Researchers exploring brainless slime moulds have uncovered a spatial memory system relying on external slime trails, challenging traditional definitions of intelligence. Led by Australian scientists studying Physarum polycephalum, published in the Proceedings of the National Academy of Sciences, the findings reveal single-celled organisms navigating complex mazes without a nervous system.
The Brainless Organism Navigating Mazes
For more than a century, conventional scientific consensus dictated that cognition and intelligence required neurons to form the pathways necessary for thought and memory. This traditional understanding placed animals at a special place within the living kingdom, separate from plants and microbes, with humans as most special of all. However, decades of research into single-celled organisms have repeatedly disrupted those boundaries. At the center of this ongoing debate is Physarum polycephalum, a bright yellow slime mould often found creeping across leaf litter in forests, earning it the colloquial nickname dog vomit mould
.
Despite lacking a brain or central nervous system, this large, cell structure has consistently astonished biologists with sophisticated behaviours. Nakagaki later placed oat flakes to map Tokyo and its satellite towns, releasing the organism from the centre to watch it stretch tubules and connect suburban food sources with a verve for efficiency that nearly retraced the route map of the greater Tokyo area’s railway system.
“A slime mould is not a fungus or mould, but is in fact a protist, which is really the odds and ends of the natural world that don’t fit in with the rest of our taxonomic grouping system.”
Christopher Reid, PhD student at the University of Sydney
How External Memory Guides Path-Finding
Building on earlier behavioural observations, researchers at the University of Sydney set out to test the navigational limits of Physarum polycephalum using a U-shaped trap—an experiment more commonly deployed to analyse the artificial intelligence of robots. The full findings were published in the journal Proceedings of the National Academy of Sciences. In the setup, scientists placed the organism on one side of a dish filled with agar gel, with a sugary food source positioned on the opposite side behind a U-shaped barrier.
The organism moves through a network of pulsating tissues that constantly expand and contract, using a similar mechanism to our own muscle cells. Chemical receptors on the cell surface detect environmental variables like food, light, or heat, causing each part to alter its pulsation speed. Neighbouring parts communicate through this throbbing action to coordinate direction.
As the organism explored the petri dish, it deposited a trail of slime wherever it moved. The research team discovered that this residue served as an external navigation marker, comparable to the breadcrumbs in the Hansel and Gretel fairy tale. Because the slime mould actively avoided areas it had already investigated, the physical trail effectively prevented pointless backtracking.
Experimental Results and the Biogenic Stance
The experimental metrics demonstrated a stark contrast in navigational success when this external memory system was disrupted. When tested normally, 96% of the slime moulds successfully located the sugary substance, completing the journey in an average of 57 hours. However, when researchers pre-coated the entire dish in slime to eliminate the distinct path markers, only 33% managed to reach the goal within the 120-hour time limit.
Without external memory markers, the organisms spent almost 10 times longer re-exploring previously visited zones. These observations mark the first to identify ‘memory’ in an organism without a brain or central nervous system.
This capability feeds into a broader philosophical and biological shift. Since the 1980s, a growing contingent of scientists and philosophers has championed what Pamela Lyon, a philosopher at Adelaide University, terms the biogenic
stance—the argument that cognitive traits are intrinsic not just to animals but to all life.
Redefining Biological Intelligence
The implications extend far beyond simple maze navigation. Researchers emphasize that while the internal memory of higher, multi-cellular organisms probably did not directly evolve from these externalised systems, studying protists offers insight into how ancient organisms behaved.
Future investigations are slated to examine what other information the slime trail may hold. For an organism operating without central command, the creature continues to challenge preconceived notions of cognitive limits.
