The revelations never cease. First, it turns out that piranhas aren't the vicious cow-stripping killers we'd been led to believe. Now it turns out that they bark when they see things they don't like. Discover the physics of how piranhas yip away like yappy dogs.
Scientists knew that piranhas could and did bark when provoked, but they hadn't figured out exactly why or how the fish made the sound. The research they conducted to find out would not be out of place in a horror movie - right down to the random maulings. Several researchers were bitten so badly that they had to go to the hospital. One woman's finger was almost bitten in half.
To be fair to the piranhas, they had cause. Occasionally, to produce the barking sounds, the researchers would actually try to hold them in their hands. Since the scientists also wanted to see when they piranhas barked in a natural environment, they suspended a hydrophonic microphone in the tank and recorded the piranhas activities on video and audio. When they replayed the tapes, they found out that the piranhas barked at each other during confrontations, but that they produced two other noises as well. They also made a 'drumming-like' sound when trying to intimidate each other, and a croaking sound when their jaws snapped together.
This didn't settle the question of how they made the sound in the first place. Underwater, generally only the air-breathers make a lot of noise. Dolphins and whales squeak and grunt, but sharks stay quiet. The only large storage facility of gas in a fish is their swim bladder - the sack of gas that allows fish to control their depth in the water. It was assumed that the piranhas contracted the muscles around the swim bladder to 'strum' it and make it vibrate, a little like a finger strums a guitar string. When scientists simulated it, though, they found that the swim bladder stopped vibrating as soon as the muscles stopped contracting. This may explain why most fish don't hum. It's not a matter of twanging the right organ, the muscles need to be actively engaged in making the sound, and they have to engage in different ways to make each different sound.
What's next in piranha acoustics? It seems that it's hard to get piranhas to mate in captivity. The scientific team, which hails from University of Liège, Belgium, will have to head out to the wild to see if piranhas also make noise when they are trying to woo the opposite sex. If so, look forward to downloading the album The Love Song of the Piranha, recorded by Fingerless Sandie Feelgood.