Piranhas less aggressive than thought

This video says about itself:

Swimming With Red Belly Piranhas – River Monsters

29 April 2015

Jeremy has a pool filled with 100 Red Belly Piranhas, to demonstrate how they will only attack when blood and flesh are thrown into the water.

The video shows something different from the traditional view on piranhas.

The red bellied piranha (Pygocentrus nattereri) inhabits all of the Amazon basin.

The animals grow up to 20 – 30 cm.

Reuters reports:

LONDON – Despite their fearsome reputation, piranhas are wimps that gather in large shoals to protect themselves from predators, scientists said on Monday.

Rather than aggressive killers, research shows piranhas are omnivorous scavengers, eating mainly fish, plants and insects, Anne Magurran of Scotland’s University of St Andrews said.

“Previously it was thought piranhas shoaled as it enabled them to form a cooperative hunting group. However, we have found that it is primarily a defensive behavior,” she said.

Piranhas face constant attack from predators including river dolphins, caiman — a relative of the crocodile — and bigger fish, such as the giant piracucu.

“Their cautious behavior is crucial to avoid being eaten,” Magurran said.

Her work with the Mamiraua Institute in Brazil shows how shoal sizes increase in relation to predation risk, especially when water levels in the Amazon basin are low, giving piranhas less room to escape attack.

The research is featured at the Royal Society‘s summer science exhibition in London.