Simple Toadfish Grunts May Contain Complex Information

Wired.com: Simple Toadfish Grunts May Contain Complex Information

The simple-sounding grunts and hoots of the toadfish contain surprisingly complex information. Sounds hidden within the toadfish calls may communicate everything from, “It’s me again, can we spawn?” to, “It’s your neighbor, but get away from my nest.”

Birds, amphibians and mammals often use hard-to-discern sounds to add an extra layer of vocal information. But these subtle harmonics and dissonances, called nonlinearities, hadn’t been observed in fish.

“Fish rarely come to mind when we talk about animal sounds. It’s always seen as sort of boring, simplistic communication,” said biologist Aaron Rice of Cornell University, who led a study of three-spined toadfish vocalizations in Proceedings of the Royal Society B May 11. But with the right kind of analysis, “complex underlying structures emerge in images of these sounds. We think they’re communicating detailed information.”

Toadfish are a widely distributed but little-studied family of droopy-faced bottom dwellers, best known for their swim bladders. These air-filled sacs typically allow fish to alter their buoyancy, but those of toadfish are unique: They’re split into two chambers, controlled independently by muscles capable of twitching 200 times per second, making them some of the fastest muscles in the animal kingdom.

By vibrating their muscles, toadfish turn their swim bladders into a sonic instrument. When a male wants to spawn with a female, he uses the organ to hoot. If something comes too close to the pile of eggs he’s fertilized, he grunts. When Rice looked at spectrogram images of these seemingly simplistic calls, “there was some weird stuff,” he said.

Rice didn’t just notice variations in pitch and volume — linear changes, which would be expected — but non-linearities as well. These are generated by the interaction of two different sounds, and can convey much more information than linear sound.

Most amphibians, birds and mammals have two-chord vocal structures able to create nonlinearities, and recent research suggests they’re crucial to everything from increasing alarm in an infant’s cry to adding extra sexiness to a seductive voice. They’re even useful in movies. In fish, however, non-linear vocalization hadn’t been observed.

“A physicist who understands vocal structures probably would say this isn’t shocking, but for me as a biologist it’s a surprising discovery,” said Tobias Riede of the University of Utah. “I’d be interested to see this data compared to other fishes that might produce nonlinearities. That could help us understand the evolution of these structures.”

To be certain the swim bladder chambers actually produced the nonlinearities, Rice’s team paralyzed one chamber in toadfish and set them loose. Sure enough, the complex signals vanished.

There’s a slim chance the toadfish’s complex calls are meaningless products of evolution, said Rice. But he thinks the nonlinearities can broadcast an individual animal’s identity, much like penguin parents do when calling their chicks.

For the next experiment, Rice plans to see if toadfish with half-paralyzed swim bladders can effectively communicate. If they can’t, non-linear signals could be a new frontier of fish communication research.

Whatever happens, Rice hopes to draw more attention to toadfish.

“We know very, very little about their basic life histories, even what they eat or how often they spawn,” Rice said. “The opportunities for gaining basic understanding of this animal are still wide open.”