Slime mold, a unicellular organism at the bottom of the food chain, can learn, a finding that has important implications for understanding the evolution of learning, as well as how many creatures can be "smart" and successful without a brain.
Learning likely even predates the emergence of nervous systems, much less brains, according to a new study, which is published in the journal Proceedings of the Royal Society B. The research could explain why something as lowly as slime fares so well.
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"Slime molds are known for their surprisingly complex cognitive abilities: finding the shortest way through a maze, anticipating periodic events, choosing the best diet or avoiding traps," lead author Romain Boisseau of Toulouse University and Paris' École Normale Supérieure told Discovery News.
Boisseau and colleagues David Vogel and Audrey Dussutour investigated whether or not slime could learn to react in a beneficial manner to quinine and caffeine, two substances that slime tends to avoid. The researchers built a gelatin bridge that the slime, Physarum polycephalum, had to cross in order to obtain an oat-based snack on the other side. In one experiment, the bridge contained quinine; in the other, it contained caffeine.
During the first runs, the quinine and caffeine-containing bridges stopped the slime in its tracks. Eventually, after several hours, it slowly crossed the bridge and obtained the snack. In subsequent runs, the slime moved ever faster over the bridge and toward the food. When quinine and caffeine were removed from the bridge, the slime went back to its original behavior of crossing the bridge with no hesitation.
The learning exhibited by the slime during the experiments is known as habituation, which is different, the researchers say, than simple sensory adaptation -- when chemical receptors change their sensitivity due to a stimulus -- or motor fatigue, when an organism is no longer able to respond because it is tired.
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Boisseau explained that basic learning requires at least three steps: a behavioral response to whatever the trigger is, memory of that moment, and future changed behavior based on the memory.
The organism must, however, "be able to recover from the process," such that it is not locked into the new behavior, senior author Dussutour added.
The researchers are not yet certain how a unicellular creature like slime exhibits learning and cognition without a brain. Prior papers authored by Simona Ginsburg of the Open University of Israel and colleague Eva Jablonka theorized the ability to switch genes on or off could potentially store past experiences in cells, allowing organisms without nervous systems to remember and learn.
Plants also have the ability to learn, previous studies have shown. For example, Monica Gagliano and Michael Renton from the University of Western Australia and their team determined that the Mimosa pudica plant could learn to fold its leaves in a protective way in response to touch. If the handling occurs gently over time, however, the plant stops wasting energy on adjusting the folding, having learned that this type of touch poses little if any threat.
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In another study, Gagliano and Renton even found that plants can "talk" with each other via nanomechanical vibrations.
Gagliano explained that "acoustic signals generated using nanochemical oscillations from inside the cell(s)" of plants could allow one plant to communicate with others that are nearby.
Still other research has demonstrated that single-celled organisms called ciliate protozoa may be able to learn. Dussutour additionally suspects that bacteria are capable of learning.
"I think that learning is one of the best ways to adapt to the environment," she explained. "It's necessary for survival in most organisms, so I would predict that it would also exist in bacteria."
She and her colleagues believe that simple forms of learning likely emerged before the evolution of specialized nervous systems. They point out that many of the processes that are considered to be fundamental features of the brain, such as integrating sensory information, decision-making, and learning, have all been displayed in various organisms without nervous systems.