Despite their dangerous reputation in horror films and the media at large, the majority of the thirty known species of piranha, which live in the fresh waters of South America, are actually herbivores.
Only the four genera Pristobrycon, Pygocentrus, Pygopristis, and Serrasalmus are considered to be true piranhas due to their specialised teeth, which are tightly packed together and interlock for rapid puncture and shearing.
However, attacks on humans and large mammals are rare and generally occur only during periods of drought when lower water levels result in fewer fish for piranhas to prey upon. Whilst these attacks usually only result in injury they can still be lethal, though most recorded cases involve small children.
The reputation of piranhas may date back to a book written by President Roosevelt in 1914, in which he recounts a visit to Brazil when local fishermen blocked off part of a river and starved a shoal of piranhas for several days. They then pushed a cow into the water, where it was quickly torn apart.