Intra- vs. inter-group variance in collective behavir

Collective motion is the synchronized spatial translation of many individuals concurrently. Its evolutionary importance is emphasized by the wide variety of organisms performing it, ranging from bacteria colonies, to insect swarms, schools of fish, flocks of birds and even human crowds. Thus, collective motion constitutes a key feature in explaining macro-ecological phenomena. In the case of pests such as locusts, understanding the emergence of swarms is also of utmost importance for agricultural planning and assuring food security.

Collective motion is the product of an efficient compromise between individual variability and group homogeneity that enables generating inter-individual synchronization and maintaining coherence. Therefore, deciphering the bi-directional interactions between the individual and the group is essential for understanding the swarm phenomenon, and for predicting large-scale swarm behaviors. 

Using locust nymphs marching in controlled laboratory settings, we have explored these individual-group interrelations. To this end, we compared single animals, locust groups, and virtual (computerized randomly-shuffled) groups, composed of the trajectories of real, though non-interacting, members. These experiments revealed two types of behavioral characteristics: those homogenized by the group, and those retaining individual heterogeneity. The latter were also found to generate distinctive group characteristics. Comparison with Markov-chain models exemplified an interplay between variance at the individual level and the emergence of inter-group variance. This suggests that the unique combination of individuals within a given group plays a critical role in the formation of collective behavior.