A brainless slime mold known as Physarum polycephalum uses its body to sense mechanical cues in its environment. Then, in a process similar to what we consider 'thinking', they decide on the best direction to grow. The study was published in Advanced Materials.
"People are becoming more interested in Physarum because it doesn't have a brain but it can still perform a lot of the behaviors that we associate with thinking, like solving mazes, learning new things, and predicting events," said Nirosha Murugan, one of the authors of the study. "Figuring out how proto-intelligent life manages to do this type of computation gives us more insight into the underpinnings of animal cognition and behavior, including our own."
Slime molds are amoeba-like organisms that can grow to be several feet long. They help break down decomposing natural matter in the environment. They move by shifting their watery cytoplasm back and forth throughout the length of their body in regular waves in a process known as 'shuttle streaming'.
In most animals, it's not possible to see what's happening inside their brain as they make decisions. However, with slime mold, we can observe its decisions in real-time by watching the shuttle streaming process, says Murugan.
To see how slime molds make decisions, the researchers set up an experiment in which slime molds would navigate around one or three small glass disks over 24 hours. The researchers noticed that in the first 12 to 14 hours, the slime mold grew evenly outwards in all directions.
Afterward, however, they grew long branches that grew directly towards the three-disc region 70% of the time. The researchers noted that they seemed to 'choose' to grow towards the greater mass without first physically exploring the area.
When investigating how they did this, the researchers found that rather than simply growing towards the heaviest thing they could sense, the molds make calculated decisions on where to grow based on relative patterns of strain they detect in their environment.
The researchers found that they can detect strain in their environment via special channel proteins in their cell membranes called TRP-like proteins that detect stretching. These same proteins have been shown to mediate mechanosensing in human cells too.
"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, behavior, and morphogenesis are," says Mike Levin, one of the study's authors.
"In this organism, which grows out to interact with the world, its shape change is its behavior. 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."