Though Piranhas are famous for their vicious bite, three Belgium scientists were interested in their bark.

Anyone who has handled a piranha knows it makes sounds when picked up, so not many people know that. But University of Liège scientists, Sandie Millot, Pierre Vandewalle and lead researcher Eric Parmentier, were curious about the conditions under which piranha would produce sounds naturally. So the three submerged an aquatic microphone (known as a hydrophone) and studied a tank full of red-bellied piranha.

They found the fish were usually silent but produced a barking sound when entering into a confrontation.

For animals, it's less expensive [in terms of energy] to make a lot of noise and impress the other guys, rather than fight, Parmentier said.

The scientists also discovered the fish making two other sounds. One is a drum-like percussive noise when fighting for food or circling an opponent. They also found a soft croaking sound created by their jaws when they snap at one another.

The researchers were also interested in how the fish make the sounds. They thought it was due to the intrinsic acoustic properties of the fishes' swim bladders - the internal organ that regulates the animals' buoyancy. But after a few experiments, the scientists discovered it was due to a muscle vibrating the swim bladder that produced the sounds.

Understanding what sounds fish make will yield insights into the fish's mating process. The researchers hope that one day fishermen will use hydrophones to determine when the fish are mating and when they aren't. If fishermen understand the mating cycle of their prey, it will mean they can give the species more time to develop and will be better for fish-kind overall.

But the researchers will have to go to Brazil for those answers, since fish don't generally like to mate in tanks.

As for what is worse, piranhas' bark or bite:
We both visited the hospital because we were bitten and Sandie's finger was nearly cut in half, Parmentier said.

The study can be found in the Journal of Experimental Biology.