Confinement and hydraulic resistance appear to separately govern motility and path selection in Physarum polycephalum.

In self-organizing biological systems, structure often emerges from the interplay between physical constraints and active behaviours. We explore this principle in the unicellular slime mould Physarum polycephalum, focusing on how it selects growth paths and modulates movement under geometric confinement. Using microfluidic devices with bifurcating channels of varying lengths and hydraulic resistances, we demonstrate that Physarum consistently favours the path of least hydraulic resistance-even when geometrically longer-indicating that transport efficiency, not distance, governs directional choice. In parallel, we show that locomotion dynamics transition from sustained growth to intermittent 'run-and-tumble' behaviour as confinement increases, with this motility regime best predicted by a confinement factor based on the channel cross-section. By decoupling resistance from geometric constraint, our work reveals that path selection and locomotion are governed by distinct physical cues. These findings offer a mechanistic framework for environment-dependent navigation in simple organisms and provide a physical basis for bioinspired routing algorithms in confined or constrained environments.