Variability and Heterogeneity in Natural Swarms: Experiments and Modeling

Collective motion of large-scale natural swarms, such as moving animal groups or expanding bacterial colonies, has been described as self-organized phenomena. Thus, it is clear that the observed macroscopic, coarse-grained swarm dynamics depend on the properties of the individuals of which it is composed. In nature, individuals are never identical and may differ in practically every parameter. Hence, intragroup variability and its effect on the ability to form coordinated motion is of interest, both from theoretical and biological points of view. This review examines some of the fundamental properties of heterogeneous collectives in nature, with an emphasis on two widely used model organisms: swarming bacteria and locusts. Theoretical attempts to explain the observed phenomena are discussed in view of laboratory experiments, highlighting their successes and failures. In particular we show that, surprisingly, while heterogeneity typically discourages collectivity, there are several natural examples where it has the opposite effect.