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I For One Welcome Our New Slime Mold Civil Engineers

Slime molds are pretty good at figuring out global trade routes and can do so without the maps and compasses that have guided past explorers. In a paper recently published on the ArXiv, Andrew Adamatzky, a researcher at the University of the West of England, let a slime mold conquer the globe in his lab, and recreated the ancient Silk Road trade route in the process.

A slime mold's tendrils spread across Asia, closely following the ancient Silk Road. Image courtesy of Andrew Adamatzky.
Adamatzky doused the continents on the globe with nutrient rich agar, and placed small oat flakes at points corresponding to modern major cities. He picked 24 major metropolitan areas around the world, including Moscow, London, Istanbul, Seoul, Tokyo, Mumbai and even New York and Mexico City. At the site of Beijing, he placed an oat flake cultured with a slime mold, and let it grow.

The way that the slime mold Physarum polycephalum grows is it spreads out tiny feeler tubes in every direction. When one tube runs into a food source, it covers it and forms a node and excretes digestive enzymes to feast on the newly discovered morsel. It continues to expand out to find more, and when another of its tiny plasmodium veins run across a scrap of foodstuff, it forms another node. The vein running between the two nodes expands and starts acting like a major artery, transporting nutrients throughout the organism.

The mold makes its way to Africa. Image courtesy of Andrew Adamatzky.
 P. polycephalum is very good at optimizing these transport routes. It's in the mold's best interest for survival to be as efficient at transporting food as possible. When there are multiple food sources spread out throughout an area, P. polycephalum will grow its own efficient little networks of nutrient highways between the different nodes of food.

The day after Adamatzky planted Beijing, the mold had grown to encompass Seoul, Tokyo and Delhi. A day later its yellow tendrils reached the streets of Tehran, Istanbul and Moscow. By the end of the 4th day, it covered almost all of Asia. Africa and Europe were the next to fall, before it headed for North America. Within ten days, Physarum polycephalum covered the entire globe.

P. polycephalum comes to America through Greenland, not quite the same route the Columbus took. Image courtesy of Andrew Adamatzky.
At least in one of the runs of the experiment. In total Adamatzky ran the test eight times on the globe, and a further thirty times on a flat map of the planet. The results were about the same between the two because the Pacific ocean is so big, the mold never made it across. Each time, the mold's route to world conquest was a little bit different.

While expanding out, it was also economizing. New veins would grow and connect two sources of food if it found a shorter route. Of course the slime mold was never consciously doing any of this, it was building efficient networks using innate evolutionary adaptations. This is in effect emergence, a kind of network dynamics where complex solutions and patterns arise out of fundamental interactions.

It turns out that the average route that the slime mold took was very close to the ancient Silk Road that opened up trade in the first millennium between Europe, the Middle East and East Asia. The biggest difference was the slime mold never established a northern route, over the Tibetan Plateau, and onto what is today Tehran. This however may be because there were no food flakes to simulate the important trading cities along that were bigger cities in ancient times, or anything to simulate the difficulty of crossing the Himalayas.

Adamatzky  also compared the route to the modern Asian Highway network, which the mold also recreated fairly accurately as well. Though in this case, the mold had a hard time finding a route directly between Beijing and Hong Kong, despite being right next to it. All together, the slime mold recreated about 76 percent of the routes between the old Silk Road and the modern Asian Highway network.

This isn't the first time scientists have compared routes of slime molds to feats of actual civil engineering. Adamatzky himself has simulated the motorways of Canada, the United States, Australia, England, Mexico, Brazil, Portugal and Spain. Mark Fricker of the University of Oxford recreated the railways of Japan, for which he won an Ig Nobel prize in 2010. He shared the award with Toshiyuki Nakagaki of Hokkaido University, who won the award in 2008 as well for showing how slime molds can navigate through mazes.


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