Single-Celled Organism Learns Like Pavlov's Dogs—Without a Brain

A new study has found that a single-celled microorganism can learn by association—a skill previously thought to require a nervous system. Researchers discovered that Stentor coeruleus, a trumpet-shaped protist, reacts to gentle taps by anticipating stronger ones. This behaviour mirrors Pavlovian conditioning, a form of learning first demonstrated in dogs over a century ago. The experiment began with 60 strong taps on a petri dish containing S. coeruleus. At first, the protist curled into a tight ball each time. But as the taps repeated, its contractions lessened, showing it had grown accustomed to the disturbance.

Normally, the organism extends in a trumpet shape to feed. When bothered, it retracts into a protective ball. To test learning, scientists set up two groups: one received a weak tap followed by a strong tap, while the other got two weak taps in a row. The weak-strong group soon began curling at the first gentle tap, as if expecting the stronger one. The weak-weak group showed no such response. This suggests the protist had linked the weak tap with the impending strong one—a clear sign of associative learning. Previously, single-celled organisms were known to show basic learning, like habituation, but not this more complex form. The study, uploaded to *bioRxiv*, challenges the idea that associative learning depends on synapses or a brain.

The findings imply that associative learning evolved long before nervous systems did. Since S. coeruleus lacks synapses, its ability to form associations suggests a cellular basis for this behaviour. This discovery could reshape understanding of how learning first emerged in living organisms.