Linus Schumacher: Common behavioral mechanisms underlie aggregation and swarming of C. elegans nematode worms

In complex biological systems, simple individual-level behavioral rules can give rise to emergent group-level behavior. While such collective behavior has been well studied in cells and larger organisms, the mesoscopic scale is less understood, as it is unclear what physical processes matter a priori. Here, we investigate collective feeding in the roundworm C.  elegans at this intermediate scale, and use quantitative phenotyping and agent-based modeling to identify behavioral rules underlying both aggregation and a novel swarming phenotype which we report for the first time. Using fluorescent multi-worm tracking, we quantify aggregation behavior in terms of individual dynamics and population-level statistics. Based on our quantification, we use agent-based simulations and approximate Bayesian inference to identify two key behavioral rules that give rise to aggregation, namely cluster-edge reversals and density-dependent switch between crawling speeds. While this leads to aggregation in simulations, extending the model with a mid-range taxis interaction improves quantitative agreement with aggregation measured in experiments. Additionally, using our extended model we suggest that dynamic swarming is driven by local food depletion and otherwise employs the same behavioral mechanisms as the initial aggregate formation. Our results suggest that mesoscopic C. elegans uses mechanisms familiar from microscopic systems for aggregation, but implemented via more complex behaviors, which is characteristic of macroscopic organisms