Density-dependent cell migration in the absence of social interactions: a case study of Acanthamoeba castellanii

Cell migration is often influenced by intercellular or social interactions, ranging from long-range diffusive cues to direct contacts that can trigger biochemical signaling within the cell and affect the cell protruding activity or direction of turns. Here we study the density-dependent migration of the amoeba Acanthamoeba castellanii (Ac), a unicellular eukaryote that moves without social interactions. Using experiments and mean free path theory, we characterize how collisions affect motility parameters in crowded environments. We identify the collision rate as a key parameter linking cell density to the collision-induced reorientation rate, and we show its consistency across multiple independent approaches. Our findings reveal that the intrinsic migration speed remains constant, while persistence time and effective diffusion are entirely governed by collisions. At high densities, cells exhibit nearly ballistic trajectories between collisions, a behavior rarely reported in eukaryotes. These results establish Ac as a minimal model for motility in the absence of biochemical signaling, with implications for testing behaviors in complex crowded environments and pre-jamming dynamics.