On January 31st, 2017 we learned about

Single-celled slime molds can somehow share memories to solve problems

As animals that have put considerable resources into evolving large brains, it’s hard to rationalize the apparent intelligence of slime. Without the brain cells, neurotransmitters and other specialized functionality we pack into our heads, slime, or more accurately, slime molds, can still negotiate their way through the world. reacting and learning about specific stimuli. The fact that they seem to also share data with each other may end up reshaping how we think about learning and memory in general.

Congealed and coordinated

Slime molds like Physarum polycephalum exist primarily a wholly-contained single cell organisms. The greenish-yellow organisms don’t grow into muscle cells, or bone cells, or brain cells, but they can wiggle and ooze along to stay out of trouble while looking for fungal spores and various microbes to eat. As individuals, there’s not a whole lot to say about their behavior. Once they congeal into a group, or plasmodium, things get a lot more complex, even thought the slime is basically staying the same.

A plasmodium is basically a sac of fluid, with all the individual mold cells squirming about inside. That squirming can become very coordinated though, and allows the group to react to the environment at speeds up to four centimeters an hour. Slime molds, appropriately, seem to dislike bright light, and if light is shined on part of a plasmodium, it will eventually move itself towards available darkness. It can do this not by developing new sensory organs or structures, but by varying the activity levels of individual cells in the group. Happy slime mold cells will generally be more excited and expand, leaving unhappy cells to contract and slow down. The net effect of these two sets of activity leads the slimy collective away from things like light, or towards things it/they do like, like food.

Sharing smarts

This somewhat mechanical form of decision making isn’t that hard to wrap your brain cells around, but it’s really only the start. Slime mold decision making has been found to be fairly efficient if given enough time, picking optimal routes through different spaces to find food. If that food is behind unpleasant but harmless barriers, like a small pile of coffee grounds, the plasmodium will be repulsed at first, but eventually suck it up and become desensitized to the normally irritating substance. If the road to food goes through coffee grounds, the plasmodium will eventually decide to roll over it anyway, although scientists have yet to understand how the slime can measure when it’s been near the coffee long enough to feel ok about it.

Weirder still is the fact that these memories of benign coffee can be shared. If a plasmodium has been in proximity to an obstacle long enough to become desensitized, a “piece” of it can be removed and added to a different plasmodium that hasn’t learned how to crawl over coffee grounds yet. Word somehow travels fast though, as cells that never had direct experience with the obstacles will react quickly once experienced slime is added, and the newly educated plasmodium will put that information to use, crawling over the coffee. This indicates that information is somehow being recorded and shared in ways we don’t really understand, or even see yet. Our models of memory all involve brain cells and synapses, but these slime molds suggest that there’s more than one way to share and store an experience in living things.

Source: A Brainless Slime That Shares Memories by Fusing by Ed Yong, The Atlantic

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