A team of researchers led by Atsushi Tero simulated how Tokyo’s rail system would have developed if slime mold was calling the shots. See the original paper if you have a subscription to Science (warning: lots of advanced math) or this helpful summary by MSNBC, which brought the story to my attention.
To make a long story short, slime mold is a unicellular organism resembling a fungus. As it grows, it oozes outward seeking out food sources, then it connects these by forming narrow veins that look suspiciously like transportation links. For economy’s sake, the mold forms the most lean and efficient network possible while maximizing its access to nutrients.
The scientists placed food deposits (oat flakes) in a pattern that mimicked the distribution of population in the greater Tokyo area. They also discouraged mold growth in areas corresponding to obstacles like ocean and mountains by placing light sources (mold’s sworn enemy) in these spots. The researchers then introduced a single deposit of the mold on their mock central Tokyo and let the slime do its thing.
The result? The mold formed a network that closely mimicked the actual Tokyo railway map. In terms of efficiency and fault tolerance, the mold performed about the same as the real Tokyo system, and it did so at a slightly lower cost. All of this was done without any guiding overall intelligence, but through a decentralized method in the mold that continually adapted to reinforce links that performed well, while eliminating those did not.
More after the link.
Of course, they probably could have gotten similar results with a lot less work using agent-based models.