Humans typically look for intelligence and memory in other species within the animal kingdom. On the surface, this makes sense as intelligence and memory generally are features associated with the brain and nervous systems. One might ask: “What would intelligence look like without a brain?”. Recent work has focussed on understanding these features in a different kingdom of life, one that lacks such a nervous system: protists. Protists are eukaryotic organisms that may be unicellular (such as amoeba) or multicellular colonies.

Slime moulds, a type of protist, are fascinating and exist as plasmodial entities (single-cell, multinucleate eukaryotes). They reproduce through spore production (much like their fungal namesake) and may feed on detritus and other bacteria. The species Physarum polycephalum (later referred to as Physarum) is particularly interesting; it displays signs of memory, decision-making, and risk management.

Physarum is motile. It moves through a rhythmic streaming of cell cytoplasm back and forth, altering the shape and volume of the cell as certain regions of it expand and others contract using actin-myosin interactions. An extended pseudopod is put forth and the plasmodium drags itself in that direction, allowing Physarum to creep along surfaces in the search for food. Whilst this nomadic instinct is not impressive in its own right, Physarum does not search at random.

The slime mould explores using a vascular network that forms its sensory front. It is chemotactic and responds to positive gradients by releasing the signalling molecule cAMP that promotes cytoplasmic streaming in that vein. This, in turn, results in contractions of veins that have explored less favourable routes. Using this as a positive feedback loop, the slime mould is self-informed and relies on its sensing ability to forage.

Inherent “decision-making” also forms part of the genius of Physarum. This is evidenced in three case studies. One case found the slime mould to remain in regions of high-quality food longer than in regions of lower quality. When feeding in the presence of “risk” (i.e. a negative stimulus such as bright light), the slime mould lingers in “safe” areas for longer than “risky” environments. In another instance, a plasmodium was given the option between two sources of nutrition: high nutrition with high risk, and low nutrition with low risk. Physarum chooses the lower-risk path unless the high-nutrition source is so rich that the protist is willing to gamble.

In the third case study, when presented with two routes of different lengths, the slime mould would take the shortest path to the food source. But then the ability of a plasmodium to select the shortest path to the food source was taken one step further. Experimentalists used an agar plate and positioned food sources such that they resembled station positions in the Tokyo railway system. The slime mould explored the network of food sources. Over time, the veins that were not fruitful were retracted and the veins providing the shortest routes were engorged. The end result: a slime mould map of the Tokyo railway. The shape and efficiency were staggeringly comparable.

The maze-solving skills of the slime mould were tested in 2000. A food source was placed at one end of the maze; the plasmodium at the other. Using its vascular sensory network, the plasmodium explored every route down the maze and made its way chemotactically to the food source. Over time, the extracellular slime and veins left behind in routes leading to dead ends were retracted, leaving a slime mould in the shape of the maze solution.

Amazingly, Physarum has shown a degree of memory. As discussed, the slime mould thickens veins in the presence of food. This may be seen as Physarum “remembering” where food can be found whilst it explores other areas. As it moves, the plasmodium secretes an extracellular slime. Physarum avoids regions covered in this slime (unless necessary) to prevent tracing over its own steps. Essentially, this form of external memory stops Physarum from searching for food where it has already been and allows it to navigate complex environments.

These are but a few accomplishments of a single slime mould species. There are many more mazes and tasks that Physarum polycephalum has been able to complete, along with other species of slime moulds that behave in a manner of different ways. However, what can be gleaned from this is that intelligence does not require a nervous system. Slime moulds may revolutionise how we think of intelligent species altogether.

Many have started looking to protists and slime moulds as a different method of sensing and problem-solving. Physarum mapped a railway system in hours – a task that could have taken days for a human. Research is currently being carried out to form mathematical models of slime moulds to better understand how they behave and make decisions.