Lacking a brain would represent a major hindrance to most organisms – yet not to a type of slime mold called Physarum polycephalum, which retains the ability to perform calculations about its environment and make important decisions about which direction to move in. According to new research in the journal Advanced Materials, the snot-like creature uses “mechanosensation” in order to respond to the shape, size, and mass of other objects and navigate its surroundings.
To be clear, Physarum is not a true mold, but is in fact an amalgamation of individual eukaryotic cells that are bound together inside a single membrane and all float around within a shared cytoplasm. The organism is able to move via a process called shuttle streaming, which involves the shifting of this watery cytoplasm back and forth in a wave-like motion.
Previous studies have revealed that, despite possessing no neural architecture whatsoever, Physarum is able to find its way to a piece of food at the center of a maze, picking up on chemical signals in order to locate its reward. However, researchers are keen to determine whether the slime can also make decisions in the absence of such signals.
To find out, the study authors placed the organism in a petri dish containing a semi-flexible agar gel, with one small glass disc at one end of the dish and three discs spread evenly across the other end. In their write-up, they report Physarum chose to move towards the three-disc area in 70 percent of trials, displaying a clear preference for this arrangement over the single disc.
The researchers hypothesized that the slime was able to detect distortions in the agar gel produced by these discs, and chose to move towards the most deformed area in the expectation of discovering a larger piece of food. However, when the experiment was repeated, this time involving three discs stacked on top of each other at one end and a single disc at the other, Physarum no longer showed a preference for the heavier object and moved towards both areas with roughly equal frequency.
Such a finding indicates that the organism does not use mass alone in order to assess which way to move, but takes more factors into account. To understand further, the researchers used computer modeling to analyze the amount of stress exerted on the agar gel by each arrangement of discs.
This revealed that while the three stacked discs created the most concentrated stress, the three dispersed discs generated a broader distribution of stress, with the slime mold appearing to favor this particular mass configuration.
By way of explanation, study author Dr Richard Novak 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."
Other organisms, including humans, possess molecules called TRP-like proteins within their cell membranes that are able to detect stretching. To determine whether Physarum navigates using this same mechanism, the researchers applied a TRP channel-blocking substance. Consequently, the slime lost its ability to discern between the various disc configurations.
"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,” explained study author Dr Mike Levin.
“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."