title: Intermittent collective dynamics emerge from conflicting imperatives in sheep herds.
abstract: Among the many fascinating examples of collective behavior exhibited by animal groups, some species are known to alternate slow group dispersion in space with rapid aggregation phenomena induced by a sudden behavioral shift at the individual level. We study this phenomenon quantitatively in large groups of grazing Merino sheep under controlled experimental conditions. Our analysis reveals strongly intermittent collective dynamics consisting of fast, avalanche-like regrouping events distributed on all experimentally accessible scales. As a proof of principle, we introduce an agent-based model with individual behavioral shifts, which we show to account faithfully for all collective properties observed. This offers, in turn, an insight on the individual stimulus/response functions that can generate such intermittent behavior. In particular, the intensity of sheep allelomimetic behavior plays a key role in the group's ability to increase the per capita grazing surface while minimizing the time needed to regroup into a tightly packed configuration. We conclude that the emergent behavior reported probably arises from the necessity to balance two conflicting imperatives: (i) the exploration of foraging space by individuals and (ii) the protection from predators offered by being part of large, cohesive groups. We discuss our results in the context of the current debate about criticality in biology.
authors.affiliation: Physics Department and Institute for Complex Systems and Mathematical Biology, King's College, University of Aberdeen, Aberdeen AB24 3UE, United Kingdom; francesco.ginelli@abdn.ac.uk.
authors.forename: Francesco
authors.initials: F
authors.lastname: Ginelli
authors.affiliation: Laboratoire J.A. Dieudonné, Université de Nice Sophia Antipolis, UMR CNRS 6621, 06108 Nice Cedex 02, France;
authors.forename: Fernando
authors.initials: F
authors.lastname: Peruani
authors.affiliation: Centre de Recherches sur la Cognition Animale, UMR-CNRS 5169, Universit Paul Sabatier, 31062 Toulouse Cedex 9, France; CNRS, Centre de Recherches sur la Cognition Animale, F-31062 Toulouse, France;
authors.forename: Marie-Helène
authors.initials: MH
authors.lastname: Pillot
authors.affiliation: Service de Physique de l'État Condensé, CNRS UMR 3680, Commissariat à l'énergie atomique et aux énergies alternatives (CEA)-Saclay, 91191 Gif-sur-Yvette, France; Beijing Computational Science Research Center, Beijing 100094, China.
authors.forename: Hugues
authors.initials: H
authors.lastname: Chaté
authors.affiliation: Centre de Recherches sur la Cognition Animale, UMR-CNRS 5169, Universit Paul Sabatier, 31062 Toulouse Cedex 9, France; CNRS, Centre de Recherches sur la Cognition Animale, F-31062 Toulouse, France;
authors.forename: Guy
authors.initials: G
authors.lastname: Theraulaz
authors.affiliation: Centre de Recherches sur la Cognition Animale, UMR-CNRS 5169, Universit Paul Sabatier, 31062 Toulouse Cedex 9, France; CNRS, Centre de Recherches sur la Cognition Animale, F-31062 Toulouse, France;
authors.forename: Richard
authors.initials: R
authors.lastname: Bon
mesh_terms: D000818:Animals; D001522:Behavior, Animal / Q000502:physiology*; D012756:Sheep / Q000502:physiology*; D012919:Social Behavior
keywords: Allelomimetism; collective behavior; computational modeling; self-organization; sheep herds