WHAT IS IT? ----------- This project is inspired by the aggregation behavior of slime-mold cells. It shows how creatures can aggregate into clusters using a very decentralized strategy, without any "leader" involved. In this example, each creature drops a chemical pheromone (shown in green). The creatures also "sniff" ahead, trying to follow the gradient of the chemical. Meanwhile, the patches diffuse and evaporate the chemical. Following these simple, decentralized rules, the creatures aggregate into clusters. HOW TO USE IT ------------- Click the SETUP button to set up a collection of slime-mold cells. Click the GO button to start the simulation. The NUMBER slider controls the number of slime mold cells. Changes in the slider do not have any effect until the next SETUP command. The next 3 sliders affect the way turtles move. Changes to them will immediately affect the model run. SNIFF-THRESHHOLD -- Turtles only look for chemical gradients if there is sufficient (greater than sniff-threshhold) chemical where they are. Otherwise, if there is not enough chemical concentration in the turtle's patch, the turtle moves randomly. This parameter enables slime mold cells to aggregate when there are enough other cells nearby. The default value is 1.0. NOSE-ANGLE -- This value is the amount turtles turn to the left and right to check for greater chemical concentrations. The default value is 45. WIGGLE-ANGLE -- This controls the variation in the randomness of turtle movement. When set to zero, the turtles remain at the same heading unless they are following the chemical gradient. The default value is 40. BIAS -- Turtle movement is randomized. When BIAS is zero, the average turtle movement is straight ahead. When BIAS > 0, turtles will tend to move more left than right. The default value is 0. There are several other critical parameters of the model that are not accessible via sliders. They can be changed by modifying the code in the procedures window. Amongst these are: the evaporation rate of the chemical -- set to .9 the diffusion rate of the chemical -- set to 1 the amount of chemical deposited at each step -- set to 2 THINGS TO NOTICE ---------------- With 100 creatures, not much happens. The creatures wander around, dropping chemical. But the chemical evaporates and diffuses fairly quickly, and the creatures do not aggregate. With 500 creatures, following the exact same rules, the result is qualitatively different. When a few creatures happen (by chance) to wander near one another, they create a "puddle" of chemical. The creatures sniff the chemical and try to stay nearby. They then deposit more chemical in the puddle, so the puddle expands and attracts more creatures. This process is a good example of positive feedback: the more creatures, the larger the puddle, and the larger the puddle, the more likely it is to attract more creatures. THINGS TO TRY ------------- Try different values for the SNIFF-THRESHHOLD, NOSE-ANGLE, WIGGLE-ANGLE and BIAS sliders. How do they affect the turtles' movement, gradient following and the formation of clumps? Change the NOSE-ANGLE slider after some clumps have formed. What happens to the clumps? EXTENDING THE MODEL ------------------- Modify the program so that the creatures aggregate into a single large cluster more quickly. How do the results change if there is more (or less) randomness in the creatures' motion? What "critical number" of creatures is needed for the clusters to form? How does the critical number change if you modify the evaporation or diffusion rate? Can you find an algorithm that will let you plot the number of distinct clumps over time? STARLOGOT FEATURES ------------------- In the uphill-slime procedure, the turtle "follows the gradient" of the chemical. That is, it "sniffs" in three directions, then turns in the direction where the chemical is strongest. The uphill-slime procedure can be useful in many other projects. You might want to try variants of the uphill-slime procedure, changing the number and placement of "turtle sniffs." CREDITS and REFERENCES ----------------------- This model was adapted from the MIT Media Lab slime model. See Resnick, M. (1994) "Turtles, Termites and Traffic Jams: Explorations in Massively Parallel Microworlds." Cambridge, Ma: MIT Press. Adapted to StarLogoT, 1997, as part of the Connected Mathematics Project.