THE ANIMAL FILES

Red-bellied piranha

Pygocentrus nattereri

A school of piranhas can be seeking protection, not organising a hunt.

The red-bellied piranha eats a mixed diet and lives among predators of its own. Shoaling, scavenging and social sounds make its ecology more varied than the familiar image of a coordinated attack.[1]

Follow the evidence ↓
Red-bellied piranha
Image: Gregory Moine from Niantic, USA · Commons · CC BY 2.0
Diet
Fish, invertebrates, plants and carrion[1]
Shoals
Grouping can reduce vulnerability[1]
Sound
Muscles associated with the swimbladder[1]

A mixed feeding strategy

Fruit, leaves, insects, molluscs, fish and carrion are recorded foods. Feeding changes with circumstances and life stage, and includes scavenging as well as pursuing prey. Sharp teeth do not by themselves define every meal.[1]

Safety in a shoal

Experiments comparing differently sized shoals found that breathing rates returned to baseline faster in larger groups after a simulated predator attack. Larger fishes, aquatic birds and caimans can themselves prey on piranhas. Grouping therefore has a defensive context.[1]

Disputes have a soundtrack

Sound production involves rapid muscle contractions associated with the swimbladder. Different sounds have been linked with social behaviours such as displays, food competition and chasing. These are observed associations, not a translated vocabulary for every call.[1]

A COMMON MIX-UP

Does a shoal prove cooperative pack hunting?

No. Grouping can provide protection, and the cited shoaling studies do not establish coordinated hunting of large prey.[1]

TRY A LITTLE REASONING

What observation would help test a pack-hunting claim?

Reveal the explanation

One would need evidence of individuals coordinating a capture, rather than merely swimming together near food.[1]

Sources & editorial notes

Adapted and translated from the cited Wikipedia revisions, with source-linked reasoning. Image credit and licence are listed separately above.

Adapted and translated from Wikipedia contributors; exact revisions are linked below. CC BY-SA 4.0

  1. Wikipedia contributors — Red-bellied piranha (fixed revision; adapted and translated)
  2. NCBI Taxonomy — Pygocentrus nattereri (identity and classification)

Last fact check:

One discovery leads to another

Channel catfishThe whiskers are sensory tools, not just decoration.Ictalurus punctatus ↗electric eelThe electricity is both a sense and a tool.Electrophorus electricus ↗Wels catfishSix barbels surround a broad mouth that explores a varied menu.Silurus glanis ↗