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Slime molds look gross and... not smart, but they definitely seem to communicate and plan even without neurons. Michael explains the science behind these clever eukaryotes.
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Slime mold: sounds like something out of a sci-fi movie or maybe what you'd find growing in your shower if you forget to clean it. But don't let the gross-sounding name put you off. These organisms may look like oozing blobs of goo, but the more scientists learn about what they're capable of, without a brain, the more fascinating they are.

The term "slime mold" actually refers to more than 900 species, found on every continent. And despite the name, they're not fungi. They're actually lumped in with protists. These guys have been around for at least 600 million years. They're a diverse group of amoeba-like eukaryotic organisms, which means they have cells surrounded by membranes and DNA tucked inside a nucleus. They come in all sorts of shapes and colors. There's even one called "dog vomit slime mold," because it kind of looks like bright yellow puke. 

All slime molds start life as single-celled organisms, but when food is scarce, they join up to form a bigger mass, using chemical signals to find each other. Plasmodial slime molds fuse together to make one giant cell with a bunch of nuclei floating around inside it, which continue to grow and divide in sync. And cellular slime molds remain separate cells, but just communicate and work together as a big swarm.

These blobs cruise around in search of things to eat, like microbes, and when the environment is safe enough, slime molds grow tiny, mushroom-like sporangia, pumping out spores that grow into the next generation of single cells. But the weirdest thing about slime molds is that they're actually kind of smart.

Not in quite the same way as animals, though. When these cells stick together, they don't magically grow a brain or use tools or anything like that. But scientists have found that slime molds can find efficient paths between lumps of food and seem to learn new things about their environment. Which is pretty impressive for an organism that doesn't have a single neuron.

One kind of experiment tested their route-finding abilities by using maps of countries and a particular plasmodial slime called Physarum polycephalum. In a 2010 paper, for instance, researchers laid out bits of food in a petri dish to mimic towns near Tokyo, Japan. First, the huge slime mold cell spread out to cover the whole map and figure out where the tasty treats were. That food-finding strategy seems pretty normal, and probably takes a good amount of energy. 

But then the slime mold contracted, leaving slimy branches between food lumps to shuttle nutrients back and forth throughout the cell. And when they compared this petri dish network to a map of Tokyo, it was really similar to the way engineers laid out the real subway lines. This happens with other maps, too. Physarum blobs find incredibly efficient pathways, thanks to chemical signals, without complicated computer algorithms or brains actively solving a problem.

A study from 2008 showed that this same species can learn to anticipate changes in their environment that keep happening. At regular intervals, like every 30 minutes, scientists would suddenly lower the temperature and humidity around a plate with slime mold growing on it. And the slime mold would slow down its movement to save energy. Eventually, though, the slime mold started to slow down before the change actually hit. It had somehow come to expect the change, meaning it had some way to sense time passing.

One researcher suggested it might have to do with the normal pulsing movements of its cell membrane or the fluids inside, but there's still a lot we don't understand about how these brainless blobs have a sort of memory.

Anther experiment in 2016 showed that Physarum slime molds can even get used to a chemical they don't like over time, such as caffeine. This is called habituation, and is considered to be a kind of learning. And when these habituated slime molds fuse with a buddy, they can share things they've learned, so the bigger new blob behaves just like the habituated one. Even with all these experiments, scientists still aren't sure how they do any of this. Something biochemical is going on to store information without the help of neurons. And doing more research to understand them could teach us more about all life on earth.

Life has existed on our planet for more than three billion years, but we think brains have only been around for a sixth of that time. Most organisms on earth are still brainless, and slime molds show how even single-celled living things can navigate complex environments and mess with our ideas about intelligence. They could also offer some insight into how the first multi-cellular organisms formed, since they started as single cells before glomming together.

So next time your stumble across a smear of yellow goo on a rotting log, know that it's not just a gross blob: it's a slime mold with a lot of chemical secrets.

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