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Can a simple organism that lives off of dead trees and that grows as a mass of protoplasm actually have intelligence? The goal of this science fair project is to test the ability of the slime moldto solve the problem of finding the shortest path through a maze.

Introduction

With a name like slime mold, it's understandable that you would expect these organisms to be ugly and disgusting. Despite their unappealing name, slime molds are fascinating and, for some observers, beautiful organisms. They form colorful gliding masses as they creep over decaying wood and leaves. A particularly interesting and well-studied slime mold is called Physarum polycephalum, the many-headed slime mold.



Figure 1. Physarum polycephalum growing in Olympic State Park. (Wikipedia, 2009.) growing in Olympic State Park. (Wikipedia, 2009.)

In its growing stage, Physarum polycephalum looks like a giant amoeba that spreads out like a fan, enveloping and eating everything in its path. It consumes bacteria and decaying organic matter. In this plasmodial stage, Physarum polycephalum is a bright yellow glistening mass that can grow to more than a foot across. If the conditions that the organism finds itself in become challenging—for example, the ground becomes dry or food becomes scarce—the plasmodium will begin to shrink and form sporangia. The sporangia produce spores that can be spread by the wind to new regions for the organism to grow. Germination of the spores leads to the formation of a new plasmodium.

A curious variation occurs when the plasmodium encounters conditions that are too severe for it to continue growing, but not severe enough to cause it to form sporangia. It goes into a kind of dry, dormant state, called sclerotium from which it can revive itself if it becomes moist. The organism is sometimes shipped as a sclerotium when purchased from scientific supply companies.

Physarum polycephalum is inexpensive to obtain and it can be easily cultured on a moist surface, such as a damp piece of filter paper or an agar plate (agar plates are easier to use because they stay moist). It can be fed rolled oats as it grows, and divided to a fresh plate when it has grown to cover the first surface.

Slime molds, like Physarum polycephalum are part of the kingdom Protoctista. This is probably the least understood of the five kingdoms of life; the others being animals, plants, fungi, and bacteria. One of the most interesting things about Physarum polycephalum is that it shows a quality that could be called primitive intelligence. Studies done by Dr. Nagakaki and colleagues in Japan have shown that Physarum polycephalum can find the shortest distance through a maze. You can watch the video below to get an idea of how this can work.

If the Physarum polycephalum organism is chopped up and dropped into a labyrinth (a maze), they put themselves back together and start to move. At the same time, if a food source is placed at the entrance and exit to the maze, they avoid dead ends in the maze and form a connection (as a single tube) between the food sources. In all cases, the Physarum polycephalum chose the path that was the shortest between the two food sources. In a way, it "solved" the puzzle of finding the shortest path through the maze!

According to the abstract of the paper, here is how the authors sum up the findings:

"The plasmodium of the slime mould Physarum polycephalum is a large amoeba-like cell consisting of a dendritic network of tube-like structures (pseudopodia). It changes its shape as it crawls over a plain agar gel and, if food is placed at two different points, it will put out pseudopodia that connect the two food sources. Here we show that this simple organism has the ability to find the minimum-length solution between two points in a labyrinth" (Nakagaki, et al., 2000).

The procedure, as described in the Nakagaki paper, is quoted below:

"We took a growing tip of an appropriate size from a large plasmodium in a 25 X 35 cm culture trough and divided it into small pieces. We then positioned these in a maze created by cutting a plastic film and placing it on an agar surface. The plasmodial pieces spread and coalesced to form a single organism that filled the maze, avoiding the dry surface of the plastic film. At the start and end points of the maze, we placed 0.5- x 1- x 2-cm agar blocks containing nutrient (0.1 mg/g of ground oat flakes). There were four possible routes (α1, α2, β1, β2) between the start and endpoints" (Nakagaki, et al., 2000).

A maze is created and filled with the slime mold Physarum polycephalum. The first image in the top-left is taken after the slime mold completely saturates all parts of the maze, and before two nutrient blocks are added to the start and end point of the maze. The second image on the bottom-left shows that the initial volume of the slime mold decreases after 4 hours and the majority of the slime mold exists and connects along every possible path from the beginning nutrient block to the end nutrient block. The third image in the top-right shows the slime mold decreasing in volume again after 4 hours. The majority of the slime mold now only exists and connects along the shortest path from the beginning nutrient block to the end nutrient block. The final image in the bottom-right is of a table that shows the frequency which the slime mold "chooses" certain paths in the maze after the experiment is repeated. In fourteen out of nineteen trials the slime mold picked the shortest or second shortest path which only differed in length by about 2%.

Figure 2. Maze-solving by Physarum polycephalum. 2.a. Structure of the organism before finding the shortest path. Blue lines indicate the shortest paths between two agar blocks containing nutrients: α1 (4151 mm); α2 (3351 mm); β1 (4451 mm); and β2 (4551 mm). 2.b. Four hours after the setting of the agar blocks (AG), the dead ends of the plasmodium shrink and the pseudopodia explore all possible connections. 2.c. Four hours later, the shortest path has been selected. Plasmodium wet weight, 90 +/- 10 mg. Yellow, plasmodium; black, "walls" of the maze; scale bar is 1 cm. 2.d. Path selection. Numbers indicate the frequency with which each pathway was selected. "None:" no pseudopodia (tubes) were put out. See Supplementary Information at the Nature website noted in the Bibliography for an animated versions of 2.a-c. (Nakagaki, et al., 2000.) Maze-solving byStructure of the organism before finding the shortest path. Blue lines indicate the shortest paths between two agar blocks containing nutrients: α1 (4151 mm); α2 (3351 mm); β1 (4451 mm); and β2 (4551 mm).Four hours after the setting of the agar blocks (AG), the dead ends of the plasmodium shrink and the pseudopodia explore all possible connections.Four hours later, the shortest path has been selected. Plasmodium wet weight, 90 +/- 10 mg. Yellow, plasmodium; black, "walls" of the maze; scale bar is 1 cm.Path selection. Numbers indicate the frequency with which each pathway was selected. "None:" no pseudopodia (tubes) were put out. See Supplementary Information at thewebsite noted in the Bibliography for an animated versions of(Nakagaki, et al., 2000.)

Quoting again for the Nakagaki paper, here is a summary of their results:

"The plasmodium pseudopodia reaching dead ends in the labyrinth shrank [Figure 2.b., above], resulting in the formation of a single thick pseudopodium spanning the minimum length between the nutrient-containing agar blocks [Figure 2.c., above]. The exact position and length of the pseudopodium was different in each experiment, but the path through α2—which was about 22% shorter than that through α1—was always selected [Figure 2.d., above]. About the same number of tubes formed through β1 and β2 as the difference (about 2%) in their path lengths is lost in the meandering of the tube trajectory and is within experimental error" (Nakagaki, et al., 2000).

In this advanced biology science fair project, you will attempt to repeat the research results of Nakagaki, et al. Independent confirmation of results is a common and important practice in science. The goal is to address the question "Does the organism Physarum polycephalum demonstrate qualities that can be termed primitive intelligence?" The Procedure in this science project idea provides an outline for approaching this research problem, but you will have to work out many of the details independently. There are plenty of ways to expand on Nakagaki, et al.'s findings, some of which are suggested in the Make It Your Own section at the end of the procedure, but you may be able to think of others.

Terms and Concepts

Slime mold

Physarum polycephalum

Plasmodium

Sporangia

Germination

Sclerotium

Agar plate

Protoctista

Primitive intelligence

Amoeba-like cell

Dendritic network

Pseudopodia

Minimum-length solution

Questions

How many species have been described in the kingdom Protoctista?

What is a pseudopod ?

? What is meant by the term dendritic network ?

? What are the various forms that Physarum polycephalum can assume in its life cycle?

can assume in its life cycle? Do you agree with the authors of the paper by Nakagaki, et al. that slime molds are capable of demonstrating a form of "primitive intelligence"?

Bibliography University of California, Berkeley. (2000). Introduction to the "slime molds.". Retrieved August 18, 2009.

Thomas, A. (2000, September 28). Slime mold solves maze puzzle. Australian Broadcasting Corporation, Science News. Retrieved August 18, 2009.

Conover, A. (2001). Hunting Slime Molds. They're not animals and they're not plants, and biologists want to know a lot more about them. Smithsonian Magazine. Retrieved August 18, 2009.

Nakagaki, T, et al. (28 September, 2000). Maze-solving by an amoeboid organism. Nature 407, 470. The abstract to the paper is available here: http://www.nature.com/nature/journal/v407/n6803/abs/407470a0.html. The complete version of the original paper is available for a fee from www.nature.com, or you could find it at a local library that carries Nature. The author may supply PDF versions of the paper on request.

407, 470. Jabr, F. and Rothschild, A. (2012, November 2). How Brainless Slime Moldes Redefine Intelligence. Nova scienceNOW and Scientific American. Retrieved October 29, 2013.