title: Silent flocks: constraints on signal propagation across biological groups.
abstract: Experiments find coherent information transfer through biological groups on length and time scales distinctly below those on which asymptotically correct hydrodynamic theories apply. We present here a new continuum theory of collective motion coupling the velocity and density fields of Toner and Tu to the inertial spin field recently introduced to describe information propagation in natural flocks of birds. The long-wavelength limit of the new equations reproduces the Toner-Tu theory, while at shorter wavelengths (or, equivalently, smaller damping), spin fluctuations dominate over density fluctuations, and second-sound propagation of the kind observed in real flocks emerges. We study the dispersion relation of the new theory and find that when the speed of second sound is large, a gap in momentum space sharply separates first- from second-sound modes. This gap implies the existence of silent flocks, namely, of medium-sized systems across which information cannot propagate in a linear and underdamped way, either under the form of orientational fluctuations or under that of density fluctuations, making it hard for the group to achieve coordination.
authors.forename: Andrea
authors.initials: A
authors.lastname: Cavagna
authors.affiliation: Istituto Sistemi Complessi, Consiglio Nazionale delle Ricerche, UOS Sapienza, 00185 Rome, Italy.|Dipartimento di Fisica, Università Sapienza, 00185 Rome, Italy.|Initiative for the Theoretical Sciences, The Graduate Center, CUNY, New York, New York 10016 USA.
authors.forename: Irene
authors.initials: I
authors.lastname: Giardina
authors.affiliation: Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas (INIFTA) and Departamento de Física, Facultad de Ciencias Exactas, Universidad Nacional de La Plata, Casilla de Correo 16, Sucursal 4, 1900 La Plata, Argentina CONICET La Plata, Consejo Nacional de Investigaciones Científicas y Técnicas, Argentina.
authors.forename: Tomas S
authors.initials: TS
authors.lastname: Grigera
authors.affiliation: Istituto Sistemi Complessi, Consiglio Nazionale delle Ricerche, UOS Sapienza, 00185 Rome, Italy.|Dipartimento di Fisica, Università Sapienza, 00185 Rome, Italy.
authors.forename: Asja
authors.initials: A
authors.lastname: Jelic
authors.affiliation: Initiative for the Theoretical Sciences, The Graduate Center, CUNY, New York, New York 10016 USA.|Department of Physics, Technion-IIT, 32000 Haifa, Israel.
authors.forename: Dov
authors.initials: D
authors.lastname: Levine
authors.affiliation: TIFR Centre for Interdisciplinary Sciences, 21 Brundavan Colony, Osman Sagar Road, Narsingi, Hyderabad 500 075, India.
authors.forename: Sriram
authors.initials: S
authors.lastname: Ramaswamy
authors.affiliation: Istituto Sistemi Complessi, Consiglio Nazionale delle Ricerche, UOS Sapienza, 00185 Rome, Italy.|Dipartimento di Fisica, Università Sapienza, 00185 Rome, Italy.
authors.forename: Massimiliano
authors.initials: M
authors.lastname: Viale
mesh_terms: D000818:Animals; D001522:Behavior, Animal / Q000502:physiology*; D001717:Birds; D005426:Flight, Animal / Q000502:physiology*; D008954:Models, Biological; D009068:Movement / Q000502:physiology*