HOW does a frog with no true ear listen out for its rivals? Apparently it
鈥渉ears鈥� with its skin. And if that鈥檚 not enough, there鈥檚 always an elaborate
semaphore system to help keep in touch. This is the conclusion of researchers in
the US who have studied the golden frog, a rare species that lives along
fast-flowing streams in the forests of Costa Rica and Panama.
In most frogs, the eardrum, or tympanum, connects with the inner ear via a
small bone that transmits the signals created by the vibrating drum. But the
genus Atelopus is different. Most of its 70 known species lack the
connecting bone, so how they hear has been a puzzle.
Now Erik Lindquist and Thomas Hetherington at Ohio State University in
Columbus have studied one of the earless species, the golden frog, Atelopus
zateki, in the Campana National Park in Panama. A. zateki is
highly territorial, and the males keep close tabs on each other鈥檚
whereabouts.
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In detailed studies of 15 males in the forest, the researchers recorded the
animals鈥� calls then played back the tapes of the calls and recorded the animals鈥�
movements. Nine of the 15 frogs turned to face the loudspeaker broadcasting the
calls, and seven answered back. This shows, says Lindquist, that the frogs are
responding to the calls rather than some other cues (Journal of
Herpetology, vol 30, p 347). 鈥淭hese guys could definitely hear,鈥� he says.
鈥淭he question was, how?鈥�
In unpublished studies in the lab, they found that the frog鈥檚 skin is 鈥渢uned鈥�
to vibrate at about 2 kilohertz, the same frequency as the frog鈥檚 main call. The
vibrations resonate in the lungs and the auditory nerve is then stimulated so
that the sound is heard, the pair believe.
And there is a second communication system. The frogs made slow, circular
movements of their front and hind limbs to signal to each other. Hetherington
says the circular movements are a kind of exaggerated stepping pattern.
鈥淪emaphoring makes the direction an animal is taking really obvious. What it
must be saying is something like: 鈥淚鈥檓 coming towards you, I鈥檓 gonna take care
of you.鈥�