What happens when you fall into piranha-infested waters?

Imagine you're gazing into the depths of the Amazon River when, unexpectedly, you lose your footing and plunge into the water. As you fall, you notice piranhas darting around in the swiftly approaching water. The question arises: are you in peril? Will your plunge trigger a deadly feeding frenzy among these voracious creatures? To predict your fate, let's delve into what we know about these intriguing fish.

There are over 30 species of piranhas, all of which inhabit the freshwater ecosystems of South America. One distinguishing feature they share is a single row of sharp, interlocking teeth on each jaw. These teeth serve a variety of purposes. Many piranhas are omnivorous, supplementing their diets with insects, crustaceans, worms, and other fish, as well as fruits, seeds, and plant matter. Some, like the red-bellied piranhas, are both hunters and scavengers. On the other hand, species like wimple piranhas exhibit specific dietary preferences, primarily targeting other fish scales.

When it comes to social behaviour, some piranhas, like redeye piranhas, tend to be more solitary, whereas red-bellied piranhas form shoals ranging from 10 to 100 individuals. Although red-bellied piranhas are often depicted as particularly aggressive, their reputation for pack hunting is somewhat inaccurate. It is believed that the primary advantage of their group living is not cooperative hunting but protection from numerous predators. Within these shoals, larger and mature red-bellied piranhas typically occupy central positions where they are safer. Interestingly, smaller groups of red-bellied piranhas exhibit faster breathing, possibly due to increased anxiety. These fish also communicate with each other through specialized muscle contractions above their swim bladders, emitting "barks" during confrontations or when captured, and producing thudding noises during aggressive interactions or competition for food.

So, when do red-bellied piranhas become aggressive towards humans? Incidents of piranha bites on humans mostly occur when the fish are being handled, during activities such as food preparation or cleaning fish in the water, or when humans disturb piranhas during mating or egg-guarding periods in the wet season. Starvation stress can also lead to bolder and more aggressive behaviour, potentially resulting in feeding frenzies where each fish competes for available food. However, such behavior is extremely rare, and the piranha's international notoriety can be traced, in part, to former US President Theodore Roosevelt.

In 1914, Roosevelt published a bestselling book in which he famously referred to piranhas as "the most ferocious fish in the world" and claimed that the scent of blood could provoke them to rapidly consume an entire cow or even a human alive. However, Roosevelt's account is generally regarded as anecdotal and misleading. The "feeding frenzy" he witnessed is believed to have been a staged event, where red-bellied piranhas were intentionally starved and then presented with the opportunity to feed on a cow carcass to create a sensational spectacle.

Now, returning to the scenario of falling into piranha-infested waters in the Amazon River, what would be your fate? Let's assume these are red-bellied piranhas, and they are not starving. Fortunately, you are not plunging into the water alongside a load of fish entrails, nor are you disturbing a piranha breeding event.

As you tumble into the water, the piranhas are likely to steer clear of you. Experts recommend calmly and gently swimming or wading to the shore because splashing can attract piranhas. Although they possess formidable teeth, piranhas seldom attack humans as they typically have more suitable prey to target. As you make your way to dry land, there is no frenzied assault, and your body remains intact. Upon exiting the water, you'll likely be relieved to find no missing chunks of flesh.