Article by Amy Deacon
“People eat fish, Grogan. Fish don’t eat people” reassures the camp leader in the film ‘Piranha’, shortly before a shoal of incredibly voracious fish turn the waters alongside the camp site red, in a savage attack on innocent bathers.
In this fictional Hollywood tale, the fish were escaped stock left over from a decommissioned military experiment, and had been genetically modified to be exceptionally good at finding and devouring human flesh.
In December, newspapers carried reports of a real-life piranha attack that seemed to turn this fiction into a true horror story. Seventy weekend holidaymakers were attacked while bathing in the river near the town of Rosario, Argentina. Thankfully, unlike the Hollywood version, no one was killed, although bathers sustained a variety of unpleasant injuries to their limbs and digits.
The species in this case was most likely Pygocentrus palometa, a close relative of the famous red-bellied piranha, Pygocentrus natterii.
Having spent many a science fair pleading innocence on behalf of this fascinating yet harmless creature, my heart sank at the news coverage of this latest, highly unusual, incident. Just when I felt the message might be getting through that piranhas are not the ferocious killers portrayed in the films, the Rosario incident reinforced the view that they are terrifying pack-hunting flesh-eaters.
The truth is that the majority of species of piranha (there are more than thirty) are vegetarian. Even the so-called ‘carnivorous’ types (including Pygocentrus spp.) are at best omnivorous, consuming a wide and varied diet, with a ‘meat’ component consisting of insects and small fish rather than cows and people. It is undeniable that some species have extremely sharp teeth, but the most common ‘victims’ of their bite are the fishermen who have to remove hooks from their mouths, shortly before turning the poor piranhas into soup (which is delicious, apparently).
The life of a piranha is a perilous one. Caiman, cormorants, river dolphin and the giant pirarucu, a fish reaching 2 metres in length, are just some of the terrifying predators that piranhas have to contend with. (If you want to know what this feels like, try playing ‘Piranharama’, a computer game we’ve used at science fairs to challenge people’s perception of piranhas as bloodthirsty hunters).
There are multiple mechanisms by which being in a group is an advantage to an individual – the confusion effect (making it hard for a predator to focus in on any one prey), the dilution effect (the reduced probability of any one member being eaten), and vigilance benefits (more eyes mean increased likelihood of spotting predators).
While people readily accept that sardines, mackerel, minnows or guppies shoal for these reasons, the ideas of Hollywood have penetrated popular culture so deeply that it is hard to convince the general public that piranha shoaling behaviour is anything other than sinister.
However, after decades of bad press (which can perhaps be traced back to Blofeld’s piranha-filled execution pool in 1976’s ‘You Only Live Twice’), science has finally cleared the name of these, surprisingly beautiful, fish by means of a neat animal behaviour experiment.
The research, which was undertaken by my colleagues at the University of St Andrews, Scotland and Brazil’s Mamirauá Institute, has quashed the notion that piranhas shoal in large groups as a pack-hunting strategy.
Having already established that piranhas appear to use the same rules as other fish when deciding who to shoal with and where to be within a shoal, Anne Magurran and Helder Queiroz decided to look more closely at fear responses. By measuring an individual’s ‘opercular rate’ (i.e. how fast they are breathing - a proxy for fearfulness) when alone and when in different sized groups, they demonstrated that piranhas in larger shoals are far less stressed than those alone or in small shoals. A second experiment revealed that those in larger groups recovered (returned to a ‘normal’ opercular rate) more quickly after exposure to a scary model predator (a cormorant).
The best explanation for these findings is that individuals feel (and are) safer in a group, and reminds us that in all probability piranhas are no different from other shoaling fish – such as the guppies that I study – in that the purpose of shoaling is primarily safety from predators.
So, if that is the case, how do we explain the Rosario incident? Well, this is where I am forced to abandon science in favour of speculation. It seems to me that this was an exceptional circumstance – an extremely hot day in an unusually hot year, which not only concentrated the fish into a smaller volume of water, but also encouraged a much higher density of bathers. Once one bather had been accidentally ‘nicked’ by a passing piranha, this may have triggered feeding behaviour in the shoal, but they certainly would not have been purposefully hunting for human flesh.
Piranhas are abundant in the South American rivers in which they live, but for some other species the bad PR from sensationalised Hollywood films can have serious implications for their conservation status. ‘Jaws’ is one of my all-time favourite films, but it has a lot to answer for even nearly forty years later when it is still seen as acceptable in places like Western Australia to cull a population of sharks after a few isolated, albeit tragic, incidents.
As with piranhas, ‘attacks’ on humans by sharks are extremely rare, and tend to be the result of highly unusual circumstances. Behavioural research confirms that shark distribution is determined by environmental conditions and prey abundance, not on the presence of humans in the water as depicted in the movies. In this case we have an opportunity to influence human behaviour by learning more, and educating people, about animal behaviour.
Science is at its most exciting and important when it can challenge deeply engrained public perceptions with facts – but it is the responsibility of scientists to convey this information more effectively than Hollywood’s sensationalised versions, and that is quite a public engagement task!
For some more evil-animal Hollywood stereotypes, check out not bad science's Top 10 horror movies featuring animals
Amy Deacon is a postdoctoral researcher at the University of St Andrews, and a visiting researcher at the University of the West Indies. She studied invasive guppy behaviour for her PhD, and now spends her time surveying the biodiversity of Trinidadian rivers. During her PhD she became involved in public outreach activities, including touring with an exhibition disseminating the results of her research groups’ work on piranha behaviour. She's also a very talented artist, and painted the picture for not bad science.
Red-bellied piranha: Cliff
Movie posters: Piranha, 1978
Magurran, A.E. & Queiroz, H.L. (2003) Partner choice in piranha shoals. Behaviour, 140, 289-299.
Queiroz, H. L. & Magurran, A.E. (2005) Safety in numbers? Shoaling behaviour of the Amazonian red-bellied piranha. Biology Letters, 1, 155-157.