How brainless slime molds solve mazes

HostI was looking at a map on my phone the other day, trying to find my way through a park, and I realized how much I lean on tools to tell me where to go. But then I heard about this yellow, gooey blob that can find its way through a maze without a brain, a map, or even a single nerve. How does something that looks like spilled paint actually figure out a puzzle?

GuestIt's pretty wild to watch. This thing is called a slime mold. It's not a plant, and it's not an animal or a fungus. It's just one giant living cell that grows in damp places, like under a rotting log. When it gets hungry, it starts to spread out to find food. If you put it at the start of a plastic maze and hide some oats at the other end, it'll find them. It doesn't just wander around blindly, either. It somehow finds the shortest path between its home and the snack, which is something even humans struggle to do without a bird’s eye view.

HostBut if it doesn't have a brain, it can't have a memory. How does it know where it has already been so it doesn't just keep going in circles?

GuestThat's the clever part. It uses its own body as a kind of logbook. As the slime mold moves, it leaves behind a trail of clear, thick goo. Think of it like a trail of breadcrumbs, but instead of eating them, the blob uses them as a warning. When the front of the slime mold touches a spot that already has that old goo on it, it knows it has been there before. It feels that slime and pulls back, then tries a different way. It's a way of storing information outside of its body. The space around the blob becomes its memory.

HostI don’t know if I would call that a memory. It sounds more like a simple physical reaction, like a ball bouncing off a wall. It's just a chemical that it doesn't like to touch.

GuestWell, it's more active than just a bounce. If you look at the blob under a lens, you can see it pulsing. The whole thing acts like a heart. It pumps a watery fluid back and forth inside its body about once every minute. When the blob finds something good, like a pile of oats, the pulsing in that part of the body speeds up. That fast pulsing makes the tubes there grow wider and stronger. In the parts where there's no food, or where it hits its own old slime trail, the pulsing slows down and the tubes start to wither away. Over a few hours, the blob literally reshapes its body into a map of the fastest route to the food. It grows the paths that work and lets the dead ends die off.

HostSo it's just a set of pipes that gets bigger or smaller? That still feels like it's just reacting to what's happening right now. Surely it can't plan ahead or deal with anything that changes over time.

GuestYou would be surprised. Some researchers in Japan decided to test if the blob could learn a rhythm. They put a slime mold in a spot and every hour, they would blow a cold, dry wind over it for ten minutes. Slime molds hate dry air because they need to stay wet to move. Every time the cold wind hit, the blob would stop growing and hunker down to stay safe. After they did this three or four times, the researchers stopped blowing the wind. But even though the air stayed warm and damp, the slime mold still stopped moving right at the hour mark. It was getting ready for the cold wind it thought was coming. It had learned the pattern.

HostWait, how can it keep track of time? There's no clock, and there are no brain cells to count the seconds.

GuestIt all goes back to that pulsing. Those pulses are like a built-in beat. When the cold air hits, it changes the speed of the pulse. That change leaves a mark on the way the fluid flows inside the cell. It's almost like a wave in a pool that keeps bouncing back and forth for a long time. The blob doesn't need to think about the time. The timing is just built into the physical swing of its body. It's a way of knowing things through movement instead of through thought.

HostOkay, so it can track a rhythm and it can find a path. But mazes in the woods aren't just clean plastic walls. There are risks, like bright light or dry patches. How does it choose between a short path that's dangerous and a long path that's safe?

GuestIt weighs those choices by a kind of internal tug of war. Imagine the blob has two different parts of itself reaching out. One side finds a snack but has to crawl over a patch of salt, which it hates. The other side finds a smaller snack in a safe, damp spot. Both sides are pulsing and pulling. The side with the better food pulses harder and tries to pull the rest of the body toward it. But the salt makes the tubes on that side shrink. The final path the blob takes is just the result of which pull was stronger. It's a perfect balance of all the good and bad things it has found. It doesn't need a leader to make a choice because the whole body is the choice.

HostIt's strange to think that a yellow smudge on a log is solving the same kind of math problems we give to computers.

GuestThose tubes and pulses are so good at finding the best way to move that they can even map out human transit systems. Researchers once put oats on a map of Japan, placing them exactly where the major cities are. They put the slime mold where Tokyo is. In just one day, the blob grew a network of tubes to connect all the oat-cities. The pattern it made was almost an exact match for the real rail system in Japan. It found the same short, strong routes that human engineers took years to plan.

HostThat yellow goo on the forest floor might be better at finding the way home than I'm with a map in my hand.

GuestThe blob manages to solve the puzzle simply by being the puzzle itself.

HostThat little smudge of slime proves you don't need a brain to be brilliant at getting where you need to go.