Frogs' Hearing
 What
structures does the frog hear with? How do they work? Is there any other animal
that hears the same way? Someone, World
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!
Label 1 points to the frog's
eardrum. Photo courtesy of Jon Glase, Cornell
University
The ear structures function much as 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 only 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.
Further Reading:
Most
poisonous creature on Earth, WonderQuest
What is a spring peeper and how do they make all that noise and why?
WonderQuest
Will frogs
glow if they eat enough fireflies?, WonderQuest
Can a frog
freeze solid and live?
Ohio State U: Earless frog uses lungs to hear
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