Pavlov’s Amoebas: They May Not Have Brains, but They Have Memory

Unicellular animals can remember where food is for up to 90 minutes and will overcome obstacles to get to it, Israeli-Spanish collaboration shows

Ruth Schuster
Ruth Schuster
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Ruth Schuster
Ruth Schuster

Trained dogs will salivate at the sound of a dinner bell, Ivan Pavlov famously discovered by accident in the 1890s. If the dogs realize that a peal portends food, they will drool at its sound and not only when the meal materializes. That is called conditioning. Now, a multidisciplinary group of Israeli and Spanish scientists found evidence that amoebas can be conditioned too — which was quite the shocker given that they are one-celled animals with no brain.

The amoebas were not producing anticipatory salivation. They do not have mouths or saliva. What they were doing is remembering the way to food for up to 90 minutes, report Ildefonso M. De la Fuente of the University of the Basque Country, Shira Knafo of Ben-Gurion University of the Negev, Be’er Sheva, María Dolores Boyano (also of the Basque university) and others, with disciplines encompassing mathematics, cancer research and pathology. Their groundbreaking study was published in the journal Nature.

The microscopic animalcules can retrace their path to food after the food stimulus is removed. Not all are unicellular Einsteins: Some lose the ability after about 20 minutes but some can retain it for 90 minutes, the scientists found. The average was 44 minutes.

Amoebas have never been suspected of smarts. Nor are they now. Knafo and her colleagues suspect the associative memory of the avenue to food is biochemical: temporarily encoded in a network of proteins.

Associative memory — the ability to learn and remember a consistent relationship between unrelated things, such as a sound and steak — was thought to be the prerogative of animals that have nervous systems. It isn’t a vertebrate thing: nonvertebrates from the clever octopus to the snail have amply demonstrated the talent. It has not been demonstrated in unicellular species, until now.

“Here we describe a motility pattern consistent with associative conditioned behavior in the microorganism Amoeba proteus,” the team begins its paper.

The conditioning stimulus wasn’t a bell, it was an electric field. And the bait (“the unconditioned stimulus”) was a yummy protein chain. Amoeba steak, in other words.

What they found is that the amoebas could link two past events: an electric stimulus and the appearance of dinner.

Crucially, the scientists created an obstacle in the amoebas’ way and demonstrated that the little things would slog on willy-nilly.

By nature — for reasons not clear, Knafo acknowledges — amoebas are attracted to negative current and repelled by positive.

The researchers put the amoeba food on the positive side of the slide — and despite their repulsion for that charge, they went there. “Chemotaxis prevailed over galvanotaxis,” Knafo observes, which means their yen for the food overcame their dislike for the charge.

If the food was taken away, the amoebas would still persistently gravitate toward the positive side of the slide, which contravenes their nature. They would do this for a time (on average, the abovementioned 44 minutes).

That’s pretty impressive for an animal without a brain.

The experiment was repeated with another one-celled animal, Metamoeba leningradensis, a cousin to the simple amoebas we know and love. The same results ensued.

Now the team is working on four more amoeba species, Knafo told Haaretz.

Since this “memory of how to find food” has now been found in two amoeba species, one may suspect that it’s universal. However, it is early days and, yes, the team does plan, generally, to test for the phenomenon in other unicellular species before seriously postulating its universality.

However, Knafo suspects that since nature is parsimonious with its biological mechanisms, this hungry amoeba study could shed light on how our own cells work.

If you wonder how an amoeba moves, here’s a video of one in action: