Signatures of directed and spontaneous flocking

TL;DR

This paper develops criteria based on fluctuation analysis to detect directed collective motion in biological systems, enabling identification without environmental correlation detection.

Contribution

It introduces two novel criteria using fluctuation and structure factor analysis to identify directed flocking from measurable signatures.

Findings

Criteria effectively distinguish directed from spontaneous flocking.

Static criterion works well in large systems.

Dynamic criterion requires long observation times.

Abstract

Collective motion - or flocking - is an emergent phenomena that underlies many biological processes of relevance, from cellular migrations to animal groups movement. In this work, we derive scaling relations for the fluctuations of the mean direction of motion and for the static density structure factor (which encodes static density fluctuations) in the presence of a homogeneous, small external field. This allows us to formulate two different and complementary criteria capable of detecting instances of directed motion exclusively from easily measurable dynamical and static signatures of the collective dynamics, without the need to detect correlations with environmental cues. The static one is informative in large enough systems, while the dynamical one requires large observation times to be effective. We believe these criteria may prove useful to detect or confirm the directed nature of collective motion in in vivo experimental observations, which are typically conducted in complex and not fully controlled environments.