Piranhas kept at Sea World in Jakarta enjoy a feeding frenzy.

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Can piranhas really strip a cow to the bone in under a minute?

When Theodore Roosevelt went on a hunting expedition in Brazil in 1913, he got his money's worth. Standing on the bank of the Amazon River, he watched piranhas attack a cow with shocking ferocity. It was a classic scene: water boiling with frenzied piranhas and blood, and after about a minute or two, a skeleton floating to the suddenly calm surface.

Roosevelt was horrified, and he wrote quite a bit about the vicious creatures in his 1914 book, "Through the Brazilian Wilderness." He recounted the stories of townspeople who had been eaten alive, and others who'd lost body parts to piranhas while bathing in the river. "They are the most ferocious fish in the world," Roosevelt announced to the world. "[T]hey will snap a finger off a hand incautiously trailed in the water; they mutilate swimmers -- in every river town in Paraguay there are men who have been thus mutilated; they will rend and devour alive any wounded man or beast; for blood in the water excites th­em to madness" [source: ESPN]. The legend of the piranha had begun.

Hollywood picked it up from there with the 1978 horror flick "Piranha" ("When flesh-eating piranhas are accidentally released into a summer resort's rivers, the guests become their next meal"), 1981's "Piranha II: The Spawning," and a remake of the original B-movie that came out in 2010 [sources: IMDb, Movie Insider]. The killer piranha has made the gory jump into the 21st century.

But is the vicious reputation deserved? Roosevelt witnessed the now-famous cow stripping incident in Brazil, where piranhas live in especially high numbers. Howev­er, they're native to and pretty common all along South America's Amazon River -- from Argentina to Colombia. So are South American bovines a regular meal for these ferocious fish? And why are there cows hanging out in the Amazon River?

Setting aside the account of a former U.S. president, piranhas stripping a cow -- or a human -- to the bone in less than a minute is a tough sell. How would that even be possible for a bunch of 10-inch, 3-pound fish?

Let's find out.

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A man displays his bloodied finger after dipping his hand in the piranha-infested waters of the Parapeti River.

H. John Maier Jr./Time Life Pictures/Getty Images

Piranha!

Roosevelt explored the exotic Brazilian Amazon as an avid hunter, but he wasn't exactly the common tourist. He was a former U.S. president, and his guides wanted him to be very pleased with his trip. But what he saw was real: piranhas stripping a cow to the bone in a shockingly short period of time. Whether it was really under a minute, we'll never know. But we do know that this type of attack is feasible for piranhas. What Roosevelt witnessed had some special circumstances, which we'll get to later. Nonetheless, it was a disturbing sight, considering the size of these fish.

A piranha bears its fearsome teeth.

Paul A. Zahl/National Geographic/Getty Images

We're not talking monsters here. While piranhas top out at about 2 feet (60 cm), most are about 8 inches (20 cm) from head to tail and weigh just a few pounds. The most vicious of the roughly 20 species found in the Amazon River, the red-bellied piranha (Pigocentrus naterreri), is on the small end of the spectrum and usually weighs about 3 pounds (1.36 kg) [source: ESPN]. The next most aggressive species is the black piranha (Serrasalmus rhombeus), which tends to be bigger than the red-bellies.

In the case of piranhas, it's not the size that counts. It's the teeth.

A piranha's teeth are only about a quarter-inch (4 mm) long, but they're like razors, and the whole jaw mechanism is designed for chomping efficiency. The teeth are spaced in an interlocking pattern, so when a piranha jaw snaps shut, the top teeth and the bottom teeth interlace like dozens of razor-sharp scissors. The jaws are incredible strong: Some people who have lost toes to piranhas have actually lost the entire toe, bone included.

The reason why piranhas can strip a large animal like a cow down to a skeleton so quickly is because of a few factors. First, piranhas don't chew. When they bite down, the big chunk of flesh they take out of the cow goes right into their bellies. They just keep snapping their jaws shut and filling themselves up. Next, that type of task is accomplished by hundreds of piranhas, not just the typical school size of 20, and piranhas are very efficient team eaters. In a feeding frenzy, they rotate continuously, so as each piranha takes a bite, it moves out of the way so the piranha behind it can get a bite, and so on. They take turns with incredible speed, which is where the boiling-water effect comes from. The piranhas are constantly changing position during a feeding frenzy.

Another important factor involved when piranhas eat a large animal in minutes has to do with the special circumstances surrounding what Roosevelt saw: Feeding frenzies happen when piranhas are starving. It's not an everyday occurrence. Roosevelt's guides in Brazil had set up the scene for their famous guest. They had set nets to close off a small part of the river and had tossed hundreds of piranhas into it, trapping them. By the time they threw that cow into the water, the piranhas had been starving for some time.

So if eating an entire cow in under a minute isn't the norm, how and what do piranhas usually eat?

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Most piranhas, like this red-bellied piranha, simply nip at other fish as the pass.

Claus Meyer/Minden Pictures/Getty Images

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Piranha Feeding: The Daily Grind

Attacking a live animal isn't out of the question for piranhas, but it's not likely they could take down a healthy, full-grown human. They have, however, been known to attack sickly, old animals that come to drink from the river. When a cow lowers its head, they'll clamp onto its face. If the cow is too weak to fight back, the piranhas will drag it into the water and eat it. But live prey isn't the mainstay of their diet. Mostly, they're scavengers. The skeletons of animals and people found in the Amazon, apparently eaten by piranhas, weren't attacked alive. They were already dead when the piranhas got to them.

As with other fish, mammals are by no means a big part of the piranha's diet. They eat other fish, mostly, and sometimes other piranhas. An aquarium in Wales that had gone to considerable trouble to acquire a male and a female piranha (piranhas are illegal to import in most parts of the world, including Britain) in hopes the two would mate, were disappointed when the female ate her potential suitor [source: BBC News]. But piranhas aren't strict carnivores. They'll eat fruits and plants, too, especially when they're young.

Contrary to legend, most piranhas don't really attack anything. Twelve of the 20 species in the Amazon survive entirely on taking small bites out of the fins and scales of other fish as they pass by. The fish swim away only slightly disturbed, and their fins and scales grow back.

While piranhas aren't quite the vicious man-eaters of myth, attacks on humans have been increasing in frequency. In South America, people have been losing fingers and toes more often than they were just 10 years ago, and experts believe it might have something to do with an increase in the number of dams on the Amazon River. Dams slow the current, and piranhas like to breed in the slowest-moving waters. Creating more placid areas along the river is an invitation to piranhas to come set up camp in large numbers. Since placid areas also attract swimmers, humans and piranhas are coming into contact more and more.

For more information on piranhas and related topics, swim through the links on the next page.

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Can piranhas really strip a cow to the bone in under a minute? Author's Note

Articles like this one, which explore a pop-culture or common-knowledge "fact" and find the truth, fiction, or abiding mystery in it, are some of my favorite ones to write. This one in particular made my heart beat a little faster when I first received the assignment from my editor, because I believed I'd seen piranhas on a scuba-diving trip some years ago.

I hadn't, as I learned when I started researching and looking at pictures of these creatures. Heck, I wasn't even in South America.

As for actual piranhas, the cow myth turned out to be surprisingly, mostly true, even if the incident described by Teddy Roosevelt had been staged (that poor animal). Those relatively petite fish had, in fact, very quickly removed all of the flesh from a cow. Sure, they don't usually feed that way, but it's somehow morbidly pleasing to know it's possible (and will likely prevent my swimming in the Amazon if I'm ever lucky enough to make it there). Whether it was one minute or three minutes seems beside the point.

Sources

  • China orders piranhas destroyed. BBC News. December 24, 2002. http://news.bbc.co.uk/2/hi/asia-pacific/2603689.stm
  • Hungry piranha seeks good catch. BBC News. June 1, 2000. http://news.bbc.co.uk/2/hi/uk_news/wales/772802.stm
  • Piranha. Encyclopedia Britannica. http://search.eb.com/eb/article-9060157
  • Piranha. Extreme Science. http://www.extremescience.com/Piranha.htm
  • Piranha. HowStuffWorks. http://animals.howstuffworks.com/fish/piranha-info.htm
  • Piranha increase "due to dams". BBC News. December 28, 2003. http://news.bbc.co.uk/1/hi/sci/tech/3346301.stm
  • Piranhas. London Aquarium. http://www.londonaquarium.co.uk/files/factsheets/Piranhas.pdf
  • Rumble in the jungle with Amazon's killer piranha. LA Times. November 22, 2005. http://travel.latimes.com/articles/la-os-piranha22nov22
  • Sutton, Keith "Catfish." Out there: Piranha! ESPN.com. http://sports.espn.go.com/outdoors/tv/columns/story?columnist= sutton_keith&page=g_col_sutton_piranha
  • The Truth About Piranha Attacks. Practical Fishkeeping. http://www.practicalfishkeeping.co.uk/pfk/pages/item.php?news=1180

Piranhas: Cheat Sheet

Stuff you need to know:

  • Piranhas are pretty small considering their predatory abilities -- only about 8 to 24 inches long. And the most vicious ones are usually toward the smaller end of the range.
  • Piranhas only occasionally kill live prey. More often, they take small, non-fatal nibbles out of passing fish or devour food that's already dead.
  • Former U.S. President Theodore Roosevelt was a wilderness adventurer!
  • Piranha feeding is an elegant example of teamwork. Each small fish takes a bite and then moves out of the way while it swallows (without chewing), making room for another fish to reach the prey and take a bite. They're constantly rotating positions, allowing for the incredible speed with which they can strip their prey and leading to that creepy "boiling" effect in the water while they eat.
  • A particularly large group of piranhas would certainly be capable of stripping a cow very quickly. But under a minute? It's unlikely.

Now, test your knowledge with these quizzes!

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