The Brainless Slime That Can Learn By Fusing

Sometimes, Audrey Dussutour enters her lab in Toulouse to find that one of the creatures within it has escaped. They tend to do so when they’re hungry. One will lift the lid of its container and just crawl out. These creatures aren’t octopuses, which are known for their escape artistry. They’re not rats, mice, flies, or any of the other standard laboratory animals. In fact, they’re not animals at all. They are slime molds —yellow, oozing, amoeba-like organisms found on decaying logs and other moist areas. They have no brains. They have no neurons. Each consists of just a single, giant cell. And yet, they’re capable of surprisingly complicated and almost intelligent behaviors. The species that Dussutour studies, Physarum polycephalum, can make decisions, escape from traps, and break out of Petri dishes. “It’s a unicellular organism but it looks smart,” she says. At its smallest, Physarum can exist as microscopic cells, which actively swim about. These cells are attracted to each other, and when they swarm together, they can merge. The result is a single giant cell called a plasmodium, which can extend for meters. It moves with a top speed of 4 centimeters per hour, by extending tendrils in any direction. A single plasmodium can tear itself into fully functioning pieces, and the pieces can fuse right back again.

Habituation is one of the simplest forms of learning, but slime molds show all its hallmarks. They get used to the chemicals with repeated exposure, and then become newly sensitized once exposure is withdrawn. Their behavior changes based on their experiences, and they retain a kind of primitive memory. “Most people thought that it was impossible for a cell to learn,” says Dussutour, “but we’ve tried this now with more than 2,000 slime molds. It can’t be an accident.” Now, using the same bridge-crossing experiment, she has also shown that slime molds can transfer what they’ve learned by merging with each other. She brought naïve slime molds that had never encountered the repellent chemicals next to habituated ones that were already used to them. As is their wont, the molds fused. And those merged molds behaved as if they were habituated—they were quicker to cross the bridge than naïve individuals. Even if three naïve molds fused with a habituated one, the resulting entity still shows signs of habituation. The habituated mold’s memories weren’t diluted. Dussutour noticed that when the fused mold starts to move across a bridge, the first tendril it sends often came from what was formerly the naïve mold. And if she separated the two molds after they had been allowed to fuse, the formerly naïve one still showed signs of habituation. So something moves across between the molds, granting the naïve ones the memories of the habituated ones. This process might allow slime molds to better adapt to their environments, allowing separate “individuals” to benefit from their collective knowledge by becoming one.