Hollywood movies have given the piranha a fearsome reputation. In fact, across the different species of this globally famous fish there is considerable diet variation and the plant eating habits of some piranha have led to them being classified as omnivorous. Piranhas have only been known to eat small mammals when those creatures were already dead or incapacitated, and they certainly do not consider humans their natural prey.
Long before Hollywood movies like the 1970s cult classic “Piranha” gave these freshwater fish global fame as all-devouring hunters, the US president Theodore Roosevelt wrote of their supposed ferocity in his bestselling travelogue “Through the Brazilian Wilderness”, in which he chronicled his time in the rainforest. Roosevelt described piranha as hunting in packs, deadlier than “sharks or barracudas”, and being capable of stripping an entire cow to the bone. In fact, it was subsequently reported that local people had arranged a demonstration for the visiting American dignitary by starving captive piranha and then setting them loose upon a cow’s carcass.
Scientists estimate that there are somewhere between thirty and sixty species of piranha in the lakes and rivers of the Amazon basin. Most species appear to be restricted to individual river basins. Piranhas tend to measure between 14 and 26 centimeters in length, although some have been reported as measuring up to sixty centimeters. While different species of piranha are difficult to tell apart and much taxonomic work is yet to be done if we are to learn more about them, they are known to inhabit the waterways of South America from Venezuela’s Orinoco River to the Paraná River in northern Argentina.
Of course, it is their sharp teeth which have given piranha their fearsome but highly exaggerated reputation. The name “piranha” comes from the indigenous Brazilian Tupi language, and it means “tooth fish”. The tooth enamel of piranhas is similar to that of sharks, and their knife-like teeth are arranged into two interlocking rows. Their bite is strong, typically exerting a pressure equal to about three times their own bodyweight.
However, piranhas are far from voracious, usually consuming only about 12% of their own bodyweight in a single day. And those piranhas which do not subsist mostly on seeds and plant material tend to seek out insects, crustaceans, worms and carrion. Rather than forming packs to make them more formidable, they gather in shoals as a method of protection, as a way of reducing their chances of falling prey to genuine rainforest predators such as birds, caimans, giant river otters and other larger fish.
While red (Pygocentrus nattereri), and black (Serrasalmus rhombeus) piranhas are considered the most aggressive to humans, in fact they really only tend to bite people when fearing for their own safety, for example when cornered in particularly shallow water.