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Slime molds prove you don’t need a brain to learn — or teach — a new trick

The slime mold Physarum polycephalum (Audrey Dussutour/CNRS)

A slime mold doesn't have eyes for watching, a mouth for communicating or a brain for thinking. It doesn't even have neurons, for crying out loud. It is exactly what its name suggests: a weird-looking, single-celled blob of goo.

But it can learn. And it can teach its friends. Which is pretty freaking cool.

This discovery comes in a study published last week in the journal Proceedings of the Royal Society B. Scientists at the National Center for Scientific Research (CNRS) in France taught some 2,000 slime molds to avoid nasty-tasting substances while searching for food — and found that the molds could then transmit this behavior to their compatriots simply by fusing with them.

Did you know Earth had a boring phase? Neither did these huge fossils.

Slime molds are protists — mostly single-celled organisms grouped into their own kingdom on the evolutionary family tree because scientists aren't sure where else to put them. They are formed when smaller cells fuse, creating a single blob with several nuclei that can grow as big as several meters in diameter.

They're also capable of amazing intellectual feats: solving mazes, maintaining a balanced diet and even “designing” an efficient railway system.

Earlier this year, Audrey Dussutour and David Vogel, along with their colleague Romain Boisseau, demonstrated that slime molds are capable of a simple form of learning called “habituation.” This is what happens when creatures stop responding to stimuli that don't matter — like a puppy who learns she doesn't need to bark every time the doorbell rings, or a mouse that recognizes a toy cat can't do it any harm.

The scientists tested slime molds' ability to habituate by challenging them with quinine and caffeine — chemicals that are bitter-tasting but otherwise harmless. The creatures were stuck in one agar dish, and their food was placed in a second. The only way to reach the tasty morsel was to cross a bridge covered in the bitter substances — something the blobs were initially reluctant to venture over.

Here's how Speaking of Science described their initial experiment when we reported on it in April:

On the first day of the experiment, the slimes waited until they'd exhausted all other options before extending a single, thin tentacle over the bridge to examine what was on the other side. Only then did they slowly and reluctantly transport their blobby bodies over the bridge, carefully keeping to the narrow path made by the initial exploratory tentacle. The whole ordeal took them 200 minutes — more than triple the amount of time it took a control group to make a crossing over a bridge that hadn't been chemically treated.
But by day six, realizing that a little quinine or caffeine wouldn't kill them, the slime molds cut their crossing time in half. They migrated over the bridge in a big blob rather than a cautious trickle, seemingly deciding that avoiding exposure to the bitter substance just wasn't worth the effort.

The researchers tested to ensure that this phenomenon was truly habituation by switching up the animals' chemicals — when confronted with a new bitter substance, they went back to their timid ways.

“I consider this learning,” Boisseau said at the time. But the scientists wondered if the creatures were capable of sharing their learning with others.

Now Dussutour, Vogel and their incredible habituating slime molds are back. This time, the scientists introduced two slime molds to each other: One had been habituated to salt (another noxious but harmless substance), the other was “naive” (that's the technical term). When the newly formed, hybrid slime mold was confronted with the salted bridge, it zipped right over as though it knew it had nothing to worry about. This held true even when the hybrid was made of four individuals, only one of which was habituated.

The scientists separated the slime molds after three hours and retried the experiment. Even when they were no longer attached to a habituated slime mold, the formerly naive molds knew they could ignore the salt. (This wasn't the case when the naive slimes were separated from the habituated ones after shorter time spans.) The more worldly slime molds had taught their naive companion a new trick.

This education seemed to be conducted via a vein that formed between the naive and worldly slime molds after two or three hours of fusion. But it's entirely unclear what is going on inside that channel. What changes within the slime molds when they're exposed to a more experienced one? Are genes switching on? Are molecules building up or being transferred? What does it suggest that information can be acquired and shared by a creature with no brain, or even a nervous system?

“I think we’re beginning to realize that brains are not prerequisites for complex and interesting behavior,” Tanya Latty, from the University of Sydney, told the Atlantic. “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.”

As is often the case with science, answering one question leads to a dozen new ones.

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