Planes, trains, and slime molds

Designers of transportation networks have to weigh the cost of serving customers against the need for an efficient, robust system. Natural organisms, too, confront tasks in which they need to balance competing desiderata. As it forages for food, for example, a slime mold must balance cost (that is, the amount of protoplasm it extrudes), efficiency, and the ability to withstand injury. Remarkably, as recently reported by Atsushi Tero and colleagues from Japan and the UK, the molds do as well as transportation engineers in balancing their analogous competing needs. Panel a of the figure recreates a 17-cm-wide map of the principal cities served by the Tokyo railway system with a slime mold (yellow) at the location of Tokyo and food flakes (white) representing other cities. In about a day’s time, the slime mold finds where the nourishment is and generates a protoplasm network with the food flakes as nodes. Standard metrics for analyzing transportation networks reveal that the mold’s foraging network and the Tokyo railway system perform similarly. Perhaps more significantly, Tero and company imitated slime-mold networks in numerical simulations that don’t incorporate detailed biochemistry. Instead, they include a feedback step in which tubular links carrying a large protoplasm flux grow wider and flux-poor links contract. By tweaking their simulation parameters, the researchers could nudge the network toward, for example, greater cost efficiency. With optimal parameters, they could even improve upon the work of slime molds and human engineers. (A. Tero et al., Science 237 , 439, 2010 http://dx.doi.org/10.1126/science.1177894 .)