The mysteries of slime mold intelligence

      Who would have guessed that an artist might have two illustrations of slime molds (Physarum polycephalum) in their portfolio? Certainly not me when I was an embryonic biology student at the University of Michigan—even though one Dr. Alfred Sussman transmitted his enthusiasm for these strange creatures when I was but a freshman. Sussman highlighted the ability of these wandering, multinucleated masses of living tissue to produce specialized reproductive cells when stressed. Now researchers are discovering that this primitive organism possesses the rudiments of intelligent behavior once considered the prerogative of more complex beings like us.

       Slime molds tend to wander on logs and forest floors in relatively moist habitats. I forgive those of you in the arid west who never encountered this biological wonder. Once classified as a kind of fungus, they really belong to a rather hodgepodge group called protists—single-celled beasties not easily classified otherwise. For those of you that are old enough, think of “The blob” in the old 1950s horror show, although slime molds are much smaller and slower. Nevertheless, slime molds are a mass of cytoplasm—the complex fluid found in all cells—sprinkled with multiple nuclei—the compact repository for a cell’s genetic blueprints. They scour logs and leaf litter for bacteria to eat. When faced with starvation, they send out signals that attract other wandering blobs. Those blobs merge, and some parts transform to create a stalk, that rises into the air and produces spores free to sail away on a draft of air to better hunting grounds.

      A Japanese researcher in the early 2000s, Toshiyuki Nakagaki, discovered that slime molds could successfully run mazes to find a food source (usually an oat flake). Physarum quickly found the most direct route to food. Eventually, the researchers discovered the mechanism for this behavior: Physarum deposits a trail of slime behind as it moves. As it wanders, if it encounters its own trail, it knows to turns toward yet unexplored territory. The slime trail becomes a kind of externalized spatial memory, not unlike the scent trails that ants and termites leave in their wake.

      Nakagaki et al came up with even more clever ideas. They placed oat flakes on the map locations of Japanese urban areas and set Physarum loose to find the most direct routes to food sources. Physarum eventually created “trunk lines” of cytoplasm essentially identical to Tokyo’s railway network. More recently, researchers have used slime mold talents to map out computer circuits. One team demonstrated that “a slime mold network can carry out XOR or NOR Boolean operations.” Whatever that means, it sounds pretty smart.

       Back in the 1960s one of my favorite authors, Loren Eiseley, compared the network of humans launching spacecraft off Earth toward other realms to slime molds. In his book, The Invisible Pyramid, he described the Pioneer probes of the day as “thistledown off to new worlds.” The Voyager spacecraft, launched just before Eiseley died, are some of our first “spores” headed off in the general direction of Alpha Centauri, our closest neighboring star, excluding our own sun, of course. They should arrive in 70,000 years or so.

       At any rate, I think slime molds are cool. More than 600 million years ago their lineage created cells on the verge of complex organizational skills. Part of that legacy was the streaming, electrically charged miracle of moving cytoplasm that held the secrets to forming the rudiments of intelligent behavior before brainy metazoans like us thought we had cornered the market in that area.

       Should anyone want to place a visual ode to the mystery of slime molds on their wall, I’ve got just the prints for you. E-mail me for more information about either the original artwork or reproductions.