Collective response to local perturbations: how to evade threats without losing coherence

TL;DR

This study explores how groups of moving individuals respond collectively to local disturbances, revealing that efficient information transfer and moderate motility are crucial for coordinated turns without fragmentation.

Contribution

It demonstrates that collective directional changes are finite-size effects and identifies key conditions for effective global responses in self-propelled particle models.

Findings

Global response time scales with group size

Efficient information propagation is necessary for coherence

High motility can prevent successful collective turns

Abstract

Living groups move in complex environments and are constantly subject to external stimuli, predatory attacks and disturbances. An efficient response to such perturbations is vital to maintain the group's coherence and cohesion. Perturbations are often local, i.e. they are initially perceived only by few individuals in the group, but can elicit a global response. This is the case of starling flocks, that can turn very quickly to evade predators. In this paper, we investigate the conditions under which a global change of direction can occur upon local perturbations. Using minimal models of self-propelled particles, we show that a collective directional response occurs on timescales that grow with the system size and it is, therefore, a finite-size effect. The larger the group is, the longer it will take to turn. We also show that global coherent turns can only take place if i) the mechanism for information propagation is efficient enough to transmit the local reaction undamped through the whole group; and if ii) motility is not too strong, to avoid that the perturbed individual leaves the group before the turn is complete. No compliance with such conditions results in the group's fragmentation or in a non-efficient response.