A Brainless Slime That Shares Memories 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.

The plasmodium is essentially a flat, liquid-filled sac, but as I wrote a few years back, “it behaves like a colony. Every part rhythmically expands and contracts, pushing around the fluid inside. If one part of the plasmodium touches something attractive, like food, it pulses more quickly and widens. If another part meets something repulsive, like light, it pulses more slowly and shrinks. By adding up all of these effects, the plasmodium flows in the best possible direction without a single conscious thought. It is the ultimate in crowdsourcing.”

This simple behavior can produce extraordinary results. The slime mold can make effective decisions, comparing different options and selecting the best one. It can balance its diet, solve mazes, and escape from traps. It can be integrated into microchips and machines and used to drive robots—not quite a driverless car, but certainly a vehicle with no brain behind the wheel.

It can also rival organisms with brains—like human engineers. In 2009, Atsushi Tero, from Hokkaido University, released a slime mold onto a Petri dish modeled on a map of the Greater Tokyo Area, with bits of food standing in for major urban centers. As the plasmodium sent its tendrils over the map, it continually changed its branches, strengthening some and weakening others. After a day, it had created a network that was almost identical to Tokyo’s actual rail network. Human designers had created that network to be as efficient as possible; the slime mold had done the same, but without any brainpower.

Andrew Adamatzky, from the University of West England, later repeated the experiment with maps of other countries including the U.K., Belgium, Mexico, Netherlands, Canada, Spain, China, Brazil, and the U.S. “We found that the slime mold approximated almost all interstates,” he wrote in the New York Times.

Earlier this year, Dussutour showed that slime molds can learn—at least, to a simple degree. She presented them with an obstacle course: To reach some food, they had to crawl over a bridge that was laced with repellents like salt or coffee. At first, the molds were clearly repulsed, and were slow to ooze across. With more repetitions, they became habituated; they got used to the chemicals, started ignoring them, and moved faster. And if Dussutour gave them a long timeout, and then reintroduced them to the bridge, they were just as reluctant to cross as they originally were.  

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.

How does that work? No one knows, but Dussutour is trying to find out. Something within the plasmodium must be changing when the molds become habituated. Is it a build-up of certain molecules, or the activity of certain genes? “We’ve talked to a lot of neuroscientists and they have no idea,” she says.

“On the one hand, it’s not terribly surprising,” says Michael Levin, of Tufts University. He has previously shown that decapitated flatworms can retain their memories after they regrow a new brain, clearly showing that memory doesn’t depend on neurons. “It has to be encoded in some biophysical change in cells; something different and perduring has to occur as a result of experience, otherwise memory wouldn’t work. Whatever that medium is, inside of cells, why wouldn’t it be transferrable?”

On the other hand, he adds, it’s important to show that it happens—and especially in a creature like Physarum, which is cheap and easy to study. The bridge experiment can be “used by everyone from high-school students to state-of-the-art labs to ask fascinating questions,” he says. “This is going to spur a lot of really interesting work.”

“I think we’re beginning to realize that brains are not prerequisites for complex and interesting behavior,” says Tanya Latty, from the University of Sydney. “The majority of life forms on Earth are brainless, but we know very little about how this brainless majority are able to adapt their behavior in changing environments. I really hope studies like this one encourage other researchers to investigate that.”