Modeling the ballistic-to-diffusive transition in nematode motility reveals low-dimensional behavioral variation across species

A quantitative understanding of organism-level behavior requires predictive models that can capture the richness of behavioral phenotypes, yet are simple enough to connect with underlying mechanistic processes. Here we investigate the motile behavior of nematodes, which execute random walks driven by undulatory propulsion on surfaces. We broadly sample the nematode behavioral repertoire by measuring motile trajectories of the canonical lab strain C. elegans N2, wild strains and distant species. We focus on trajectory dynamics over timescales spanning the transition from ballistic to diffusive movement and find that much of the statistics of nematode exploratory behavior are captured by a random walk model with independent dynamics in the speed and bearing. Furthermore, the parameters describing this behavioral model vary among individuals and across species in a correlated way that suggests a common mode of behavioral control and is consistent with a trade-off between exploratory and exploitative behavior.