Collective behaviour at intermediate scales: aggregation and swarming of the roundworm C. elegans
In complex biological systems, simple individual-level behavioural rules can give rise to emergent group-level behaviour. While collective behaviour has been well studied in cells and larger organisms, the mesoscopic scale is less understood, as it is unclear which sensory inputs and physical processes matter a priori. Here, we investigate collective feeding in the roundworm C. elegans at this intermediate scale, using quantitative phenotyping and agent-based modelling to identify behavioural rules underlying both aggregation and swarming—a dynamic phenotype only observed at longer timescales. Using fluorescent multi-worm tracking, we quantify aggregation in terms of individual dynamics and population-level statistics. Then we use agent-based simulations and approximate Bayesian inference to identify three key behavioural rules for aggregation: cluster-edge reversals, density-dependent switching between crawling speeds, and taxis towards neighbouring worms. Thus, collective behaviour of C. elegans links physical mechanisms familiar from microscopic cellular and active matter systems with the behavioural repertoire of larger organisms. Our model further suggest that swarming is simply driven by local food depletion but otherwise employs the same behavioural mechanisms as the initial aggregation. Informally, the cluster moves like a self-avoiding random walk that emerges from the interaction of attractive persistent random walkers at shorter spatial and temporal scales.