Amazing maze. A slime mold finds the shortest route between two food sources.

Slimy, But Not Stupid

Slime molds, brainless amoebalike organisms that live in forests and plant beds, aren't likely to win the Nobel Prize. But Japanese and Hungarian researchers report that they may yet possess a shred of something akin to intelligence. When placed in a maze between two sources of food, the slime seeks out and finds the shortest path through the maze. Researchers say this ability shows that even lifeforms as primitive as a single cell can perform computations.

For many biologists, slime molds provide simple models for studying cell development and mobility. Toshiyuki Nakagaki, a biologist at the Institute of Physical and Chemical Research (RIKEN) in Japan, thought the organism might be capable of more. Five years ago, he got the idea to test whether a particular type of one-celled slime mold, Physarum polycephalum, could navigate through a maze.

In the 28 September issue of Nature, Nakagaki and his colleagues describe a series of experiments with a 4-centimeter-by-4-centimeter maze, using plastic film to separate the passageways. After letting the mold spread through every passageway, the researchers placed two cubes of ground oat flakes at opposite ends of the maze and waited 8 hours for the mold to respond. In each of 17 out of 19 trials, the slime shrank back to a thick strand that connected the oat cubes via the shortest route.

To do this without a brain or nervous system, says Ken Showalter, a chemist at West Virginia University, the organism relies on proteins and nutrients that "swish back and forth" through the cell to communicate the location of the food and allow the organism to change shape.

Although Showalter balks at calling the mold's behavior intelligent, he says that "putting [the mold] in a maze is a demonstration of how fantastic this optimization is," and a novel way to look at complex behavior. The mold's ability to optimize its shape shows that "you don't need neurons to do information processing," adds John Tyson, a biologist at Virginia Polytechnic University in Blacksburg.

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