How Does a Slime Mold Make Decisions Without a Brain?

Pretend you didn't have a brain but could still move around. How would you decide where to go with your buddies on a Friday night? How would you solve a maze? How would you remember how to get to your favorite restaurant?

You're probably thinking, Um, I couldn't do any of that stuff. Well, it's too bad for you, then, that you're not a slime mold, because even without a brain, they can do all that stuff. Or some of it, anyway.

You're most likely to encounter a slime mold sitting on a log in the forest, just a vibrantly colored clump of gelatinous lace. But what you might not know at first glance is that this pile of forest goo is an ancient anomaly: Slime molds branched off from every other known group of organisms before animals parted ways with plants and fungi. Some of them are single-celled with multiple nuclei, while others live in a colony that moves as a single organism. They can chase food like an animal, but can produce spores like a fungus. Without a brain or nervous system they exhibit the executive wherewithal of an 8-month-old human baby — they're just amazing.

Research published in Proceedings of the National Academy of Sciences in February 2021 investigates how one giant-bodied single-celled slime mold makes decisions. Physarum polycephalum is made up of a network of stretchy tubes that can stretch to a length of several meters — it's the largest single cell in the world, according to the Guinness World Records. Even though it has no organizational center, it can solve a laboratory labyrinth to get to some food. The researchers found Physarum polycephalum absorbs memories of food directly into its network of tubes — the researchers found an encounter with food generates the production of a chemical at the area of the slime mold's body that came in contact with it, softening the tubes in that area and causing the entire organism to veer in its direction.

"The gradual softening is where the existing imprints of previous food sources come into play and where information is stored and retrieved," said author Mirna Kramar of the Max Planck Institute for Dynamics and Self-Organization, in Göttingen, Germany, in a press release. "Past feeding events are embedded in the hierarchy of tube diameters, specifically in the arrangement of thick and thin tubes in the network. For the softening chemical that is now transported, the thick tubes in the network act as highways in traffic networks, enabling quick transport across the whole organism. Previous encounters imprinted in the network architecture thus weigh into the decision about the future direction of migration."

Slime molds don't like to cover the same ground twice. They leave themselves a path of pheromones so they can avoid places they've traveled over before.

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