A Brainless Slime Mold Rebuilt Tokyo's Rail Map

Atsushi Tero and Toshiyuki Nakagaki, working at Hokkaido University, put a Tokyo-shaped wet dish on a bench in 2009. They placed oat flakes at the geographic positions of 36 cities around the Greater Tokyo Area, and shone bright light over the spots where mountains and water sit on the real map — Physarum polycephalum avoids light, so the light functioned as terrain. Then they put a blob of slime mold on the dot for Tokyo and walked away.

Physarum is a single yellow cell large enough to see. It has no brain, no nerves, no specialized tissues. What it does have is a habit of fanning out in every direction, then pruning back the tubes that don't pay off. Within hours it had wrapped tendrils around every oat. Within about 26 hours, those tendrils had thinned into a sparse, weighted network linking the food sources.

The network looked startlingly like the actual JR rail map. When Tero's team scored both — the mold's network and the engineered one — on total length, average path between any two stations, and resilience to a single broken link, they were within a few percent of each other. The mold had reinvented the line that runs between Yokohama and Chiba.

The paper landed in Science on January 22, 2010, and the result has been replicated for road networks in Spain, Iberia and the U.S. Midwest. The slime mold isn't smart. It just runs a local rule — reinforce the tubes that move the most cytoplasm, starve the rest — and that rule, at scale, designs a tolerable subway.