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Dual bladder helps toadfish ‘talk’

CORNELL (US) — A swim bladder that is split it two allows the toadfish to be heard above the roar of the underwater crowd.

The findings show that the vocal behaviors and underlying mechanisms for sound communication in fish, including control by the brain, have far more in common with other vertebrates—including primates—than previously thought.

Researchers first reported in 2009 on the three-spined toadfish (Batrachomoeus trispinosus), a species with a swim bladder that is split in two. An analysis published online in the journal Proceedings of the Royal Society B: Biological Sciences, reports that of some 1,300 toadfish recordings, 35 to 40 percent of the calls had a nonlinear aspect.

Nonlinear sounds are dissonant and acoustically complex and have been previously observed in reproductive, territorial, and distress calls of birds, amphibians, and mammals.

“A large proportion of the sounds looked noisy, which means in spectrographic analysis the sound is not neatly contained in frequency ranges,” says Aaron Rice, science director of the Bioacoustics Research Program at Cornell University and lead author of the study.

Nonlinear vocalizations have been noted in alarm calls of meerkats, crying and screaming of human babies, and calls that serve to identify individuals in penguins.

Co-author Bruce Land, senior research associate in neurobiology and behavior and in electrical and computer engineering, used a computer program to compare the structure of nonlinear calls of the fish with those of birds and nonhuman primates.

“Different types of nonlinear calls may have different functions,” Rice says. In this way, “examples from the animal kingdom could offer reasonable hypotheses for what these sounds mean in our toadfish.  This fish has evolved a unique vocal organ that is analogous to our larynx and the syrinx of birds.”

The role of the fish’s dual bladders in vocal output was directly shown by clipping the nerve going to one bladder to render it inactive; the fish then only made linear sounds like those of fish with one bladder, Rice says.

“The dual swim bladders offer the behavioral advantage of allowing toadfish to make enriched sounds that travel farther and increase the likelihood of being heard in a potentially noisy aquatic environment.”

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