Because complex organisms are usually made up of trillions of cells, it is easy to ignore single-celled organisms. But a new study from Harvard University suggests that some of these organisms may have more complex decision-making skills than we think. In the experiment, the researchers found that a single-celled organism could choose a different option and “change its mind.”
The creature, known as Stentor roeselii, is a freshwater single-celled organism shaped a bit like a trumpet. Its relatively large body has a mouth-like opening at one end, surrounded by fine hairs called cilia. These cilias rotate in precise motion, forming a vortex that draws food particles into the “mouth”. At the other end, S. roeselii gradually slims it into a holding agent, anchoring it to a solid surface.
In experiments with organisms, the Harvard team found that it could choose from a hierarchy of behaviorbasedd on responses to stimuli and “change its mind” when action didn’t help. Stimulation is the pulse of microplastic beads.
The researchers observed that S. Roeselii reacts to stimuli in many different ways. First, these creatures bend away from the beads. If that doesn’t work, it starts to shake the cilia in the opposite direction, pushing the particles away from its mouth. The S. Roeselii can also choose to shrink himself back into the fixer and, as a last resort, escape and swim away.
These behaviors suggest that the behavior and decision-making of single-celled organisms is more complex than scientists generally believe. After all, for a creature without a brain, changing ideas is a pretty impressive ability.
The team is not the first to do such an experiment – instead, they set out to reproduce the results of the study more than a century ago. Zoologist Herbert Spencer Jennings described the same basic experiment as early as 1906, but the results were similar.
However, the researchers in the new study did find some slightly different results than Jennings’s. Jennings reports that these behaviors always have to occur in the same order. But the new study found that not all single-celled organisms react in the same way or in the same order. Some single-celled organisms may bend and then contract, while others may resort to cilia, then bend, and then shrink.
However, when the team analyzed the results, they found an interesting pattern. Although each organism may bedifferent, they have a potential hierarchy. In response, S. roeselii in most cases bends and swings the cilia first, and if the stimulation persists, they contract and eventually detach. They almost always follow this rule.
Jeremy Gunawardena, lead author of the study, said: “They do simple things first, but if you keep stimulating, they ‘decide’ to try other things. S. Roeselii does not have a brain, but once the sensation has been going on for too long, there seems to be a mechanism that can make it ‘change its mind’. This hierarchy gives a vivid sense of some form of relatively complex decision-making calculation in organisms, weighing whether it is better to execute one behavior than to execute another.
The researchers speculate that single-celled organisms like S. roeselii may have some form of cellular “cognition” that allows them to process complex information and make decisions.
The study was published in the journal Current Biology. The team described the findings in the video below.