Using underwater microphones, the scientists recorded the sounds the fish made as they swam around and competed for food. They found that the well-fed fish were mostly quiet. But when the piranhas threatened each other over food, they produced a barking sound. It was the same sound they made when the scientists picked them up with their hands.

BOB DOUGHTY: At first the scientists thought the fish made just this one sound. But they learned that the piranhas produce two or more other sounds. When the fish are fighting for food and circling an opponent, they make a short, drum-like sound. When they bite each other, their jaws make a softer, croaking sound.

A red-bellied piranha

Piranhas produce noises by vibrating, or moving their swim bladders. These gas-filled organs help them control their position. The scientists studied how the fish used muscles to vibrate their swim bladder, creating the sounds.

The findings were published in the Journal of Experimental Biology. Professor Parmentier and the study’s co-author, Sandie Millot of the University of Algarve in Portugal, did not complete their study without falling victim to the aggressive fish. Both scientists suffered bites while holding piranhas.

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BARBARA KLEIN: This SCIENCE IN THE NEWS was written by Brianna Blake, Christopher Cruise, Jerilyn Watson and June Simms, who also was our producer. I’m Barbara Klein.