the room. By contrast, the high-frequency tweeters and midrange speakers must be separated and properly placed to give the stereo illusion of an orchestra in the room.
Humans hear only up to 20,000 Hz, and the highest note on the piano is only a little above 4,000 Hz. But many mammals hear extremely high frequencies, up to 60,000 Hz, quite well. These sound waves are much shorter than even the tiniest mammal heads. Frogs’ high frequency hearing is not as sharp. They top out at 10,000 Hz, making the distance between their eardrums too short to localize the highest pitches they hear. So how do females find their mate? Scientists studying the coqui frog of Puerto Rico, Eleutherodactylus coqui, think they have a lead.
One way frogs can localize sound is by hearing a sound twice in the same ear. Because of the frog’s head anatomy, sound can travel from the middle ear down the Eustachian tube, across the frog’s mouth cavity, up the other ear’s Eustachian tube and into the middle ear. Sound can then affect the near ear of a frog, for example, twice. First when the sound strikes the eardrum, and second when it arrives a bit later by way of the mouth and Eustachian tubes from the far ear. These two sounds may arrive out of phase, with peaks and troughs interacting in a way that gives the frog’s brain a clue as to where the sound came from. This pathway doesn’t affect mammals because our mouth and ear anatomy differ from that of frogs.
By mapping vibration in the frog’s body with lasers, the scientists have found that a particular spot over the lungs of some species vibrates as well as the eardrum. They propose that the body wall here acts like a large eardrum, providing yet another pathway for sound to travel to the eardrumfrom lung to mouth to Eustachian tube to middle ear. This may further increase the precision with which female frogs find their princes.