title: Interaction ruling animal collective behavior depends on topological rather than metric distance: evidence from a field study.
abstract: Numerical models indicate that collective animal behavior may emerge from simple local rules of interaction among the individuals. However, very little is known about the nature of such interaction, so that models and theories mostly rely on aprioristic assumptions. By reconstructing the three-dimensional positions of individual birds in airborne flocks of a few thousand members, we show that the interaction does not depend on the metric distance, as most current models and theories assume, but rather on the topological distance. In fact, we discovered that each bird interacts on average with a fixed number of neighbors (six to seven), rather than with all neighbors within a fixed metric distance. We argue that a topological interaction is indispensable to maintain a flock's cohesion against the large density changes caused by external perturbations, typically predation. We support this hypothesis by numerical simulations, showing that a topological interaction grants significantly higher cohesion of the aggregation compared with a standard metric one.
authors.affiliation: Centre for Statistical Mechanics and Complexity, Consiglio Nazionale delle Ricerche-Istituto Nazionale per la Fisica della Materia, Dipartimento di Fisica, and Sezione Instituto Nazionale di Fisica Nucleare, Universita' di Roma La Sapienza, Roma, Italy.
authors.forename: M
authors.initials: M
authors.lastname: Ballerini
authors.forename: N
authors.initials: N
authors.lastname: Cabibbo
authors.forename: R
authors.initials: R
authors.lastname: Candelier
authors.forename: A
authors.initials: A
authors.lastname: Cavagna
authors.forename: E
authors.initials: E
authors.lastname: Cisbani
authors.forename: I
authors.initials: I
authors.lastname: Giardina
authors.forename: V
authors.initials: V
authors.lastname: Lecomte
authors.forename: A
authors.initials: A
authors.lastname: Orlandi
authors.forename: G
authors.initials: G
authors.lastname: Parisi
authors.forename: A
authors.initials: A
authors.lastname: Procaccini
authors.forename: M
authors.initials: M
authors.lastname: Viale
authors.forename: V
authors.initials: V
authors.lastname: Zdravkovic
mesh_terms: D000374:Aggression / Q000502:physiology; D000465:Algorithms; D000818:Animals; D001522:Behavior, Animal / Q000502:physiology*; D001717:Birds / Q000502:physiology*; D005426:Flight, Animal / Q000502:physiology*; D011235:Predatory Behavior / Q000502:physiology; D012931:Social Environment; D013997:Time Factors; D014785:Vision, Ocular / Q000502:physiology