Frogs Use Eardrums to Amplify Calls
Posted February 1, 1998
"Nee-deep, nee-deep." "Chug-a-rum." Frog calls such as these have long been thought to be produced by the frogs' vocal cords and amplified by the large vocal
sacs underneath their jaws. But recent studies show that the American bullfrog uses its eardrums rather than its vocal sac to amplify the sound that originates in the vocal cords. Why
do frogs need to amplify their calls? Researchers think male frogs amplify their calls to attract female frogs for mating.
The use of eardrums to amplify sounds was discovered when researchers were investigating exactly how frogs use their vocal sacs. Alejandro Purgue, who studies animal
acoustics at the University of California at Los Angeles (UCLA), designed a sound generating device that could be placed into a frog's mouth. Purgue then turned on the device and measured
how different body parts amplified the sounds it produced. The vocal sac did pick up the vibrations, but to his surprise, the eardrums amplified the sounds by 70 to 80 percent! He now
hypothesizes that the vocal sac is actually used to store the air used by the vocal cords to produce sound, but that the eardrums are the primary means used by all frogs to amplify their
calls.
Armed with this new information, researchers are taking a look at the ears of other frogs. One scientist at UCLA, Peter Narins, collected an African frog named Petropedetes
parkeri in Cameroon. He was interested in this frog because it appeared to develop an enlarged middle ear bone that protruded from the eardrum during the mating season. However,
he discovered that this protuberance was in fact a spongy growth, not a bone. Because this growth appears only during the mating season, and only on the eardrums of male frogs, Narins
wondered if it, too, had a role in the amplification of frog calls. Based on acoustic analysis, Narins suggested that this growth helps to tune the eardrum to resonate at the dominant
frequency in the frog's call. Because the dominant frequency in the call is different from the eardrum's natural resonating frequency, Narins thinks that the growth helps the eardrum
match the call more closely. Another scientist thinks the structure might be used to tune the frog's hearing at different frequencies as well.
References
"How Male Animals Gain an Edge in the Mating Game: Croaking With All Ears." Science, Vol. 277, July 18, 1997, p .317.
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