Frog finds empty bandwidth, then croaks.

Frog finds empty bandwidth, then croaks

Like radio stations vying for crowded air waves, some singing frogs complete for broadcast frequencies. Alejandro Purgue, a herpetologist at the University of Utah in Salt Lake City, suggests that the South American frog Leptodactylus ocellatus evolved an underwater mating call in response to airwave competition from another frog that uses the same frequency above water.

Scientists have suspected that a few frog species might, through some unknown mechanism, produce underwater calls. Purgue appears the first to document it thoroughly, using U.S. Navy hydrophones and computer analysis. He interprets L. ocellatus' underwater calls as a previously unrecognized dimension of "niche partitioning" -- the means by which species living close to each other minimize competition.

The male L. ocellatus produces calls in the frequency range of 250 to 500 hertz, presumably to maintain territoriality and to attract mates. But the frog Physalaemus cuvieri calls in a similar range at many of the same ponds, and its song is about 40 decibels louder. Taking advantage of the fact that very little sound crosses the air-water interface, L. ocellatus apparently avoids competition by channeling its call underwater. "This underwater channel has no other species calling in a close frequency range, providing an excellent alternative way of communication," Purgue says.