Experiment: The Tokyo Subway Map

In 2010, the scientific world was stunned by a simple experiment published in Science.

Researchers at Hokkaido University placed oat flakes on a map of Japan, corresponding to the locations of Tokyo and its 36 surrounding subway stations. They then set a Physarum polycephalum loose on the map.

The result? The slime mold created a network of veins that was almost identical to the actual Tokyo subway system, but in some cases, it was more robust and cost-effective than the one designed by human engineers.

Why it works: Optimization through Pruning

The blob doesn’t “plan” the network. It uses a three-step process:

Exploration: The blob grows in every direction, covering the entire map with a fine web.
Detection: It finds all the “stations” (oat flakes).
Optimization: It strengthens the veins that are being used frequently to pump food and prunes (dissolves) the ones that are redundant or inefficient.

How to Run the Experiment

Step 1: The Map

Print out a map of a complex transit system (London Tube, NYC Subway, or your local city). Scale it down to fit your large petri dish or container.

Step 2: Setting the Barriers

In the real Tokyo experiment, researchers used Intense Light to represent Mountains and Lakes. Since blobs are photophobic, they will avoid light.

Shine a bright LED or use a mask to create “light barriers” where geographic obstacles should be.

Step 3: Placing the “Stations”

Place a single oat flake on every major city hub/station on your map.

Step 4: Releasing the Engineer

Introduce a piece of active plasmodium at the central hub (e.g., Tokyo Central or Grand Central Station).

Step 5: The Time-Lapse

This experiment takes 2-4 days.

Day 1: The blob will look like a messy yellow cloud covering everything.
Day 2: It will begin to find the oats.
Day 3-4: Watch the “magic” pruning happen. The cloud will disappear, leaving behind a crisp, efficient “railway” of thick yellow veins.

The Engineering Metric: Resilience

Human engineers design for efficiency (lowest cost), but also for resilience (if one track breaks, can passengers still get home?).

The slime mold is a master of resilience. It often leaves “loops” in its network. If you cut one of the blob’s veins with a needle, the fluid will simply re-route through a different path. This is why urban planners are now using “Slime Mold Simulations” to design more reliable fiber-optic cables and road networks.

Keeper’s Challenge: Try mapping your own neighborhood. Place oats on the grocery store, the park, and your house. See if the blob finds a shortcut you didn’t know about!

Want to know how it learns? Check out our article on Habituation and Memory.

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