Slime Has Memory but No Brain

By leaving behind gooey deposits (left), Physarum polycephalum "remembers" where it's been.

Photograph courtesy Audrey Dussutour

Sasha Ingber

Published October 8, 2012

The living slime that may have been the muse for the 1958 science-fiction film The Blob just got creepier: Evidence has emerged that slime mold, a brainless single-celled organism, has a form of memory.

In experiments with the slime mold Physarum polycephalum, scientists at the University of Sydney noticed that the life-form avoided retracing its own paths. They began to suspect that the slime was using "externalized spatial memory" to navigate.

"The slime mold leaves behind a trail of slime everywhere it goes, which it can then detect later to recognize areas it has already been," said biologist Chris Reid.

To test this theory, researchers placed Physarum in a U-shaped trap. On an untreated surface, 96 percent of the specimens were able to steer through the trap to find a sugar solution before the time limit of 120 hours.

But when the trap had already been coated with slime, so that the specimens could not distinguish their own trails, only a third of the organisms reached the goal before the time limit and spent ten times longer returning to areas they had already been.

The team's current research also suggests that Physarum can recognize and react to the trails left by other species of slime mold. (Video: searching treetops for slime mold.)

Reid said externalized spatial memory could have been used by primitive organisms to solve the same types of problems our brains confront today—the start of the evolution of memory.

Previous research has shown that slime mold can also solve mazes and anticipate periodic events. In light of all this, Reid concluded: "I, for one, welcome our new gelatinous overlords."

More: Explore an interactive brain and map memory in 3-D >>

The new slime mold study was published online Monday by the journal Proceedings of the National Academies of Science.

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