Ode to the Slime Mould

Now I don’t plan on deep diving into the philosophical debate of defining ‘intelligence’, but I must warn you that this the following information may challenge your current definition of the word.

Planet earth is home to over eight million species, all of which evolved from a common ancestor billions of years ago. To better understand this, we've created the tree of life – a giant family tree that traces evolution back to the common ancestor.

Over time, branches formed, with groups of species popping up with similar characteristics – some branches stayed simple (think bacteria), while other branches became more complex – think humans, dolphins and cats for example.

The main character of our story today however belongs to a branch of the tree which could be described as the “too hard basket” for scientists... essentially the branch that is home to all species that didn’t quite fit in anywhere else – the protists.

One protist called Physarum polycephalum (also affectionally known as ‘The blob’) is a slime mould that has intrigued scientists for many years.

Despite the name, a slime mould isn’t actually a mould. You can usually find it in damp, decaying environments, like on a rotting tree or the moist floor of a forest. It resembles moss but stands out with its bright yellow colour. It’s a single-celled organism that behaves in surprisingly smart ways.

Think of it like a giant living blob made up of one cell with lots of nuclei (yep, just one enormous cell!). It can stretch, move, and ooze its way across surfaces in search of food - kind of like a blob-shaped explorer.

A quick Google search of "traits of intelligence" gives you the usual suspects: adaptability, problem-solving, curiosity, empathy, and memory. Surprisingly, our brainless blob ticks almost all these boxes:

• Adaptability? Absolutely – it changes shape and behaviour based on its environment.
• Problem-solving? It’s out here designing train networks more efficiently than humans. A study in 2000 showed its uncanny ability to solve mazes, navigating numerous dead ends to find a food a source (1). Then came the famous Tokyo study, where scientists placed oat flakes (basically cat nip to the blob) on a petri dish arranged to mimic the pattern of Tokyo’s train stations. After exploring the whole dish, connecting the oats as it went the slime mould reorganised itself. Strengthening some connections and removing others. The result? A network of pathways that not only closely mirrors Tokyo’s real train system, but in some cases was even more efficient (2).
• Curious? You bet – it explores every inch of a new space before making big resource intensive decisions. So, add level-headed to its list of traits!
• Memory? It remembers where food is and even reinforces paths it’s used before (3).
• Empathy? Ok, not in the emotional sense… but it does transfer learned behaviour to other blobs by physically fusing with it. Researchers trained a slime mould to cross a salty barrier to reach food - a big deal, since slime moulds usually avoid salt. After it learned the trick, they let it fuse with an untrained slime mould for a few hours. Amazingly, the second slime mould also started crossing the salt like it had learned the behaviour itself (4).

Turns out, intelligence doesn’t always come with a brain.

Cassie Stylianou

1. Nakagaki, T., Yamada, H., & Tóth, Á. (2000). Maze-solving by an amoeboid organism. Nature, 407(6803), 470-470.
2. Tero, A., Takagi, S., Saigusa, T., Ito, K., Bebber, D. P., Fricker, M. D., ... & Nakagaki, T. (2010). Rules for biologically inspired adaptive network design. Science, 327(5964), 439-442.
3. Kramar, M., & Alim, K. (2021). Encoding memory in tube diameter hierarchy of living flow network. Proceedings of the national academy of sciences, 118(10), e2007815118.
4. Vogel, D., & Dussutour, A. (2016). Direct transfer of learned behaviour via cell fusion in non-neural organisms. Proceedings of the Royal Society B: Biological Sciences, 283(1845), 20162382.