Though there is no consensus on what it means to be intelligent, most would agree that having a nervous system is a prerequisite.
That notion has recently been challenged with the publication of a study showing that a simple slime has the capacity to learn — and could redefine what we consider to be necessary for intelligence.
The unassuming candidate for an intelligent creature is Physarum polycephalum, also known as "many-headed slime". Though it can live freely as a single-celled organism, individuals often aggregate and move together as a single yellow blob that changes shape as it crawls.
A team of scientists from Belgium and France was interested in testing whether a species lacking a central nervous system, such as P. polycephalum, could exhibit habitual learning — a process by which an original behavior changes in response to repeated stimulus.
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The researchers grew samples of the slime in petri dishes containing a jelly-like substance called agar gel, and placed another petri dish containing food for the slime nearby. The two dishes were connected by an agar gel bridge that the slime could crawl across, which it tended to do within about 2 hours.
When researchers placed quinine or caffeine on the bridge, the slime “showed clear aversive behaviour” to these bitter substances at first. It hesitantly crossed the bridge in a much slower manner and avoided touching the irritants by taking a narrow path around them.
Over the next few days, the researchers were amazed to find that the slime was beginning to cross the bridge more quickly — it was becoming habituated to the quinine or caffeine in an intelligent way.
Experiments previously demonstrated that this slime is able to find the shortest route connecting food sources placed at the start and end of a maze — a challenge thought to require complex calculations.
"Our results point to the diversity of organisms lacking neurons, which likely display a hitherto unrecognized capacity for learning,” the authors wrote.
The ability to learn is often considered a hallmark of intelligence — a trait that even a blob of slime without a nervous system can exhibit, according to this research.
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