Department of Fisheries and Allied Aquacultures / Auburn University
While observing red-bellied piranhas swimming around the tank and competing for food, the researchers noted that the fish produced three distinct combative sounds.
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updated 10/13/2011 3:46:53 PM ET 2011-10-13T19:46:53

Piranhas are already feared for their sharp teeth and meat-eating ways, and now a ferocious "bark" can be added to the list. Aggressive red-bellied piranhas produce bark-like sounds to scare off other piranhas, a new study shows.

Researchers from the University of Liège, Belgium, had noticed that red-bellied piranhas (Pygocentrus nattereri) make barking noises when they are picked up by humans. To figure out why and how, the team suspended a hydrophone into a tank containing piranhas and recorded any sounds they made throughout the day. They also filmed the fish so that they could later match up the sound recording with the film footage.

The recordings showed that the fish were generally silent and non-combative. As soon as one was confronted by another, however, silence turned to barking.

While observing the piranhas swimming around the tank and competing for food, the researchers noted that the fish produced three distinct combative sounds. The first was a bark-like noise that they made during so-called frontal displays, when two piranhas would swim rapidly toward each other and remain face to face. "This sound could therefore be interpreted as a warning signal during an intimidation phase between two individuals," the researchers write in the current issue of the Journal of Experimental Biology.

This was the same barking that the piranhas produced when picked up by the researchers.

The second was described as a short, drum-like percussive sound that the fish produced when fighting over food and circling its opponent. The third noise was described as a softer "croaking" sound that they made with their jaws when snapping at each other.

Hear the piranhas bark (WAV audio from Parmentier and colleagues):

Previous research had found that piranhas produce noises using muscles attached to their swim bladders, but scientists weren't sure how the swim bladder was involved in sound production. To find out, study researcher Eric Parmentier and his team stimulated the red-bellied piranhas' swim-bladder muscles to contract.

Results showed that the swim bladder stopped vibrating as soon as the muscles finished contracting. This meant that the muscles were directly driving the swim bladder's vibrations, the researchers said. The frequency, or pitch, of the bark and drum sounds was determined by the muscles' contraction, not by any resonant properties of the swim bladder itself.

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While the well-fed piranhas were generally peaceful, they occasionally attacked each other and even nipped at the researchers' fingers.

"We both visited the hospital because we were bitten, and Sandie's finger was nearly cut in half," Parmentier said, referring to his colleague Sandie Millot, in a statement.

Even so, the Encarta encyclopedia and other sources note that tales of vicious attacks on humans are mythical. The fish are known instead to eat worms and small fish.

"There are no documented human deaths from piranha attacks," according to the Encarta. "A common feeding behavior is to nip off parts of the fins or scales from other types of fish. This cropping tactic allows the victim to survive and regrow the injured parts, providing a kind of renewable food resource for piranhas."

Parmentier said the researchers now aim to study whether the fish are vocal when mating. They may have to travel to the piranhas' native waters of Brazil to find out, since the fish are not prone to reproduce in a tank setting.

You can follow LiveScience writer Remy Melina on Twitter @remymelina. Follow LiveScience for the latest in science news and discoveries on Twitter @livescience  and on Facebook.

© 2012 LiveScience.com. All rights reserved.

Explainer: Ten of nature's scariest animals

  • Stephen Chernin  /  AP file

    Humans seem to love a good scare, particularly on Halloween. Ghosts, zombies, vampires and other fictional characters are good for putting a chill down your spine. But if you're looking for something truly scary, let nature be your guide. Click on the "Next" label to see 10 of the animal world's scariest creatures.

  • Box jellyfish pack a deadly sting

    Anders Garm

    The box jellyfish is ghostly and squishy, with 24 eyes and a tangle of tentacles, each equipped with about 5,000 stinging cells. The creatures pack a special type of venom — the most deadly in the animal kingdom — that is activated by contact with certain chemicals found in fish, shellfish and humans. The venom can cause cardiac arrest, cripple the nervous system, and eat away skin. Several victims stung at sea die before they reach shore.

  • Black mamba: Speedy snake with lethal venom

    Eric Marquette

    Even people who are fearless around snakes should be careful in the presence of a black mamba. Native to sub-Saharan Africa, the territorial and aggressive snakes grow up to 14 feet long and travel at speeds of more than 12 miles per hour. Fortunately, black mambas are shy and usually race away when humans approach. But when threatened, they attack with repeated strikes to deliver lethal venom. Only a quickly administered antidote saves victims from death. The black mamba gets its name from the color of the inside of its mouth, seen here.

  • Saltwater crocodiles eat people

    Greg Wood  /  AFP - Getty Images

    Saltwater crocodiles are aggressive and territorial. And unlike their North American alligator cousins, they regularly eat people. They live in saltwater estuaries, and freshwater rivers and swamps, ranging from Australia north to Southeast Asia. The biggest males weigh in excess of 2,200 pounds and measure 20 feet from toothy snout to scaly tail, making them the world's largest reptiles. Though they mostly dine on smaller prey such as fish and shorebirds, adults will occasionally tackle larger animals, including careless people. Tourists lure this croc out of the water with a chunk of meat.

  • Polar bears: a poster child best left alone

    USFWS

    Polar bears are a poster child for groups campaigning to save the world from the ravages of global warming. But the cute and cuddly image that polar bears project are an icy disguise. Polar bears are the world's largest land predators; some males top the scales at 1,500 pounds and wield 2-inch-long claws. They normally kill seals and the occasional walrus twice their size. Human attacks are extremely rare but potentially fatal.

  • Wolves are revered and feared

    AP file

    Revered by some, feared by others, the gray wolf is the world's biggest, most powerful dog. Strong jaws and sharp teeth help them rip into their prey. Wolves primarily hunt deer, moose and bison, but when wild supplies are tight, domestic cattle and sheep are easy targets — hence the wolf's uneasy relationship with humans. In the 20th century, the predators were nearly hunted to extinction. Continued fear of the wolf continues to muddy their road to recovery.

  • Lions take a mean bite

    Daniel Munoz  /  Reuters

    In 1898, a pair of lions reportedly ate 135 people working on a railroad in Kenya. Though lions continue to make the occasional human meal, most of the slaughtering now goes the other way. Today, the cats are vulnerable to extinction, due to their human predators. Conservationists are racing to keep the survivors alive. In this image, two lions at a Sydney zoo gaze at each other before a non-human meal.

  • Shark attacks: Very feared, very rare

    Image: Casa Rinconada
    Reuters

    Maybe the 1975 film "Jaws" is to blame, but whatever the cause, many humans are terrified of the great white shark, the largest predatory fish in the ocean. The fear is understandable: These giants have a mouth full of razor-sharp teeth, they can reach 20 feet in length, and they top the scales at 5,000 pounds. Despite all that, scientists insist the predators pose little risk to humans. They say the chances of winning the lottery are higher than the chance of being attacked by a great white. Moreover, most attacks are not fatal. In this image, a great white swims past a cage holding tourists.

  • Scariest spider is a recluse

    Image: Chankillo
    Justin Sullivan  /  Getty Images file

    Tarantulas give lots of people the creeps, but scientists say most of the big and hairy spiders are harmless to humans. The real spider to fear is the brown recluse, a six-eyed spider with a violin-shaped head and a venomous bite that can lead to necrosis — the death of skin tissue. Fortunately, as their name suggests, the spiders are reclusive and seldom aggressive, only biting when threatened. Most bites have little or no effect on their human victims, but some are nasty and even fatal.

  • Mosquitoes kill millions every year

    USDA

    The world's deadliest animal? The mosquito. In some parts of the world, these pesky, bloodsucking insects spread diseases such as malaria, dengue fever and West Nile Virus that kill nearly 3 million people a year. Many of the malaria deaths occur in sub-Saharan Africa, where scientists and international aid organizations are busy developing strategies to stop the disease's lethal spread.

  • Vampire bats feed on blood

    Ho  /  AFP - Getty Images

    Finally, what list of scary animals would be complete without the vampire bat? The thumb-sized flying mammals with eight-inch wingspans feed exclusively on blood in the dark of night. Their prime targets are cattle and horses, but they are known to attack humans, too. Heat sensors in the bat's nose help it find flowing blood. They bite through the skin with razor-sharp teeth and lap up what oozes out.

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