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, 1 (2), 155-7

Safety in Numbers? Shoaling Behaviour of the Amazonian Red-Bellied Piranha


Safety in Numbers? Shoaling Behaviour of the Amazonian Red-Bellied Piranha

Helder Queiroz et al. Biol Lett.


Red-bellied piranha (Pygocentrus nattereri) shoals have a fearsome reputation. However, the variety and abundance of piranha predators in the flooded forests of the Amazon in which they live indicate that an important reason for shoal formation may be predator defence. Experiments using wild-caught piranhas supported the hypothesis that individual perception of risk, as revealed by elevated ventilatory frequency (opercular rate), is greater in small shoals. Moreover, exposure to a simulated predator attack by a model cormorant demonstrated that resting opercular rates are regained more quickly by piranhas in shoals of eight than they are in shoals of two. Together, these results show that shoaling has a cover-seeking function in this species.


Figure 1
Figure 1
Opercular rate (per minute) of the focal individual as a proportion of the singleton's opercular rate (indicated by the line through unity) in a set of four tests. Mean value (± s.e.) is shown.
Figure 2
Figure 2
Mean opercular rate (± s.e.) of the focal individual in shoals of two and eight, in the 5 min period following predator attack, as a proportion of its baseline value (indicated by the line through unity). Diamond symbols represent shoals of two, and square symbols represent shoals of eight.

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