As creatures made up of trillions of cells, it’s easy for us to look down on single-celled organisms, but new research from Harvard suggests that some of them may be capable of more complex decision-making than we give them credit for. In experiments, a single-celled organism was found to consult a hierarchy of options and be capable of “changing its mind.”

The creature in question is called Stentor roeselii, a freshwater, single-celled organism that’s shaped a bit like a trumpet. At one end of its relatively large body is a wide mouth-like opening, with tiny hairs called cilia surrounding it. These hairs swirl in a precise motion to create a whirlpool that sucks particles of food into its mouth. At the other end S. roeselii tapers into a holdfast, where it anchors itself to a solid surface.

In experiments on the organism, the Harvard team found that it could choose from a hierarchy of behaviors in response to stimuli, and change its mind if an action wasn’t helping. The stimulus was a pulse of microplastic beads.

The researchers observed the S. roeselii responding to the irritant in a number of different ways. At first, the creatures would bend away from the beads. If that didn’t help, it would begin to flail its cilia in the opposite direction, pushing the particles away from its mouth. The S. roeselii may also choose to retract itself into its holdfast, and as a last resort it would detach itself and swim away.

A Stentor roeselii contracting in defense Bill Porter

The way these actions escalated in response to an unpleasant situation suggested more complex behaviors and decision-making than normally ascribed to single-celled organisms. After all, changing your mind about a decision is a pretty impressive ability for something without a brain.

The team wasn’t the first to conduct such an experiment – on the contrary, they specifically set out to reproduce results from a previous study from more than a century ago. Zoologist Herbert Spencer Jennings described the same basic experiment, with similar results, way back in 1906. Unfortunately, other scientists were unable to reproduce the results, and so they were largely discredited.

The researchers on the new study did find a couple of slightly different results to Jennings’, though. Jennings reported that the behaviors always had to happen in the same order. But the new study found that not all individuals reacted in the same way, or the same order. Some might bend then contract, while others might resort to flailing cilia, then bend, then contract.

But when the team ran statistical analysis on the results, they found an intriguing pattern. Though the individual behaviors might vary, there was an underlying hierarchy to them. In response, most of the time the S. roeselii would bend and wave cilia first, and if the irritation continued, then they would contract, then finally detach and flee. They almost always followed this general rule, and would never detach before contracting.

If other defense mechanisms don't work, S. roeselii will detach itself and swim away as a last resort Dexter et al, 2019

“They do the simple things first, but if you keep stimulating, they ‘decide’ to try something else,” says Jeremy Gunawardena, lead author of the study. “S. roeselii has no brain, but there seems to be some mechanism that, in effect, lets it ‘change its mind’ once it feels like the irritation has gone on too long. This hierarchy gives a vivid sense of some form of relatively complex, decision-making calculation going on inside the organism, weighing whether it’s better to execute one behavior versus another.”

The researchers speculate that single-celled organisms like S. roeselii may possess some form of cellular “cognition” that allows them to process complex information and make decisions.

The research was published in the journal Current Biology. The team describes the work in the video below.

Can a single cell "change its mind"?

Source: Harvard