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COMPUTING SCIENCE

In Search of the Optimal Scumsucking Bottomfeeder

Brian Hayes

Following the Paper Trail

Surely the least abstract simulation of trace-fossil worms was described in 1997 by Tony J. Prescott and Carl Ibbotson of the University of Sheffield. They did it with hardware—specifically, a robot built from Lego parts, which marked its trail by paying out a roll of toilet paper behind it. Arms extending from each side of the body carried pairs of optical sensors, which could detect the white paper against a dark floor. An on-board computer steered the machine by applying the now-familiar rules of thigmotaxis, phobotaxis and strophotaxis. The basic idea was to stay parallel to an existing trail by keeping the edge of the paper between the two sensors on one side of the chassis. If both sensors lost contact with the trail, the robot would steer toward it; if both sensors detected the presence of paper, the robot would veer away to avoid a crossing.

Figure 3. Lego robot built byClick to Enlarge Image

Getting the robot to produce a spiral was particularly easy. In fact, a spiral is the natural product of the rules just stated, given a bias favoring the sensors on one side or the other. Initially there is no trail to detect, and so the robot turns continually toward the favored side. After making a 360-degree loop, it encounters its own trail and begins spiraling outward.

Making zigzag traces is only slightly more complicated. All that's needed to initiate a U-turn is to transfer control from the sensors on one side of the body to those on the other. But the robot must still decide when to turn. Prescott and Ibbotson used a simple timer: The worm always turns after a fixed interval. With appropriately chosen parameters, they were able to generate the common motif of a spiral that evolves into a meander. At the outset, when the radius of the spiral is small, the worm can complete a few full revolutions before the internal timer runs out; later, at a larger radius, the timer triggers a U-turn before a full revolution is completed.

Back in the immaterial world of software, a model published in 1998 by Oyvind Hammer of the Paleontological Museum in Oslo returns to the simulated-evolution methods of Papentin. A population of 400 worms is let loose in an environment with limited food resources, and only the most successful grazers are allowed to pass on their genes. Each worm's movements are controlled by a network of components such as sensors, oscillators, memory elements, adders and multipliers, which initially are wired up randomly. It is the connections between these modules that are altered by mutation and recombination.

Under conditions of intense competition, Hammer's worms evolve from random toward systematic foraging. In particular, the worms learn to move rapidly between isolated patches of food, but they dawdle within a patch. Hammer's worms did not develop the kind of tightly organized meanders produced by ichnospecies such as H. labyrinthica. They also seem to have a habit of nibbling around the edges of a patch of food, rather than spiraling from the inside out.




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