The exceptional reactivity of animal collectives to predatory attacks is
thought to be due to rapid, but local, transfer of information between group
members. These groups turn together in unison and produce escape waves.
However, it is not clear how escape waves are created from local interactions,
nor is it understood how these patterns are shaped by natural selection. By
startling schools of fish with a simulated attack in an experimental arena, we
demonstrate that changes in the direction and speed by a small percentage of
individuals that detect the danger initiate an escape wave. This escape wave
consists of a densely packed band of individuals that causes other school
members to change direction. In the majority of cases this wave passes through
the entire group. We use a simulation model to demonstrate that this mechanism
can, through local interactions alone, produce arbitrarily large escape waves.
In the model, when we set the group density to that seen in real fish schools,
we find that the risk to the members at the edge of the group is roughly equal
to the risk of those within the group. Our experiments and modelling results
provide a plausible explanation for how escape waves propagate in Nature
without centralised control
