Putting the bite on piranha myth

Published on Monday 2 July 2007 00:15

PIRANHA may not deserve their reputation as the ferocious man-eaters that almost made mincemeat of James Bond, new research shows.

Scientists found that the legendary South American fish swim in dense shoals not to hunt down victims but to protect themselves.

The fish, which inhabit flooded Amazon forests, are usually depicted as merciless killers which overwhelm large animals by force of numbers and strip them to the bone with razor-sharp teeth.

In the Bond film You Only Live Twice, super-villain Ernst Blofeld uses piranha to dispose of his enemies.

But researchers from St Andrews University claim the creatures are seriously misunderstood.

The researchers have been studying piranha behaviour in their natural habitat, working in a reserve that covers an area over half the size of Wales. And the biologists have not only discovered why piranha shoal, but also how this behaviour changes in relation to the level of danger from predators.

Professor Anne Magurran, of St Andrews University, who is investigating the fish in collaboration with Dr Helder Queiroz of the Mamiraua Sustainable Development Institute in Brazil, said: "Contrary to popular belief, and their sharp teeth, piranha are omnivores. They are scavengers more than predators, eating mainly fish, plant material and insects.

"Piranha 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, such as the pirarucu, which can grow up to three metres long. Their cautious behaviour is crucial to avoid being eaten.

"We have grown up with this image of them on the movie screen ripping people apart, but that is not being wholly fair to them. They can be aggressive if they are provoked or have limited space ... but that sort of behaviour is the exception rather than the rule.

"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, although they always deserve our respect."

The team also identified a distinct pattern to the shoals: larger, more mature piranha place themselves in the middle of the group, surrounded by smaller, younger fish.

Prof Magurran said: "Our study has given us a better understanding of the function of shoaling. Previously it was thought piranha shoaled as it enabled them to form a co-operative hunting group. However, we have found that it is primarily a defensive behaviour, and quite a complex one.

"There is a distinct structure. But there is no 'lead fish' or co-operation - the piranha pretty much look out for themselves. The bigger, older fish tend to swim in the middle as they are reproductively mature and need to keep safe, whereas the outer layer of the shoal is made up of smaller, younger piranha.

"Being at the edge of the group means they can get access to food sooner, essential if they are to grow more quickly."

The researchers also noticed that the size of the shoal changed in relation to predation risk. The water level of the flooded forest is dependent on seasonal rainfall and ice melt from the Andes, changing by up to 12m during the year.

Prof Magurran added: "We realised that the shoaling behaviour of the piranha depends on the water level. At high water the fish swim in small shoals, as there is more space to avoid predators, and the threat is low. However, when the water level drops, isolating the flood water into smaller lakes and channels, shoals can grow to over 50 fish due to the close proximity of predators such as caiman and pirarucu."

The scientists, working with the Zoological Society of London - who are providing a tank of live piranhas - are displaying their findings at the Royal Society Summer Science Exhibition, which runs in London until Thursday.

• THE word "piranha" comes from the South American Guarani language, meaning "devil fish" or "tooth fish".

Local people traditionally used the teeth of the fish to make both tools and weapons.

Piranha are said to be quite tasty, although anyone catching one should reel them in quickly, as a piranha on a hook is likely to be attacked by others in the shoal.

There are numerous species of piranha, including some who feed only on fruit. Piranhas only rarely attack mammals in the water; attacks on humans are rarer still. But anyone keeping them in a fish tank should not wiggle their fingers in the water.