LONDON, July 2 (Reuters) - 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 behaviour," 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 behaviour 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.

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