A unifying model for the structure of animal groups on the move Emiliano Cristiani * , Benedetto Piccoli † March 17, 2009 Abstract In this paper we present an agent-based model for the simulation of animal groups on the move in two and three dimensions. The model takes into account only two forces: a long-range cohesion and a short-range re- pulsion. We also consider the visual field of the animals and the different zones where cohesion and repulsion are active. Interactions between ani- mals are both metric and topological. Unlike most of the existing models, we do not take into account the alignment of the velocities. The main result is that, despite the simplicity of the model, we can obtain almost all known patterns commonly seen in nature for animal groups (globu- lar, extended and front clusters, columns, Vees, Jays, echelons) without adding extra animal-specific features, disclosing the basic causes of pat- tern formation. Keywords Flocks, schools, herds, self-organization, agent-based models. * CEMSAC, Universit` a di Salerno, Italy and IAC-CNR, Rome, Italy. E-mail: emiliano.cristiani@gmail.com. Tel.: +39 349 26 39 591. † (Corresponding author) IAC-CNR, Rome, Italy. E-mail: b.piccoli@iac.cnr.it. Mail: IAC-CNR c/o Dipartimento di Matematica, Universit` a di Roma Tor Vergata, Via della Ricerca Scientifica, 00133, Rome, Italy. Tel.: +39 329 10 48 974. 1