Red-bellied piranhas often travel in shoals as a predatory defense but rarely exhibit group hunting behavior.

The red-bellied piranha belongs to the family Serrasalmidae, which is a group of medium to large-sized characids and includes other closely related omnivores such as pacus.

[6] Within the family, red-bellied piranhas are classified in the genus Pygocentrus, which is distinguished by the unusual dentition and differing head width dimensions.

[6] The red-bellied piranha is distributed widely throughout the South American continent and is found in the Neotropical rivers of Argentina, Brazil, Bolivia, Colombia, Ecuador, Guyana, Paraguay, Peru, Uruguay, and Venezuela.

[7] They live in the warm freshwater drainages of several major rivers including the Amazon, Paraguay, Paraná, and Essequibo, as well as numerous smaller systems.

[14] As their name suggests, red-bellied piranhas have a reddish tinge to the belly when fully grown, although juveniles are a silver color with darker spots.

[7] A significant amount of lore regarding the ferocity of the red-bellied piranha can be traced to President Theodore Roosevelt following the completion of the Roosevelt-Rondon Scientific Expedition.

This event was later found to be staged by local guides, eager to provide Roosevelt with a spectacle worthy of the journey.

It later surfaced the fish had been corralled and starved for over a week, into a section of the river, into which the unfortunate cow was driven for the event.

[citation needed] Pygocentrus nattereri encompasses a larger geographic area than any other piranha species, covering much of the Neotropical region.

When red-bellied piranhas are introduced to other parts of the American continent, there are usually negative consequences for the local fish fauna,[15] partially due to its generally aggressive behavior.

[17]: 288 The typical diet of red-bellied piranhas is omnivorous[18] including fruit,[19][18] leaves,[18] insects,[18] mollusks,[18] carrion[18] and fish.

Despite the piranha's reputation as a dangerous carnivore, it is actually primarily a scavenger and forager, and will mainly eat plants and insects during the rainy season when food is abundant.

When scavenging, the piranha will eat a wide variety of food, ranging from pieces of debris, insects, snails, fish fins, scales, and plants.

[21] When individuals are ready to become sexually active, they will lose their red coloration and select habitats that are conducive to spawning, such as flooded marginal grasses and vegetation within lakes.

[22] Most likely, this shoaling behavior is a defense against predation from larger animals such as dolphins, large piscivorous fish, caimans, and aquatic birds such as storks, herons, and anhingas.

[22] Piranhas will travel to their nesting sites in shoals in order to reduce the likelihood that any single individual will be attacked by a predator.

[23] Acoustic communication among red-bellied piranhas is exhibited along with aggressive behaviors, such as biting, chasing, conspecific confrontation, and fighting.

Type one calls are made up of harmonic sounds, last approximately 140 milliseconds at 120 Hz, and are associated with frontal display behavior between two fish.

Type two sounds last approximately 36 milliseconds at 40 Hz, and are associated with circling and fighting behavior related to food competition.

Type three sounds are made up of a single pulse lasting just 3 milliseconds at 1740 Hz, and are highly associated with chasing behavior toward a conspecific individual.

Films such as these, and stories of large schools of red-bellies attacking humans, fuel their exaggerated and erroneous reputation as being one of the most ferocious freshwater fish.