Frogs’ Supersensitive Inner Ear : When a White-Lipped Thumps, Others Listen
White-lipped frogs may not have much to say to one another, but a pair of University of California scientists believe that the tiny amphibians have developed an unusual way to communicate--with seismic vibrations.
In an article in the current issue of Science magazine, Edwin R. Lewis and Peter Narins wrote that laboratory tests and field studies in the mountains of Puerto Rico indicate that white-lipped frogs can attract females and warn away other males by thumping the ground with their throat pouches.
‘This Is Very Exciting’
Lewis, a professor of electrical engineering at the University of California, Berkeley, said that he and Narins, an associate professor of biology at UCLA, believe that their work is the first to demonstrate that the inner ear of amphibians and perhaps reptiles is sensitive enough for communicating through vibrations in the soil.
“What’s exciting is just finding a new mode of communication,” said Lewis, who runs the inner-ear biophysics laboratory at UC Berkeley. “Behavioral biologists have been poking around out there for a long time, and to find something new like this is very exciting. . . . That’s why science is such a blast.”
Lewis said the tiny white-lipped frogs apparently sense vibrations by using a part of their ears, called the saccule, in much the same way as engineers detect movement in sophisticated guided missiles.
Essentially, he said, even extremely slight vibrations in the soil--much less than an angstrom, or one-100-millionth of an inch--will jar the frog’s body and disturb its saccules, small sacs of calcium crystals attached to a bundle of sensitive auditory nerves.
The saccule is the largest part of the white-lipped frog’s ear, Lewis said. Other amphibians and some reptiles have saccules that are equally well developed, indicating that they, too, may be able to communicate by thumping the ground.
That some animals respond to vibrations has been known since the 1930s, Lewis said, but the extreme sensitivity of the white-lipped frog and the apparent order of its communication is a new discovery.
Saccules in humans and other mammals are very small, Lewis said, and are believed to be used to detect balance. In fish, he added, the organs are used to hear underwater sounds.
The frogs apparently make sense of thumps ranging from four beats per second to 10 beats per second, a pace which Lewis described as roughly analogous to clucking chickens. The thumps themselves, he said, are about as strong as the light tap of a human finger.
A beat of four thumps per second is used by males to “advertise” their position, Lewis said, while more rapid “clucking” is apparently intended to intimidate other males who encroach on another’s territory. Not all thumping exchanges between males are hostile, however. Lewis said groups of males have been seen joining together in a sort of “chorus.”
Thumping is not used by all white-lipped frogs, nor is it used by any frog all the time. Lewis said that he and Narins cannot explain this, but they do note that thumping is used in conjunction with audible chirping when frogs converse. By sensing the time lag between the audible and seismic signals, he said, frogs may be able to detect the proximity of other frogs.
Lewis said thumping is distinct from the ability to detect approaching predators by feeling vibrations from their footsteps.
“It’s pretty clear that this is an advertisement call,” Lewis said. “It could say to another male, ‘Join in if you want to, but don’t get too close.’ Or it could say to a female, ‘Here’s a male and a likely place to (mate); why don’t you come over?’ ”
