By Stuart King (@StuartRFKing)

An oozing, blob engulfs Tokyo. Mindlessly, it rapidly spreads, overcoming the surrounding cities in less than a day. Although this sounds like the plot of a science fiction movie, in another example of life being stranger than fiction, it was actually part of an experiment conducted back in 2009. Fortunately for the citizens of the great Japanese metropolis, Tokyo was represented by a miniature model on an agar plate. Further, the slimy blob was not the creation of some comic book super-villain, but in fact a species of slime mould, Physarum polycephalum.

Slime moulds are clearly not plants (or even fungi, or animals, for that matter), and as such are perhaps out of place appearing on this blog. However, the complex behaviour of these bizarre organisms is nevertheless remarkable.

Physarum spends most of its life as a single gigantic cell, or plasmodium. Vivid yellow in colour, vein-like tendrils spread out from its central mass. This branched living network continually pulsates, growing or shrinking depending on what it encounters as it forages through the leaf litter on its woodland floor home. If a part of the slime mould contacts something favourable, such as a potential food source, it pulses more quickly and grows. In contrast, if it contacts something repulsive, like direct sunlight that can cause the slime mould to dry out, the pulsing slows and the slime mould shrinks back. By integrating all these different effects, the plasmodium forms an adaptable, branched network that allows the effective exploitation of resources, whilst avoiding potentially harmful conditions; all without any conscious thought.

As such, despite lacking a nervous system, Physarum exhibits apparently ‘intelligent’ behaviour. Researchers have put this brainless slime’s ability to solve complex problems to the test. They have shown how it can easily navigate mazes, quickly finding the shortest path between two exits that have been baited with food. To achieve this, the slime mould first lays down a dense network of tendrils, exploring all routes through the maze. It then subsequently trims away the dead-ends, leaving behind a single thick tube that covers the most direct path. If the conditions are changed such that part of this route is flooded with unpleasant bright light, Physarum finds a new route that keeps to the safety of the shadows.

The ‘Tokyo’ experiment aimed to test further Physarum’s ability to find the quickest route between different places. The miniature city model used oat-flakes (an attractive source of food for a growing plasmodium) to represent the major population centres of Tokyo and the surrounding cities. The model simulated prohibitive terrain, such as mountains or water, by using areas of bright light. In this model, Physarum first extended a dense network of tendrils before thinning out into selective connections between the carefully placed food sources. Interestingly, this final network bore a strikingly resemblance to Tokyo’s existing railway system; one of the world’s most extensive urban rail networks.



In similar experiments, Physarum has also recreated the motorway networks of Great Britain (although it re-routed the M6/M74 into Scotland via Newcastle rather than Carlisle, see image) and the United States with reasonable levels of success.

The ability of a mindless slime to achieve similar solutions to complex transport problems as human city planners, without any prior knowledge or forward planning, is quite astounding. Although its approach is haphazard (analogous to a city planner building a railway network by first covering the countryside in tracks before upgrading those that are used the most, and decommissioning those that are not), by following simple rules Physarum is able to generate resilient, yet efficient, networks that closely resemble human-designed counterparts.

Of course, no-one is suggesting that city planners be replaced by slime moulds (although paying salaries in oat-flakes would represent a major cost cutting opportunity). However, looking to nature we have shown that effective solutions to complex problems can emerge by following simple rules. By applying these principles elsewhere we might be able to design networks that self-organise in an “intelligent” fashion without human intervention. As such, there is possibly a great deal that this brainless slime can teach us.

About the Author: Stuart King is a former PhD student at the John Innes Centre. He studied the molecular interactions between the most economically important pathogen of potatoes, the late blight pathogen, and its host plants. He has always been interested in evolution’s weird and wonderful creations, and is now pursuing a career in scientific writing and publishing. Follow him on Twitter @StuartRFKing

References

Tero, A. et al. (2010) Rules for Biologically Inspired Adaptive Network Design. Science 327 (5964), 439-442.

Adamatzky, A. and Jones, J. (2010) Road planning with slime mould: If Physarum built motorways it would route M6/M74 through Newcastle. International Journal of Bifurcation and Chaos, 20 (10), 3065-3084.

Adamatzky, A. et al. (2012) Are motorways rational from slime mould’s point of view? International Journal of Parallel, Emergent and Distributed Systems, 28(3), 230-248.