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 They get used to the chemicals with repeated exposure, and then become newly sensitized once exposure is withdrawn.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.”
Original article by Ed Yong published in The Atlantic (here