In Hollywood films piranhas have a reputation for being so aggressive that they can strip a body of flesh in just minutes - they were, for example, Bond villain Blofeld's favoured instrument of death in You Only Live Twice.

But it seems these Latin American freshwater fish are not the insatiable man-eaters of folklore, after all.

From today the new findings about piranhas - plus a tank full of them - will be on display at the Royal Society Summer Science Exhibition in London.

According to research, the main reason the scary fish patrol in shoals is for protection from their own predators.

The widely held view that piranhas form "co-operative hunting groups" is a myth that has helped turn them into film legends.

However, a research team from the University of St Andrews and the Mamiraua Sustainable Development Institute in Brazil say piranhas are not dangerous. The researchers have been studying piranha behaviour in the flooded forests of the Amazon.

Prof Anne Magurran, of St Andrews, said: "Contrary to popular belief, piranhas are omnivores. They are scavengers more than predators, eating mainly fish, plant material and insects.

"Piranhas are under constant attack from a number of large predators including river dolphins, caiman (a relative of the crocodile) and some of the world's largest fish. Their cautious behaviour is to avoid being eaten."

The professor added that generations of film-goers have grown up with an image of piranhas ripping people apart, but said: "That's not wholly fair. They can be aggressive if they are provoked or have limited space, but that sort of behaviour is the exception rather than the rule.

"You would not want to mess with them, and they do have very sharp teeth.

"If you throw them a chicken, they will strip it to a carcass in seconds. They are very impressive creatures and you can see why Hollywood loves them. But the legend has been exaggerated."

The scientists, working with the Zoological Society of London, which is providing the live piranhas, will be on hand at the Royal Society exhibition, at 6-9 Carlton House Terrace, central London, until Thursday.