The Audacity of Brainless Slime Mold

Dispatches from the weird world of unicellular biology, where essentially brainless critters do a fair job of helping us understand aspects of human behavior--such as how we use memory.
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Dispatches from the weird world of unicellular biology, where essentially brainless critters do a fair job of helping us understand aspects of human behavior--such as how we use memory.


As any proud iPhone 5 owner knows, even genius takes a day off once in a while: When Apple decided to ditch Google and rebuild its popular Maps app using a proprietary platform, the result was a colossal cartographic #fail that still gets cited in Apple’s current share price travails. The sloppy coding directed users onto airport runways, into the ocean, and even inspired its own Tumblr.

Brainless slime mold—yes, that’s the official scientific nomenclature—has no such problems. In fact, it doesn’t even have a central nervous system. But in a paper published last fall in the Proceedings of the National Academy of Sciences, Australian and French researchers showed that the unicellular protist knows how to “map” its surroundings and navigate complex mazes in order to find food.

Such behavior, from an organism with no consciousness, sheds light on the origin and evolution of memory among us brainier creatures.

“External memory,” the authors write, is a common phenomenon in nature: ants use pheromone trails to find their way from food source to nest; bees use landmarks to guide their in-flight navigation. Such tactics, biologists believe, likely preceded “internal memory” and allowed simple life forms to solve spatial problems, long before they could “think” or “feel.” Even robots learn this way. A robot can, of course, come pre-loaded with a map of its environment, or be programmed to build one as it explores—but such capabilities are technologically expensive. A simpler robot might instead employ “reactive navigation,” and solve spatial problems by keeping track only of its immediate surroundings.

So it is with brainless slime mold: no fancy on-board computer, no nervous system, no memory. But that doesn’t prevent it from making smart decisions.

The researchers report that when Physarum polycephalum “senses” food—via the activation of membrane surface receptors—and begins to flow in its direction, the mold leaves behind a “thick mat of nonliving, translucent, extracellular slime” in its wake. (Consider the slug.) When it begins to “forage” again, it will “choose” to explore new territory by avoiding its old, slime-covered path. (Only if there are no virgin swaths of Petri dish to traverse will the mold double back on its previous turf.) Such behavior, the researchers write, “strongly suggests that it can sense extracellular slime upon contact, and uses its presence as an externalized spatial memory system to recognize and avoid areas it has already explored.”

To test this, the biologists ran a series of Petri dish trials involving U-shaped mazes. Half the dishes contained only agar. The other half were pre-coated in slime, effectively jamming the mold’s radar. On these dishes, the mold had no way to find (and thus avoid) its own navigational trail. The results were striking: when the mold’s “external memory” was blocked, it failed to escape the U-trap and find its way to a food source two-thirds of the time; when its external memory was uncompromised, however, the slimy subject performed almost flawlessly. It also spent dramatically less time exploring tracts of Petri dish where it had already been, and hewed closer to the optimal search route. By taking advantage of an “externalized spatial memory system,” the researchers conclude, the mold greatly enhances its navigational ability.

This is not the first time scientists have used brainless slime mold to shed light on how higher-order creatures like us humans interact with our spatial environment. In 2010, Japanese researchers at Hokkaido University devised a clever experiment that modeled the Tokyo rail system in a Petri dish of Physarum polycephalum. They distributed 36 food sources to match the geographic locations of cities around the Japanese capital, and turned the slime mold loose to see how efficiently it would connect the dots. The authors found that the resulting network closely matched the city’s real-world railways—a neat trick considering that Physarum has no capacity for central planning or urban design.

The takeaway? Great minds and brainless slime mold think alike.