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Piranhas: Toothy Nippers With a Bad Reputation

piranha
The red-bellied piranha (Pygocentrus nattereri) feeds on bits of flesh and fins nipped from the tails of other fish, insects, smaller fish and some plant material, such as figs or other fruits. Sylvain Cordier/Getty Images

Sometimes an animal just captures the collective imagination in a way that, while sensational, might not be entirely justified. Take the piranha, a toothy South American freshwater fish that has been the subject of not one, not two, but five Hollywood horror movies. Sure, they've got big teeth, but are they really as ferocious as their reputation?

Piranhas occur naturally only in the Amazon, but have been introduced into freshwater systems in other parts of the world, mostly through the tropical fish trade. They are illegal to keep as pets in most U.S. states, although most piranha species wouldn't be able to survive in a place much colder than Florida anyway.

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Not All Piranhas Are Carnivores

It's very difficult to say how many piranha species there are in the Amazon basin, though around 40 are recognized by the scientific community. We don't know a lot about the habits of piranhas in the wild — much of what we know, we learned from keeping them in aquaria, and most animals tend to act differently in captivity. The largest piranhas grow to be about 2 feet (0.6 meters) in length, but many species max out at about half that length. Their jaws are lined with a single row of saw-like interlocking teeth that resemble shark teeth — razor sharp and always exposed due to a pronounced underbite. Most piranha species eat meat, but just because you have a jaw three times more powerful than a similarly-sized alligator and can bite through the plates of the most heavily-armored fish of the Amazon doesn't mean you have to eat meat all the time.

Piranhas, like most wild animals, eat what they can get. A typical piranha diet includes a lot of worms and insects, other fish, some carrion they find in the river, as well as seeds and vegetation. Depending on the species, the ratio of these elements vary — some piranha species are entirely vegetarian, while others, like the infamous red-bellied piranha, reportedly wait in the weeds every evening in groups of 20-30 to ambush prey.

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Piranhas "Bark"

An underwater environment is full of sounds just like the ones in your neighborhood, and in the Amazon, piranhas are pretty noisy neighbors. They make distinctive "barking" noises by squeezing their swim bladders into different shapes. According to Rodney Rountree, a marine biologist and an expert in fish sound production, who has studied piranha vocalizations in the Amazon basin in Peru, the patterns of each species' underwater barks are very distinctive, which can help scientist looking for piranha in murky waters figure out which species are in an area without even catching them — all that's needed is a mic in the water.

Not only do different piranha species vocalize differently, piranhas make different types of noises to communicate different messages. For instance, a "bark" might be a warning for another fish to back off, while a group of piranhas may emit an intimidating chorus of grunts as they gather around a prey item.

"You hear all sorts of fish in the soundscape," says Rountree. "There are many different species of catfish, for instance. In the areas where I was catching piranhas for my research, I was hearing other fish sounds, and it became obvious that it was the sound of other fish screaming because they were being bitten by piranhas."

Red bellied piranha
A school of red-bellied piranha (Pygocentrus nattereri), which can have a lifespan of 10 years or more.
Pikrepo

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Piranhas Are "Nippers"

Although you don't want to tangle with the mouth end of a piranha, a lot of popular lore around these fish can be a bit overblown. For instance, that classic situation in movies wherein a cow steps into a river and is immediately pulled under and stripped to the bone by a pack of hungry piranhas isn't really something that happens in real life.

The waters of the Amazon are often very opaque — full of silt and tannins — and fish can't see very well, so they find their prey by smell or proximity. Most piranhas are "nippers," meaning they feed by sneaking up on their prey and shearing off a single chunk of flesh. It doesn't much matter to the piranha who it is — they'll even take bites out of each other.

According to Rountree, this "nipping behavior" isn't unique to piranhas — in fact, it's fairly common among fish in the Amazon.

"There are all kinds of fish that just take a little chunk out of other fish," he says. "Many of the fish are armored — especially some of the catfishes have heavy, boney scales — and part of this is for defense."

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Teddy Roosevelt and the Piranha

The Amazon is full of fearsome underwater killers — electric eels, for one, are arguably a good deal more deadly than piranhas. However, in 1913, former U.S. president Teddy Roosevelt took a pleasure cruise in the Brazilian Amazon ( and it very nearly killed him). At one settlement, locals showed Roosevelt a pool full of starving piranhas, and reportedly fed a live cow to the fish to show off their ferocity. Roosevelt was really struck by their killing prowess and wrote about the piranhas in his journal, describing the experience and calling them, "the most ferocious fish in the world."

According to Rountree, while a piranha might take a single chunk out of you, the devouring of a cow is virtually unheard of:

"In some areas during the dry season, when water levels draw down to pools and shallow areas where the piranhas get concentrated — those situations could probably be quite dangerous. This is when feeding frenzies happen — not like stripping animals to the bone or anything, but you wouldn't want to go swimming in them."

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