title: Boundary information inflow enhances correlation in flocking.
abstract: The most conspicuous trait of collective animal behavior is the emergence of highly ordered structures. Less obvious to the eye, but perhaps more profound a signature of self-organization, is the presence of long-range spatial correlations. Experimental data on starling flocks in 3D show that the exponent ruling the decay of the velocity correlation function, C(r)~1/r(γ), is extremely small, γ<<1. This result can neither be explained by equilibrium field theory nor by off-equilibrium theories and simulations of active systems. Here, by means of numerical simulations and theoretical calculations, we show that a dynamical field applied to the boundary of a set of Heisenberg spins on a 3D lattice gives rise to a vanishing exponent γ, as in starling flocks. The effect of the dynamical field is to create an information inflow from border to bulk that triggers long-range spin-wave modes, thus giving rise to an anomalously long-ranged correlation. The biological origin of this phenomenon can be either exogenous-information produced by environmental perturbations is transferred from boundary to bulk of the flock-or endogenous-the flock keeps itself in a constant state of dynamical excitation that is beneficial to correlation and collective response.
authors.affiliation: Istituto dei Sistemi Complessi, CNR, via dei Taurini 19, I-00185 Roma, Italy.
authors.forename: Andrea
authors.initials: A
authors.lastname: Cavagna
authors.forename: Irene
authors.initials: I
authors.lastname: Giardina
authors.forename: Francesco
authors.initials: F
authors.lastname: Ginelli
mesh_terms: D000818:Animals; D001522:Behavior, Animal; D003198:Computer Simulation; D008954:Models, Biological; D013037:Spatial Behavior