Imagine yourself as an urban planner for Tokyo’s public transport system in 1927. Imagine mapping out the most efficient paths through dense urban sprawl, around obstructing rivers and mountains. And imagine meticulously designing the most efficient possible model, after years of study and expertise… only to find your design prowess, 83 years later, matched by a slime mould: a creature with no eyes, no head nor limbs, nor nervous system.

Of course, this is anachronistic. For one, the Tokyo railroad system developed over time, not all at once. But it was designed to meet the needs of the city and maximise efficiency. Yet in 2010, when researchers exposed the slime mould Physarum polycephalum to a plate mimicking Tokyo city (with population density represented by oat flakes) it almost exactly mimicked the Tokyo railroad system (1). This became one of the most iconic slime mould experiments, ushering in a flood of research about biological urban design asking the question: Could a slime mould, or other similar organisms, map out human cities for us?

But a slime mould doesn’t know what cities are. They’re single-celled organisms; they don’t understand urban planning, or public transport, or humans. They are classified as protists, largely because we’re not sure how else to categorise them, not because they’re particularly ‘protist-y.’ They have no brain and are single-celled for most of their life; so they can’t plan routes, have preferences, or make memories. Right? 

Except, perhaps they can. Slime moulds are extremely well-studied organisms because they exhibit precisely these behaviours. But how do they think? And what does it mean—to think?

Slime moulds have evidenced memory and learning. The protoplasm network they form is really just one huge cell that eventually develops into a plasmodium, growing and releasing spores. While plasmodial slime moulds (like P. polycephalum) do this during reproduction, cellular slime moulds (dictyostelids) are able to aggregate together into one cell like this when food is scarce or environments are difficult (meaning they must be able to detect and evaluate if these things are true). Most slime mould behaviour is understood through cell signalling and extracellular interaction mechanisms; responding to chemical gradients using receptors along their membrane, which signal to the cells to move up the concentration gradient of a chemoattractant molecule and away from a chemorepellent.

This makes sense; bacteria (like almost every other living organism) do this all the time and it’s the chief way that they make decisions. But what about memory and preferences? What about stimuli beyond the immediate detected chemicals? Slime moulds can, for example, anticipate repeated events and avoid simple traps to reach food hidden behind a U-shaped barrier (2,3). These are beyond input-to-output; something more complex must be happening. Something conscious? Thinking?

The idea of consciousness requiring complex neuronal processes is becoming rapidly outdated as we observe patterns of thinking in organisms that, according to classical definitions, really should not be able to. Using the slime mould as an example, Sims and Kiverstein (2022) argue against the ‘neurocentric’ assumption that an organism must have a brain to be cognisant.

Instead, P. polycephalum is suggested to exhibit spatial memory, with cognition being suggested to sometimes include external elements (3). They showed it may undergo simple, habitual learning and hypothesised it uses an oscillation-based mechanism within the cell (3). Similarly, oscillator units along the slime mould’s extending tendrils oscillate at a higher frequency at higher concentrations of food source molecules (like some tasty glucose), signalling to the slime mould to move in that direction (4). Sims and Kiverstein (2022) also posit that the slime trail left by slime mould could function as an external memory mechanism. They found that P. polycephalum avoids slime trails as they represent places it has already been; suggesting a method of spatial memory (4). This was further proved as not a pure input-output response by showing that the avoidance response could be overridden when food is placed on or near slime trails (5). They suggest that the slime mould was able to balance multiple inputs, including oscillation levels and slime trail signals, exhibiting simple decision-making.

Should we count these processes as thinking? This topic is debated by philosophers as much as biologists. Sims and Kiverstein (2022) use the Hypothesis of Extended Cognition, being that mind sometimes extends into the environment outside of the brain and body, to argue firmly that it does count. But at the end of the day, despite understanding the chemical and electrical processes between neurons signalling and the cellular makeup of the brain, we still don’t understand how electrical signals through a series of axons make the leap to complex consciousness. Rudimentary and external cognition pathways, as seen with the slime mould, may also be an evolutionary link in the building blocks to more complex, nerve-based consciousness and decision making (3). We don’t yet understand the phenomena inside our own skulls—how can we hope to define it across all other organisms?

Slime moulds clearly have something beyond simple chemical reactions. This begs the question: Aren't our own minds also fundamentally just made of simple chemical reactions? And if a slime mould is able to evaluate multiple inputs, how wonderfully complex must such processes be inside (and outside) a sea anemone, a cockroach or a cat? There’s no way to know what such a consciousness would look like or feel like to our frame of reference. When a slime mould, moving as a network around an agar plate, ‘looks up’ (or an equivalent slime mould action) and perceives unfathomable entities, how does it process that? What does the slime mould think of us?

Bibliography

1. Kay R, Mattacchione A, Katrycz C, Hatton BD. Stepwise slime mould growth as a template for urban design. Sci Rep. 2022 Jan 25;12(1):1322.

2. Saigusa T, Tero A, Nakagaki T, Kuramoto Y. Amoebae Anticipate Periodic Events. Phys Rev Lett. 2008 Jan 3;100(1):018101.

3. Sims M, Kiverstein J. Externalized memory in slime mould and the extended (non-neuronal) mind. Cognitive Systems Research. 2022 Jun 1;73:26–35.

4. Reid CR, Latty T, Dussutour A, Beekman M. Slime mold uses an externalized spatial “memory” to navigate in complex environments. Proc Natl Acad Sci U S A. 2012 Oct 23;109(43):17490–4.

5. Reid CR, Beekman M, Latty T, Dussutour A. Amoeboid organism uses extracellular secretions to make smart foraging decisions. Behavioral Ecology. 2013 Jul;24(4):812–8.