The red-bellied piranha or red piranha (Pygocentrus nattereri) is a species of piranha native to South America, found in the Amazon River Basin, coastal rivers of northeastern Brazil, and the basins of the Paraguay and Paraná. They are not a migratory species, but do travel to seek out conditions conducive to breeding and spawning during periods of increased rainfall. Red-bellied piranhas often travel in shoals as a predatory defense, but rarely exhibit group hunting behavior. Acoustic communication is common, and is sometimes exhibited along with aggressive behaviors. The red-bellied piranha is distributed widely throughout the South American continent and is found in the Neotropical freshwater rivers of Argentina, Brazil, Bolivia, Colombia, Ecuador, Guyana, Paraguay, Peru, Uruguay, and Venezuela. They live in the warm freshwater drainages of several major rivers including the Amazon, Rio Paraguay, Rio Paraná and Rio Essequibo, as well as numerous smaller systems. It is acclimated to waters that are between 15 and 35 °C and have a pH of 6 to 7 and is typically found in white water rivers and some freshwater streams and lakes. In the Brazilian Amazon, the red-bellied piranha may sometimes inhabit flooded forests. The red-bellied piranha has a popular reputation as a ferocious predator, despite being primarily a scavenger. As their name suggests, red-bellied piranhas have a reddish tinge to the belly when fully grown, although juveniles are a silver colour with darker spots. They grow to a maximum length of 33 centimetres (13 in) and a weight of 3.5 kilograms (7.7 lb). The rest of the body is often gray with silver-flecked scales. Sometimes, blackish spots appear behind the gills and the anal fin is usually black at the base. The pectoral and pelvic fins may vary from red to orange. Females can be distinguished from males by the slightly deeper red color of their bellies.