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

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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.

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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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