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Route 20, autobahn 7 and Physarum polycephalum: Approximating longest roads in USA and Germany with slime mould on 3D terrains

Acellular slime mould Physarum polycephalum is a monstrously large single cell visible by an unaided eye. It shows sophisticated behavioural traits in foraging for nutrients and developing an optimal transport network of protoplasmic tubes spanning sources of nutrients. The slime mould sufficiently approximates man-made transport networks on a flat substrate. Does slime mould imitate man-made transport networks on three-dimensional terrain as well as it does on a flat substrate? We simplified the problem to approximation of a single transport route. In laboratory experiments with 3D Nylon terrains of USA and Germany we imitated development of route 20, the longest road in USA, and autobahn 7, the longest national motorway in Europe. We found that slime mould builds longer transport routes on 3D terrains, comparing to flat agar plates yet sufficiently approximates man-made transport routes studied. We discuss how nutrients placed in destination sites affect performance of slime mould, how the mould navigates around elevations, and offer an automaton model of slime mould which might explain variability of the protoplasmic routes constructed. We also display results of scoping experiments with slime mould on 3D terrains of Russia and United Kingdom.