What happens when you fall into piranha-infested waters? - Antonio Machado-Allison

You’re peering into the Amazon River when, suddenly, you lose your footing and fall. Piranhas dart about in the rapidly approaching water. So, are you doomed? Will your fall trigger a fatal feeding frenzy?

To forecast your fate, let’s see what we know about these fish. There are more than 30 piranha species. All live in the fresh waters of South America and have a single row of sharp, interlocking teeth on each jaw. They use their teeth in a variety of ways. Many are omnivorous and supplement diets of things like insects, crustaceans, worms, and fish with fruits, seeds, and other plant matter. Some, like red-bellied piranhas, both hunt and scavenge. And others, like wimple piranhas, have specific dietary predilections, almost exclusively going after other fish’s scales.

Species like redeye piranhas are more solitary, while red-bellied piranhas form shoals of 10 to 100. Red-bellied piranhas are among the most popularly depicted and commonly regarded as especially aggressive. However, their reputation for rapacious pack hunting is misinformed. It’s thought that the main benefit of their group-living isn’t cooperative hunting but instead protection from predators, of which they have many. Larger, mature red-bellied piranhas tend to assume privileged positions at the shoal’s center, where it’s safest. And scientists have observed that red-bellied piranhas in smaller groups breathe faster, probably because they’re more anxious.

Interestingly, they also communicate. By rapidly contracting specialized muscles above their swim bladders, they repeatedly “bark” when they’re facing off with one another or when captured. They make thudding noises when they’re aggressively circling each other, fighting, or competing for food. And when things escalate further, they chase each other while snapping their jaws together. Researchers suspect that these sounds are just a sampling of their overall repertoire, which might also have some special uses during mating.

But when do red-bellied piranhas get aggressive with humans? Well, when they do bite people, it seems to mostly happen in scenarios when they’re being handled; when people are spilling food or cleaning their fishing catch in the water; or when people disturb piranhas while the fish are mating or guarding their eggs during the wet season. Starvation stress is also thought to lead red-bellied piranhas to increasingly bold, aggressive behavior. And this could theoretically result in feeding frenzies where each fish tries to get some of whatever finds its way into the water.

Despite this kind of behavior being extremely rare, rumors of it launched the piranha’s international infamy. And this was in no small part thanks to former US President Theodore Roosevelt. In 1914, he published a bestselling book in which he called piranhas “the most ferocious fish in the world” and wrote that the scent of blood could incite them to rapidly devour an entire cow—or human—alive. But Roosevelt’s account is generally considered circumstantial and misleading. The “feeding frenzy” he witnessed is suspected to have been the result of people purposefully starving red-bellied piranhas, then giving them the opportunity to feed on a cow carcass—all to put on an exciting show.

But where were we? Ah, yes. Falling into piranha-infested waters. So, what’s your fate? Let’s assume these are red-bellied piranhas. This being the Amazon River, they should be doing alright for themselves and not starving. Thankfully, you’re also not hitting the water alongside a bunch of fish guts. And ideally, you're not disrupting a piranha breeding extravaganza.

You fall in, and the piranhas most likely avoid you. Calmly, softly swimming or wading to shore is generally recommended because splashing is thought to attract piranhas. Indeed, they’re equipped with the dentition to do damage, but they rarely attack humans. They usually have better things to eat. As you make your way onto dry land, there is no feeding frenzy where they skeletonize your body within minutes. And upon exiting the water, you're probably pleased to find no chunks of flesh missing.