One of the reasons that science is fascinating is the opportunity it gives to explore how the natural world works. That exploration, though, sometimes provides opportunities for humility, when we realize how little we really understand compared to everything that is out there. Most of you will have heard the story of the engineers’ proof that the bumblebee cannot fly.
A single-celled organism called a slime mold, Physarum polycephalum, has just provided us with another occasion for humility. According to a report at Ars Technica, researchers have discovered that P. polycepahlum can design a complex transportation network just about as well as human engineers — not too bad for an organism that doesn’t even have a nervous system.
The slime mold, when placed on a new surface, gradually spreads itself out looking for sources of food. When it finds them, it constructs an intra-cellular network of tubular structures to link them together. The researchers, whose paper appears in the current issue of Science [abstract], hypothesized that the criteria for the construction of this network were very similar to those for the construction of a transportation network: minimizing the number of connections while maintaining enough redundancy for the organism or network to survive if one connection is damaged.
The design of the experiment was simple. The researchers placed oat flakes (apparently, a delicacy for slime molds) on a clean substrate at 36 locations that replicated the layout of population centers in and around Tokyo. They then turned the slime mold loose in this environment:
Initially, the Physarum began to spread out over the entire available area but, over time, it concentrated its network on the tubes that connected the food sources. The resulting network topology “bore similarity to the real rail network.”
The researchers then modified the experiment to focus light (which inhibits the growth of the mold) in places corresponding to geographic obstacles, such as mountains or lakes, to the real network. The similarity between the mold’s network and the real Tokyo rail network increased. When compared to the minimum spanning tree [MST] (the shortest collection of paths connecting all the “stations”), the real network was 1.8 times as long as the MST, and the slime mold network was 1.75 times as long. The engineers of the rail network did do a bit better at making their design robust against connection failures, but the difference was not large.
This is another example, like fractal geometry, of how quite complex emergent phenomena can result from the repeated application of very simple rules. The experimenters were able to develop a simple fluid-flow model that could replicate the network constructed by the slime mold. They suggest that it might make a good starting point for network design, especially for networks that need to be self-organizing. Nature has had millions of years of experiments to come up with a method that works pretty well; it would be silly to ignore it just because it was Not Invented by Us.
Update Sunday, 24 January, 23:16
There is another article on this research at Wired Science.