Frogs can hear without ears says Chile RPCV Peter Narins
Frogs can hear without ears
Q: What structures does the frog hear with? How do they work? Is there any other animal that hears the same way? I am doing a project on the physiology of frogs' hearing.
Jon Glase, Cornell University
Label 3 points to the frog's eardrum. (1 is the external nares or nostrils, 2 the floor of the oral cavity.)
A: It's true frogs do not have outside ears that direct sound inward to the ear drum as ours do. But they do have an ear drum of sorts (see figure), an inner ear, a brain, and most frogs have a middle ear. They hear with these structures and one more-their lungs!
The ear structures function much like ours. The eardrum of most frogs is a membrane surrounded by a cartilage ring. Sound waves vibrate the eardrum, which wiggles a rod connected to the eardrum, which sloshes fluid in the inner ear, which waves hairs in hair cells. The hair cells contact nerve fibers, which generate electrical pulses. Nerve fibers carry the signals to the brain, which interprets the nerve signals as sound. That's how the ear structures work.
The lungs are a different story. Suppose a tree falls in the night. And suppose a frog squats within hearing range of the noise. Then, just as we discussed, his eardrums vibrate in response to the noise. But there's more: his lungs do, too. Indeed, his lungs are only slightly less sensitive than his eardrums.
In 1988 Peter Narins, a physiological science professor at the University of California at Los Angeles, and his colleagues from Germany found that a frog has an unbroken air link from the lungs to the eardrums. Narins thinks this link serves two purposes: to help the frog locate sound and to possibly protect its ears from its own raucous calls.
Suppose the falling tree is directly to the left of the frog. When the tree-fall sound reaches the frog's left ear, it also reaches his left lung and this causes a pressure difference across the left eardrum. The pressure difference is different for the right ear since the sound must travel farther to get there. The frog can sense direction by this difference.
Locating a sound is important to frogs. A female frog locates her mate by the direction of his booming calls. Similarly, a male respects another male's territory by not moving too far in the call direction.
Frog calls are extremely loud. The forested areas of Puerto Rico are dense with male coqui frogs: one every ten square meters. So each male stridently calls his loudest to drown out the others and attract a distant female. If you wander within a half a meter of one of the little creatures, you hear a croak near the pain threshold: between 90 and 95 decibels-almost as loud as a jack hammer (100 dB).
The frog's lungs protect his ears by equalizing pressures between the inner and outer surfaces of the eardrum. The eardrum does vibrate in response to his own call but with a very small amplitude.
You ask if any other animal uses its lungs to hear. Many fish do. They hear with an lung-like air bladder, specialized for sound reception. Sound travels underwater to the air bladder, vibrates the air sac, which, in turn, vibrates the fish's inner ear. Frogs may be using the same system as their ancient ancestors, the fish, do.
(Answered by April Holladay, science correspondent, April 25, 2001)
