In 1914, former U.S. President Theodore Roosevelt was traveling in Brazil when locals staged a protest of sorts. They had barricaded a school of piranhas in part of a river and then starved the fish for days. As Roosevelt watched from the river bank, they pushed a cow into the water to show how quickly the piranhas were devouring it.
Roosevelt told the story in a book and also claimed that piranhas attacked defenseless humans. Readers were horrified, and piranhas were long considered man-eating monsters.
In recent years, scientists have analyzed the mechanisms behind the piranha’s ferocious bite and determined which parts of Roosevelt’s claims were fact or fiction.
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Piranha Jaw Strength
In response to Roosevelt’s book, a famous zoologist questioned whether piranhas existed, according to the Theodore Roosevelt Center. The little fish and the deadly force of its jaw seemed strange.
Piranha species like the black piranha (Serrasalmus rhombeus)However, are as powerful as Roosevelt promised. A study carried out in 2012 Scientific reports measured the bite force of the black piranha at 320 Newtons or 70 pounds.
How can such a small fish have such a powerful jaw? Scientists better understand how the piranha’s unique shape promotes its powerful bite. Piranhas have a disc-shaped body, which gives them space for a larger jaw as well as jaw muscle movements.
“This available space on their skull, which is larger in piranhas than in other fish of the same size, allows for a greater area of attachment and movement of the jaw muscles,” says Robert May, an aquarist at the Long Island Aquarium who oversees the piranha exhibit, which is home to red-bellied piranhas (Pygocentrus nattereri).
The Piranha’s jaws also have adductor muscles, which work with other muscles, so the greatest bite force impact occurs on the rear teeth. May says this allows the piranha to “clench its jaws with incredible power.”
The very sharp teeth of the Piranha
In his book, Roosevelt claimed that the school of piranhas stripped a cow of its meat and muscles in a matter of minutes. This event was organized so that the piranhas were hungry, anxious and aggressive. But their teeth are indeed capable of tearing animal flesh and devouring it quickly.
Piranhas’ teeth are spaced well apart, which helps them chew their food quickly. Because they have a larger head surface area, May says the teeth are distributed throughout their jaw in a way to avoid biting while chewing.
Working together, a piranha’s jaw strength and sharp teeth are so powerful that if it were larger, its bite would be comparable to that of some apex predators.
“So even though modern large sharks and alligators have a greater bite force than piranhas, if there were a piranha the same size as these large sharks and alligators, then the bite force of piranhas could certainly rival others,” says May.
Although piranhas have a powerful bite force and knife-like teeth, many potential piranha prey evolved with protective armor.
In a study carried out in 2023 in Integrative Organismal Biologyresearchers observed red-bellied piranhas in captivity as they attempted to prey on three-banded corydoras (Corydoras trilineatus). Three-banded corydoras had natural armor that often resisted piranha attack. They survived 37 percent of bites without injury, and it typically took the piranha nine shots to pierce the prey.
The Piranha’s reputation as a fish
Not every bite is a quick kill, but piranhas have developed a reputation for man-eating similar to the bad reputation sharks have received.
“They’re not the flesh-eating machines that Teddy Roosevelt’s journeys through the Amazon rainforest or Hollywood movies would have us believe, even though they eat mostly meat in a particularly showy way,” says May.
Horror stories about humans swimming in rivers and being attacked by piranhas have not been validated by science, and a study of injuries caused by aquatic animals in Brazil found that 88 percent came from stinging fish like stingrays or jellyfish, according to a study by the Journal of the Brazilian Society of Tropical Medicine.
Although it may seem that piranhas may rush to attack humans as soon as they become aware of their presence, well-fed piranhas are not particularly interested in humans. At the Long Island Aquarium, for example, an aquarist regularly climbs into the piranha habitat to clean and maintain it. The piranhas keep their distance and seem relieved when the human finally leaves.
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