Today I read a fascinating piece on the Guardian‘s website about a study conducted by Andrew Adamatzky. He and his colleagues have conducted a series of great experiments on slime mould and the way in which this one-celled organism grows and builds a network or system linking different food resources. Its relative simplicity in evolutionary terms notwithstanding, this organism is known to behave ‘smartly’ and build its networks more or less rationally.

Adamatzky and colleagues poured agar on a globe and cut out the seas and oceans, so that the agar configuration resembled Earth’s land masses. They also placed oat flakes on the positions of the world’s megacities and other urban concentrations. They placed the slime mould on the flake representing Beijing and then observed how it built its network. Details are available here or here. They have previously used this methodology in a study of Australia’s major population concentrations and the road networks connecting them, available here.

A key finding of the study on the globe as a whole is that the slime mould’s network “approximates over 76% of the Silk Road routes and the Asian Highway routes”. The implication being that these historically emerged transport networks are quite rational — if not optimal — in terms of their structure. Indeed, Adamatzky writes that his research and findings “will help to design future transcontinental pathways”.

For a long time I have been fascinating by the idea of optimality in transport networks and the specific way in which optimal travel behaviour has been defined in travel behaviour studies (i.e. either in terms of minimising travel times or in terms of the optimisation of the ‘costs’ of travel and the ‘benefits’ to be reaped at the destination). So I very much welcome work such as Adamatzky’s because it sheds new light on the ‘trick’ by neo-classical economists to transpose the principle of least effort from physics to human behaviour. Perhaps there is an evolutionary edge to this principle, which the neo-classical economists grasped intuitively but were unable to prove? There must be literature on addressing this question and I would grateful for any references! Anyway, I mention neo-classical economics here because this body of thought has had such a profound influence on the history of transport and travel behaviour modelling.

At the same time, I am rather sceptical about Adamatzki’s claim that slime mould behaviour can really help us understand the configuration of surface transport networks. For there is a big difference between slime moulds’ quest for food and transport-land use interactions. At least in Adamatzki’s oat flaked world, the location and size of food sources are given (as an inevitable if understandable consequence of his experimental set-up). Though I clearly am no expert on slime mould, I would think that its spatial behaviour is essentially responsive: it reacts to a given state of affairs. And this differs fundamentally from ‘real world’ developments: as decades of transport geography research tell us, urban developments and transport infrastructure development co-evolve. There is no given and static world of cities; their size and growth is very much a consequence of how they are positioned and articulated in major transport networks (not only land-based transport but also maritime and aviation networks, and increasingly internet backbone networks).

So, to really help us understand the evolution of land-based transport networks, Adamatzki should re-design his experiment and add flakes at nodes in the networks built by the slime mould (and a set of rules would need to be deviced regarding how much the extra flakes would need to be added, although deriving those rules would be a less than straightforward task!). In this way feedback effects and positive reinforcement and ultimately the co-evolution of urban development and transport networks could be mimicked.

My hunch is that the slime mould’s networks would develop rather differently if that co-evolution were taken into account.