Scientists Discover Brainless Slime Mold That Thinks

In a new study published in Advanced Materials, scientists from the Wyss Institute of Harvard University and the Allen Discovery Center of Tufts University have unearthed a brainless organism called Physarum polycephalum that uses its body to receive mechanical cues from its environment and perform commutative actions akin to ‘human thinking’ to decide its next direction of movement based on the information received. In the observation, the organisms were not influenced with chemical signals of food to control its behaviour, implying that it reasons on its own.

First author of the report and Assistant Professor at Algoma University, Ontario, Nirosha Murugan analysing the organism behaviour said:

“People are becoming more interested in Physarum because it doesn’t have a brain but it can still perform a lot of the behaviours that we associate with thinking, like solving mazes, learning new things, and predicting events. Figuring out how proto-intelligent life manages to do this type of computation gives us more insight into the underpinnings of animal cognition and behaviour, including our own.”

What are Physarum Slime Molds?

They are amoeba-like organisms that can grow several feet long, helping to break down decomposing matter like logs, mulch and dead leaves in the human environment. A polycephalum creature is made up of a membrane that has many cellular nuclei forming a structure known as syncytium, floating within a shared cytoplasm. The movement of the organism happens when  it shuttles its watery  cytoplasm up, down and sideways in regular waves, in a process called ‘Shuttle Streaming’

Murugan continued:

“With most animals, we can’t see what’s changing inside the brain as the animal makes decisions. Physarum offers a really exciting scientific opportunity because we can observe its decisions about where to move in real-time by watching how its shuttle streaming behaviour changes”.

Taking cognizance of earlier studies that implied Physarum moves in response to light and chemicals, the team proceeded to find out how the organism could make informed decisions on how and where to move, taking physical cues from the environment.

Murugan and her team placed the Physarum specimen at the center of semi-flexible agar gel coated petri dishes and further placed either one or three small glass discs next to each other atop the gel on opposite sides of each dish.

The organisms were allowed to grow freely in the dark for up to 24 hours, while their growth patterns are being tracked.

It was observed that for the first 12 to 14 hours, the organism shoot outwards equally in all directions, nut after that, it extended a long branch that sprout up directly on the surface of the gel, aligning toward the three-disc region a large percentage of the time. The specimen chose to grow toward the bigger mass while it has not physically explored the area to know if it actually has the larger object.

The scientists were determined to find out how the physarum was able to accomplish this exploration of its surroundings before physically going there and so they experimented with several variables to see how they impacted Physarum’s growth decisions, and noticed something strange.

When  three same discs were stacked on top of each other, the organism seemed to lose its ability to distinguish between the three discs and the single disc. It grew toward both sides of the dish at roughly equal rates, despite the fact that the three stacked discs still had greater mass. Clearly, Physarum was using another factor beyond mass to decide where to grow.

To find out how changing the mass of the discs would impact the amount of stress (force) and strain (deformation) applied to the semi-flexible gel and the attached growing Physarum, the computer modelling method was used to create a stimulation of the experiment.

It was discovered that the amount of strain was increased by larger masses, while the simulation revealed that the strain patterns the masses produced changed, depending on the arrangement of the discs.

Co-author Richard Novak, Ph.D., a Lead Staff Engineer at the Wyss Institute further explaining the process said:

“Imagine that you are driving on the highway at night and looking for a town to stop at. You see two different arrangements of light on the horizon: a single bright point, and a cluster of less-bright points. While the single point is brighter, the cluster of points lights up a wider area that is more likely to indicate a town, and so you head there. The patterns of light in this example are analogous to the patterns of mechanical strain produced by different arrangements of mass in our model. Our experiments confirmed that Physarum can physically sense them and make decisions based on patterns rather than simply on signal intensity.”

The research  team were able to show that the brainless physarum creature was not simply growing toward the heaviest thing it could sense – it was making a calculated decision about where to grow based on the relative patterns of strain it detected in its environment.

 “Our discovery of this slime mold’s use of biomechanics to probe and react to its surrounding environment underscores how early this ability evolved in living organisms, and how closely related intelligence, behaviour, and morphogenesis are. In this organism, which grows out to interact with the world, its shape change is its behaviour. Other research has shown that similar strategies are used by cells in more complex animals, including neurons, stem cells, and cancer cells. This work in Physarum offers a new model in which to explore the ways in which evolution uses physics to implement primitive cognition that drives form and function,” said corresponding author Mike Levin, Ph.D., a Wyss Associate Faculty member who is also the Vannevar Bush Chair and serves and Director of the Allen Discovery Centre at Tufts University.

Reference: “Mechanosensation Mediates Long-Range Spatial Decision-Making in an Aneural Organism” 15 July 2021, Advanced Materials.
DOI: 10.1002/adma.202008161