It’s a completely different type of learning than what we’re used to.

Fusion of venous network of two slime blobs. Image credits: David Villa / CNRS Photothèque.

If the slime mould has intelligence, it’s completely different from ours. It has no brain and no nervous system, and yet it can navigate its environment, learn and teach — clear indications of intelligence. Now, scientists have shown the process which enables this alien-like creature to learn: absorption.

Physarum polycephalum, literally the “many-headed slime”, is a complex single-cell organism which inhabits shady, cool, moist areas, such as decaying leaves and logs. But there’s much more to this creature than meets the eye. In a 2010 paper, researchers showed that the slime is capable of solving the Shortest path problem, which has real-life applications. For instance, when researchers dispersed oat flakes to represent Tokyo and 36 surrounding towns. P. polycephalum created a network similar to the existing train system, and “with comparable efficiency, fault tolerance, and cost”. Similar results have been shown based on road networks in the United Kingdom and the Iberian peninsula (Spain and Portugal). The creature has also been shown re-allocate resources to apparently maintain constant levels of different nutrients simultaneously, and it is also capable of teaching other members of its own species what foods to eat and what to avoid.

So how is it able to do all of these without a nervous system?

Observations have shown that slime moulds only exchange information when their venous networks fuse. But researchers still weren’t sure how the flow of knowledge manifests itself physically, and how the slime’s memory effectively works.

In order to answer these questions, a team of scientists forced the slime moulds to cross salty environments for six days. The process can be harmful to the moulds, but it was not fatal (at least in a lab environment). Naturally, though, slime moulds avoid passing over salt if they can. After these six days, these slime moulds had ten times more salt than before the experiment but were much more inclined to pass over salty environments.

The researchers then placed the habituated slime moulds in a neutral environment. It took the moulds two days to eliminate the excess salt — and with it, the memory and inclination to cross over salty environments were also lost. To further test this theory, scientists introduced the “memory” into naive blobs by injecting a salt solution directly into the organisms. Two hours later, these organisms were behaving just as the ones who had undergone six days of training.

This strongly suggests that absorption of substances has a lot to do with the slime mould’s memory — it may even be how the process works. This is useful in nature, as the slimes memorize a substance and then avoid it, passing the information on to other individuals at will — all of this without a nervous system.

The study has been published in Philosophical Transactions of the Royal Society B.