Brainless Slime Molds Can Navigate Mazes to Find Food
Physarum polycephalum is a single-celled organism without a brain, nervous system, or organs, yet it exhibits remarkable problem-solving skills. When placed in a maze with food sources at two ends, the slime mold stretches across all open pathways before retracting from dead ends. It leaves behind a thick network along the absolute shortest route, transferring nutrients with pulsing cellular fluids.
The Giant Single Cell
Physarum polycephalum belongs to the amoebozoans, specifically the class of plasmodial slime molds known as Myxomycetes. Despite its macroscopic size, which can easily span several square centimeters or even meters across a forest floor, the entire organism is a single cell known as a plasmodium. This plasmodium is a syncytium—a continuous mass of protoplasm containing millions or billions of diploid nuclei within a shared membrane, dividing synchronously without cell walls partitioning them.
In its natural habitat, Physarum polycephalum thrives in cool, dark, and humid environments such as decaying logs, leaf litter, and moist soil. It functions as a decomposer, engulfing microscopic food particles, fungal spores, and bacteria through phagocytosis. The organism moves and feeds by spreading out as an intricate, branching network of interconnected tubular veins, continually reorganizing its shape in response to environmental gradients.
Solving Mazes Without a Brain
The cognitive-like abilities of Physarum polycephalum gained widespread scientific attention through experiments testing its navigational capabilities in artificial mazes. When a plasmodium is introduced into an agar maze, it initially spreads out indiscriminately, sending exploratory tendrils into every corridor and dead end to locate food. Once nutrient sources—typically agar blocks infused with ground oat flakes—are placed at distinct exits of the maze, the organism completely restructures its spatial footprint.
Within hours of contacting the food sources, the slime mold begins withdrawing its protoplasm from dead ends and redundant pathways. The veins that do not lie along the direct route between the food sites gradually thin out and disappear, while the vein connecting the two food sources along the shortest possible path thickens and strengthens. By maximizing nutrient absorption while minimizing the energy required to maintain unnecessary biomass, the brainless organism reliably computes the minimum path length through complex geometric layouts.