Despite the lack of a nervous system, slime molds can learn and share what they learn with other slime molds by joining together for a time.

Introduction

Neither animal nor plant, the slime mold Physarum polycephalum is a large-scale single-celled organism that lives in damp forests. At a glance it may look like a splash of paint, but patient observation reveals it creeping across surfaces by oozing forward in fingerlike projections.

Even though it doesn’t have a brain, the slime mold exhibits a simple form of learning by changing its behavior based on past experience. It also can pass what it learns to another slime mold simply by fusing with it for a couple of hours. Scientists discovered this by giving different slime molds a chance to creep across a tiny bridge made of salt (which they usually avoid) to reach a food reward.

The Strategy

The setup of the experiment was simple: A group of slime molds was taught to cross a bridge without salt. Next, half of those slime molds were exposed to a bridge with salt. They were repelled at first, but eventually crossed anyway. The other half were not exposed to a salt bridge.

When both groups were later given a chance to cross a salt bridge, the slime molds that had experience with a salt bridge traveled across the “yuck” to get to the “yum” more quickly than the others. Then, when a slime mold that had learned to tolerate the salt in order to reach the treat merged with another slime mold, the second slime mold also readily crossed the salt bridge—even after being separated from its partner—as long as the two had been together longer than an hour and had formed a connecting structure between them.

Image: Richard Hoyer / CC BY NC SA ‑ Creative Commons Attribution + Noncommercial + ShareAlike

The slime mold Physarum polycephalum creeps slowly along surfaces, gathering information on its way.

Three slime molds (bottom three circles) are challenged to cross a salt bridge to a feed reward (top three circles). The two outside slime molds have previously learned they will find a treat on the other side of the bridge. The center one has not, but crosses anyway — thanks to information shared by merging with a neighbor. Photo courtesy of Audrey Dussutour CNRS 

Close‑up shows vein formation between a slime mold that has experience crossing a salt bridge to find a reward (H) and one that does not (N). Photo courtesy of Audrey Dussutour CNRS

The Potential

How slime molds learn and share learning is still a mystery. Some scientists think it might be related to how genes are expressed, to the structure of the veins, or to how chemicals interact within the slime. But even though we don’t know how it works, this ability to learn without a brain offers valuable insights for innovation. Researchers have turned to slime mold for help solving difficult computational problems such as finding shortest paths and building better networks. And efforts to design artificial intelligence and machine learning systems can use the bright but brainless creature as inspiration for developing new approaches to learning—and sharing knowledge—without the need for a central hub.

Last Updated November 12, 2020