Piranha Shoals

In 1913, US President Theodore Roosevelt went on a hunting expedition in the Amazon where he witnessed a group of hungry Piranhas attacking a fully-grown cow, devouring it in minutes. Roosevelt would later depict them as vicious killers in his 1914 book Through the Brazilian Wilderness, and thus their reputations as bloodthirsty predators were sealed in the popular imagination. But like all popular legends, the truth is less straightforward.

While it was previously thought that Piranhas shoaled because it enabled them to hunt more effectively as a cooperative group, research has shown that their shoaling is actually a defensive behaviour as they seek safety in numbers. Far from the voracious villains that eat human victims alive in James Bond films, Piranhas, much like many other fish, school to minimize the threats posed to them by larger predators such as dolphins, aquatic birds and larger fish.

The size of the shoal itself also depends on the level of threat they face - when water levels are high and the risk of attack is lower, shoal sizes are smaller, but in drier conditions when Piranhas are vulnerable to attack, the size of the shoal increases.

The shoals also have a defined social structure, with older and larger fish being protected in the centre and younger and smaller fish keeping towards the edges where they have easier access to food. In short, contrary to their reputations as frenzied killers, Piranhas are actually far more cautious and opportunistic than previously realised, and it is their cautious natures that help them avoid becoming dinner themselves.