Slime mold points to origin of memory

NEW YORK — A single-celled, amoebalike creature called a slime mold is capable of navigating through a maze to food, despite lacking a brain.

The slime mold leaves behind a trail of goo as it oozes along. That trail forms a kind of external memory system, according to a paper released Monday in the Proceedings of the National Academy of Sciences. The study showed that creatures that couldn’t use their slime as a marker spent almost 10-fold the amount of time exploring a maze to find food.

Slime molds avoid areas where their trail already appears unless there’s no unslimed area available, the authors wrote. That suggests the creatures use their goo as a marker for places they already know. The use of externalized memory may have been a first step toward the development of recollection in other organisms, the researchers said.

“Many insects, including species of ants, bees, and wasps, use landmarks to memorize their route to and from the nest,” the authors, led by doctoral student Chris R. Reid of the University of Sydney in Australia, wrote in the paper. “We go a step further by showing that even an organism without a nervous system can effectively navigate complex environments.”

Slime molds, which aren’t actually mold, spend the bulk of their lives as huge single cells containing many nuclei. This is called a plasmodium. It searches for food by contracting and expanding. When foods are nearby, the contractions speed up, causing the slime mold to flow toward the food. When salt or light is detected, the contractions slow, allowing the plasmodium to move away from irritants.

The species used in this experiment, Physarium polycephalum, originates in forests and can range in size from a few millimeters to more than 12 inches across, according to Washington University at St. Louis.

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