Information spread enhanced by criticality in high-responsive groups of fish

Collective dynamics in animal groups has been studied in recent years intensively. Recent works have suggested that such multi-agent systems should operate in a special parameter region, close to critical points. This is relevant because critical systems exhibit unique properties like maximal responsiveness to external stimuli and optimal propagation of information within the group. However, empirical support for critical dynamics in animal groups is still very limited. We study high-density giant schools of fish (sulphur mollies; up to 3000 fish/m2) which in their natural habitat (sulphuric ponds/streams) are mostly confined to the surface. These fish exhibit a collective wave-like diving response to potential threats. We measure and quantify the "resting state" of the collective diving activity driven by environmental fluctuations, strongly resembling noise-driven excitable systems. Our quantitative analysis of experimental data suggests that the system operates close to criticality. By a systematic comparison of the system dynamics with a generic mathematical model, we conclude that this natural system operates indeed in a special parameter region close to a critical point. We explore potential functional benefits with respect to collective predator evasion.