title: Emergent oscillations assist obstacle negotiation during ant cooperative transport.
abstract: Collective motion by animal groups is affected by internal interactions, external constraints, and the influx of information. A quantitative understanding of how these different factors give rise to different modes of collective motion is, at present, lacking. Here, we study how ants that cooperatively transport a large food item react to an obstacle blocking their path. Combining experiments with a statistical physics model of mechanically coupled active agents, we show that the constraint induces a deterministic collective oscillatory mode that facilitates obstacle circumvention. We provide direct experimental evidence, backed by theory, that this motion is an emergent group effect that does not require any behavioral changes at the individual level. We trace these relaxation oscillations to the interplay between two forces; informed ants pull the load toward the nest whereas uninformed ants contribute to the motion's persistence along the tangential direction. The model's predictions that oscillations appear above a critical system size, that the group can spontaneously transition into its ordered phase, and that the system can exhibit complete rotations are all verified experimentally. We expect that similar oscillatory modes emerge in collective motion scenarios where the structure of the environment imposes conflicts between individually held information and the group's tendency for cohesiveness.
authors.affiliation: Department of Physics of Complex Systems, Weizmann Institute of Science, Rehovot 7610001, Israel.
authors.forename: Aviram
authors.initials: A
authors.lastname: Gelblum
authors.affiliation: Department of Chemical Physics, Weizmann Institute of Science, Rehovot 7610001, Israel.
authors.forename: Itai
authors.initials: I
authors.lastname: Pinkoviezky
authors.affiliation: Department of Physics of Complex Systems, Weizmann Institute of Science, Rehovot 7610001, Israel.
authors.forename: Ehud
authors.initials: E
authors.lastname: Fonio
authors.affiliation: Department of Chemical Physics, Weizmann Institute of Science, Rehovot 7610001, Israel ofer.feinerman@weizmann.ac.il nir.gov@weizmann.ac.il.
authors.forename: Nir S
authors.initials: NS
authors.lastname: Gov
authors.affiliation: Department of Physics of Complex Systems, Weizmann Institute of Science, Rehovot 7610001, Israel; ofer.feinerman@weizmann.ac.il nir.gov@weizmann.ac.il.
authors.forename: Ofer
authors.initials: O
authors.lastname: Feinerman
mesh_terms: D000818:Animals; D001000:Ants / Q000502:physiology*; D001522:Behavior, Animal; D003299:Cooperative Behavior; D008954:Models, Biological; D015233:Models, Statistical; D009038:Motion; D009068:Movement; D009425:Nesting Behavior; D011336:Probability
keywords: animal groups; collective motion; emergent behavior; social insects; statistical physics