Initiation and spread of escape waves within animal groups

TL;DR

This study investigates how escape waves originate and spread in animal groups, demonstrating through experiments and models that local interactions and small individual responses can produce large, coordinated escape behaviors without central control.

Contribution

It reveals the mechanism by which local interactions trigger large-scale escape waves in animal groups, supported by experimental and simulation evidence.

Findings

Escape waves are initiated by small percentage of individuals detecting danger.

Escape waves propagate through densely packed groups, affecting most members.

Model shows local interactions alone can generate arbitrarily large escape waves.

Abstract

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.