A particular slime mould, Physarum polycephalum (the “many-headed slime”), that has shown promise in mapping ideal routes to minimize energy, and this ability has been used by human engineers to model transportation routes. Until recently, how an organism without a brain could accomplish such a feat was poorly understoon. P polycephalum is a plasmodial, single-celled organism which grows outward from a single point as it searches for food. Once it locates a food source, the many branches it has sent out die back, leaving only the most efficient route between food source.
Is it possible to know where you’ve been without a brain? Depending on how one defines “know,” the answer may very well be yes. Recent research has demonstrated that slime molds, organisms devoid of a nervous system and individual cells, exhibit remarkable navigational abilities. They can connect food sources in optimally spaced networks and now, researchers have revealed that these organisms can fill their environment with markers indicating areas they have already searched for food, effectively allowing them to “remember” past foraging efforts and concentrate on unexplored routes.
Slime molds are fascinating entities: they possess a nucleus and complex cells but are evolutionarily distinct from multicellular animals and plants. When food is abundant, they exist as single-celled, amoeba-like organisms that forage. However, in times of starvation, these cells signal one another to aggregate and fuse, forming a visible organism filled with nuclei from many former individual cells. This collective structure enhances their foraging efficiency, and during this process, they leave behind a trail of slime.
In their studies, researchers observed that slime molds tend to avoid areas they have previously traversed. To quantify this behavior, they set up an experiment with two equal food sources at either end of a Y-shaped container. One arm was coated with slime while the other was not. The results were striking: in 39 out of 40 trials, the slime mold avoided the pre-slimed arm. This led to the hypothesis that the slime serves as an externalized spatial memory system, helping the mold recognize and avoid previously explored areas.
To further test this idea, researchers created a three-sided box with a glucose treat placed behind it. The slime mold could not take a direct route due to the box’s walls but had to explore its interior, eventually finding its way around. Remarkably, 96% of the slime molds successfully navigated this challenge. However, when the entire setup was coated in slime, only about one-third succeeded in reaching the food source. This indicates that the presence of slime interfered with their ability to track explored areas.
Latest research has expanded our understanding of slime molds’ cognitive capabilities. A study published in 2023 highlighted that Physarum polycephalum can navigate complex environments by constructing networks between multiple food sources and optimizing paths through exploratory behavior. This is akin to path-planning strategies used by autonomous systems[2]. Furthermore, recent findings suggest that slime molds can make decisions based on physical cues from their environment without relying on chemical signals or food[5].
These studies challenge traditional views of cognition and memory by illustrating how even brainless organisms can exhibit complex behaviors such as navigation and memory formation through non-neuronal mechanisms. The implications of this research extend beyond biology; understanding these processes could inspire new approaches in fields such as robotics and urban planning.
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[1] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8789834/
[2] https://link.springer.com/article/10.1007/s10071-023-01782-1
[3] https://pubmed.ncbi.nlm.nih.gov/23045640/
[4] https://www.nature.com/articles/nature.2012.11811
[5] https://wyss.harvard.edu/news/thinking-without-a-brain/
[6] https://www.cnrs.fr/en/press/slime-mould-absorbs-substances-memorise-them
[7] https://www.pnas.org/doi/full/10.1073/pnas.1215037109
[8] https://www.researchgate.net/publication/232223792_Slime_mold_uses_externalized_spatial_memory_to_navigate_in_complex_environments
