You don't need a brain to learn something new – not if you're a slime mold, anyway. Scientists who watched Physarum polycephalum search for food found that the slime mold could learn to ignore certain chemical threats.
The findings, described in the Proceedings of the Royal Society B, contradict the idea that learning always requires neurons, and may shed light on the early evolution of learning in living things.
Learning and memory are essential tools in this critter-eat-critter world; they allow animals to use information from their past experiences to make better decisions in the present. And for a long time, scientists thought only creatures with nerves and noggins truly had access to these special skills.
"We usually think of learning as a trait that is limited to organisms with brains and nervous systems," the study authors wrote. "Indeed, learning is often equated with neuronal changes such as synaptic plasticity, implicitly precluding its existence in non-neural organisms."
But that view has been changing in recent years as scientists have been confronted with the astounding abilities of brainless creatures. Take the slime mold, for example. It's an amoeba-like, single-celled organism filled with multiple nuclei, part of a primitive lineage that's been munching on bacteria, fungi and other forest detritus for hundreds of millions of years. And yet, this very simple living thing manages all kinds of intellectual feats.
For example, Japanese researchers have found that slime molds can accurately "design" an efficient rail system when yummy oats are placed where major cities would be on the map. Slime molds can also solve mazes, backing up from dead ends until they find the food at the end, and even anticipate predictable changes, such as a light turning on at regular intervals. They're capable of remarkable physical feats as well, able to create tubular structures called pseudopods (meaning "fake foot") and crawl along until they find a more satisfactory spot. And they grow fast: Given enough food, they can double their surface area in a day.
"What's interesting about slime molds is they appear to be simple, because there is only one cell, but they are capable of amazing stuff, things that we thought were only possible with nervous systems or brains," said lead author Romain Boisseau, a master's student studying evolutionary biology at Ecole Normale Superieure in Paris. "These guys are very cool."
Cunning as the slime mold may seem, can it actually learn? To find out, scientists at Toulouse University in France tested slime molds' behavior in the lab, focusing on a very basic form of learning: habituation, when a living thing's behavioral response decreases to a repeated stimulus — whether good or bad — over time.
The researchers placed the slime molds near a bridge; across the bridge, they placed a delicious pile of oats. Some of the bridges were made of plain agar gel, and the slime molds crossed those with ease. But for other slime molds, the scientists left an unpleasant surprise: bitter-tasting quinine or caffeine, which in large amounts can be toxic for some creatures.
At first, there was a clear difference between the slime molds with a bitter bridge and those without. With a plain agar bridge, the slime molds sped across and pounced on the oats in about an hour. With quinine, slime molds entered the bridge only after two and a half hours, and it took them four hours in all to cross. On caffeine-covered bridges, the slime molds took almost five hours to enter the bridge but then quickly sped across.
For both bitter bridges, the slime mold didn't simply move its body across; it extended a long, thin tendril across the bridge, minimizing the area that touched the surface, as if it were trying to tiptoe over hot sand. When it reached the oats, it quickly moved the rest of its body over through that tendril and over to the oats. Once the slime mold had consumed the food source, the scientists connected it to another bridge, with a fresh food source at the other end. If the slime mold wanted its next meal, it would have to brave the bridge again.
Here's the strange thing: The slime molds dealing with the alarmingly bitter compounds seemed to get used to it, realizing that it wasn't a threat. With every bitter bridge they crossed, they moved more quickly and easily and seemed less concerned with minimizing their "footprint" that touched the surface. By the sixth day, Boisseau said, the slime molds were acting essentially as if the bitter compounds were not there.
So had the slime molds learned anything in the first place? Or was it simply that their receptors became dulled to the chemical onslaught, or that they grew too tired to keep their bodies away from the bitter compounds?
To make sure, the scientists took slime molds that had learned to cross a quinine bridge without flinching and exposed them to caffeine. After all, if the slime molds were simply just tired from the effort of carefully crossing the bridge, they should react to the caffeine the same way they did to the quinine, with nonchalance. But no dice: Slime molds that had been habituated to the quinine reacted with extreme prejudice to the caffeine. The slime molds, it seemed, really had learned a specific reaction to a specific chemical.
The researchers also gave the slime molds a couple of days of rest, allowing them to potentially "forget" this lesson. Sure enough, after a couple of days away from the bitter compounds, the slime molds reacted to a quinine or caffeine-laced bridge as if they had never touched one before. They had forgotten that the bitter bridges were safe.
"They were behaving as if it was the first day they had ever encountered the bitter compound," Boisseau said.
How these critters manage this feat is still a great mystery to scientists, Boisseau said, and will have to await future study. But it does show that we may have to start thinking about the nature of this particular aspect of intelligence in a very different light.
"That's what is exciting here, because maybe this mechanism appeared really early in the history of life," Boisseau said. "Probably learning abilities evolved first, before the evolution of neurons and nervous systems."
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