by Vanshika Mistry1, Abhinn Gupta1, Charvi Trivedi1, Prakshal Parekh1, Vishva Patel1 | December 14, 2023.

1Biological Research Laboratory, School of Arts and Sciences, Ahmedabad University, Gujarat, India.

Edited by ScienceRely

What defines intelligence? This question may seem simple, but it unfolds into a labyrinth of thoughts and perceptions. We toss this word around so effortlessly, a trait woven into the tapestry of our perceptions of others. As if intelligence were a crown of cognitive royalty. Yet, what do we truly mean by intelligence? Our society teaches us to prize intellect, even as it leads us down a bumpy road of assumptions and expectations.

But here’s what is intriguing – there is a whole other realm of intelligence beyond humanity. Think about dolphins. We dub them intelligent, for we have a mental list of feats that intelligent beings should perform and dolphins fit those boxes. Where do our loyal companions, the dogs, fit into this narrative? Is a dolphin the measure against which we judge all animal intellect? Before we know it, an unspoken hierarchy is taking root in our minds. Now, consider an alternate reality, one where the animal kingdom storms the gates of our “intelligent” society. What will the invite list look like? Dear reader, prepare to be stunned.

The insane world of slime moulds

Within the expansive realm of living organisms, a single-celled slime mould will be part of our list. This blob-like organism thrives on decaying leaves and logs. Its scientific name is Physarum polycephalum (Fig. 1).

It is capable of skillfully solving mazes, simulating the arrangements of human transportation networks, and correctly selecting the most nourishing foods. This remarkable organism achieves all of this without depending on a brain or neural structure. Let’s explore how its intelligence enables it to identify the most efficient routes through complex mazes and make decisions to ensure optimal nourishment.

 Fig 1.  Physarum polycephalum (slime mould). ©Björn S. Licensed as Attribution-ShareAlike.
Available online: https://www.flickr.com/photos/40948266@N04/38675943684/ 

Let us get to know the slime mould a little better. It is a unicellular fungus with multiple nuclei, sustaining itself by consuming bacteria and small organic particles. It gets this nourishment through a process known as phagocytosis i.e., engulfing its food. Slime mould looks for food in its environment via slow motions fueled by the flow of the jelly-like cytoplasm within it, termed “cytoplasmic streaming.” While performing this process we see tube-like arms extending from its body, also known as plasmodial tubes.

Lost in a maze? A slime mould can show you the way!

Now, let’s dig deeper into its problem-solving and decision-making skills. A prime example is its ability to find the shortest path in a given situation, for example, in a maze. Navigating mazes to find the optimal path is challenging even for higher-order animals. Yet this seems to be effortless for these blobs. To gain insight into this intriguing behaviour, Toshiyuki Nakagaki and his team designed an experiment: imagine a simple ring marked by two distinct points. An arrangement designed to see how these moulds construct plasmodial tubes between the points while favouring the shortest path. To incentivise the process, food was placed at these points. By manipulating the distance between the food sources via changing the circle’s central angle (Θ), Nakagaki gained a window into the mould’s decision-making process (Fig. 2).

Fig 2. The image describes the ring arrangement. It shows both the long and short routes. Theta (Θ) depicts the central angle.

The results showed intriguing patterns. The mould consistently opted for the shorter route (in cases with angles like 90 or 135 degrees). However, for an angle of 160 degrees, the choice between the 2 potential pathways became similar since both the routes were almost equal. These results instilled Nakagaki and his team with a sense of confidence. Therefore, the team ventured into more complex maze scenarios and connected two points by four possible routes. Connections with longer routes died, leaving only the most promising paths intact. Curiously, the plasmodial tube can stretch even beyond the maze’s confines under specific conditions. They can cross the walls of the maze creating paths even shorter than expected. This occurs when the amount of organisms present is high or increased growth occurs.

How does a slime mould determine the shortest path?

Yoshihisa Mori conducted experiments using a more complex ring configuration with two points, varying either the ring’s radius or central angle. Different ring configurations were tested with varying diameters and central angles. This revealed that as the central angle expanded, accuracy in selecting shorter paths declined, highlighting the significance of length ratio over absolute difference in guiding P. polycephalum‘s decision. These outcomes led to the idea that the mould seemed to possess an innate grasp of magnitude differences between distinct stimuli.

The above-mentioned phenomenon is similar to the principles of Weber’s law, which is also observed in various organisms including us humans. Weber’s law states that our perception of the difference between two things (like distance, sounds, or brightness) depends more on how much they change in relation to their original amount rather than the exact amount of the things themselves. Since these moulds lack neural systems, such cognitive mechanisms are both unexpected and captivating.

The adaptive intelligence of slime moulds

Now, let us make a transition and venture into the domain of gastronomic decision-making. Physarum polycephalum’s extraordinary skill in making optimal dietary choices is yet another dimension of intelligence that defies our understanding. Decision-making, being a complex process, often needs a specialised centre in the brain of most species. How do these slime moulds perform such a coordinated task? Audrey Dussutour and her team aimed to crack this mystery. They showed that a slime mould can strategically extend its tubes to connect with patches of varying nutrient qualities, achieving an optimal diet composition.

Defining an “optimal” diet becomes an intriguing pursuit here. Dussutour’s team first determined the ideal carbohydrate-to-protein ratio across 35 diets. Then they observed how slime moulds responded when exposed to foods with different nutritional compositions, to understand shifts in their growth patterns. While the essential nutrient is carbohydrate, slime moulds display distinct behaviours on diets with varying protein densities. Light growth is observed on low-protein substrates. Therefore, they will continue searching for an optimal source leading to the generation of new tubes. On finding a high protein source it yields dense, thick tubes. On finding extremely protein-heavy diets, slime moulds fragment.

Two experiments were performed to test their ability to choose. In the first experiment, refer to Fig. 3(a),  two foods were presented, their choice here revealed that they can grow in a precise ratio towards both foods to get optimal nutrition. The second experiment, refer to Fig. 3(b), involved 11 different foods with varying carbohydrate-to-protein ratios. Here it chose the food that provided better growth rates. This proved its capacity to select nutritional options.

Fig 3 (a) shows a diverse food pairing experiment. (b) shows the Multiple-choice experiment. In both experiments, the slime mould was initially placed at the centre of the petri dish.

These experiments show that slime mould intelligence is adaptive. Therefore, depending on its environment, it can choose how it grows.

The Vast Spectrum of Intelligence in Nature

To navigate and thrive, these simple organisms distinguish nuances in their external environments. Whether using Weber’s law as a guiding force or choosing the most optimal nutritional choice, these behaviours hint at an intricate cognition system. While our understanding has been limited, such organisms challenge us to expand our views. In the grand tapestry of existence, this amoeboid entity shows us that intelligence is not isolated but spread throughout the myriad forms of life. Intelligence is multifaceted and continues to be an enigma that enriches our exploration of the natural world.

In conclusion, the fascinating world of adaptive intelligence in slime moulds stands as an extraordinary testament to the wonders of nature. Have you ever considered the existence of such small but fascinating intelligences?

References:
  1. Physarum mold. (n.d.). Hampshire College. https://www.hampshire.edu/academics/faculty/physarum-mold
  2. Briggs, G. M. (n.d.). Physarum: a plasmodial slime mold. Pressbooks. https://milnepublishing.geneseo.edu/botany/chapter/physarum/
  3. Nakagaki, T., Yamada, H., & Tóth, Á. (2001). Path finding by tube morphogenesis in an amoeboid organism. Biophysical Chemistry, 92(1–2), 47–52. https://doi.org/10.1016/s0301-4622(01)00179-x
  4. Mori, Y., & Koaze, A. (2013). Cognition of different length by Physarum polycephalum: Weber’s law in an amoeboid organism. Mycoscience, 54(6), 426–428. https://doi.org/10.1016/j.myc.2013.01.008
  5. Dussutour, A., Latty, T., Beekman, M., & Simpson, S. J. (2010). Amoeboid organism solves complex nutritional challenges. Proceedings of the National Academy of Sciences of the United States of America, 107(10), 4607–4611. https://doi.org/10.1073/pnas.0912198107
  6. Latty, T., & Beekman, M. (2009). Food quality affects search strategy in the acellular slime mould, Physarum polycephalum. Behavioral Ecology, 20(6), 1160–1167. https://doi.org/10.1093/beheco/arp111

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