Piranha! The true story of one of nature's most feared animals

Ms Amy Deacon, Professor Anne Magurran and Dr Iain Matthews, School of Biology,University of St Andrews
Dr Helder Queiroz, Instituto de Desenvolvimento Sustentável Mamirauá
Ms Alison Shaw and Mr Brian Zimmerman, Zoological Society of London

Do Piranhas deserve their reputation as vicious predators?

Researchers from the University of St Andrews, Mamirauá Institute (Amazonas, Brazil) and the Zoological Society of London are conducting research on an often misunderstood animal – the piranha.

Fish usually form schools to gain protection from predators. However, rather than schooling for its ‘safety in numbers’ advantages, it was thought that red-bellied piranhas congregate in cooperative hunting groups.

“Piranhas have a fierce reputation, but through our research we are finding many of their behavioural choices are driven by their fear of being eaten,” says Professor Anne Magurran from the University of St Andrews. “Piranhas face a number of predators and are influenced by the availability of food and cover.” “One interesting aspect of our research in Amazonia’s flooded forests has been the seasonal differences in the piranhas’ behaviour,” says Anne. “During high water levels piranha can find food and protection easily. During the low water season, they are more at risk of attacks from predators such as dolphins and caiman.”

“We are showing that piranha schooling behaviour is tuned to the seasonal cycle of predation risk and food availability"", explains Anne. ""We are also examining the changes in an individual piranha’s behaviour throughout its life-cycle, and the way in which these choices influence its chances of breeding and passing genes onto the next generation. Our investigation is giving us new insights into animal decision making in the wild.”