If you think that the answer is obvious, well, think again. Because a few brainless creatures are gifted with outstanding cognitive skills that allow them to calculate the most efficient route to food, learn and even teach!
Slime moulds are not animals, plants, or even fungi. They are protists, which is a group that mostly comprises microscopic single-cell organisms. Slime moulds, however, are not necessarily unicellular nor microscopic. Some species can intriguingly shift between unicellular and multicellular forms, while others rather live as single cells so huge that you can spot them with your naked eye. But for some reason, they all seem to rock a weird gluey look. Actually, have you ever noticed a yellowish bad curry spilling over the side of rotting logs during an otherwise really nice walk? Yeah… That probably was a slime mould!
Still, do not let their modest looks fool you. Although they do not possess a brain or any neuron cells, which are usually regarded as a sign of intelligence, these gluey creatures display incredible intellectual capabilities. In fact, the slime mold species Physarum polycephalum has recently become the focal point of several cognitive studies.
The yellowish masses in this picture are single-cell slime moulds belonging to the species Physarum polycephalum. Unlike most unicellular creatures, this species is visible to the naked eye during part of its life cycle. Photo by Bill Sheehan (CC BY-SA 3.0 license).
In one of them, scientists placed oat flakes on a flat surface, carefully arranging them so they resembled the map of Tokyo and its neighbouring cities. Then, P. polycephalum was added. I bet the researchers were baffled when this slime mould constructed networks of nutrient-channelling tubes strikingly similar to the layout of the Japanese railway. I guess you do not need a brain to be a gifted engineer after all!
Being able to find the most efficient route to food sources is not the only impressive skill these brainless creatures can brag about. You see, learning is often considered as a signature move of organisms with a neural network, but these slime moulds strongly disagree.
In another experiment, researchers placed one P. polycephalum individual and its food in opposite sides of a Petri dish. Then, they made sure that this slime mould could only reach its tasty reward by crossing a salt bridge. Despite being harmless to slime moulds, salt has a repellent effect. Hence, scientists expected them to exhibit an aversive behaviour towards the bridge. And that was exactly what happened on the first day to all tested individuals. However, after being fed for six days in environments rich in salt, several slime moulds crossed that bridge without showing any signs of aversion. This means that these brainless organisms actually learned not to “fear” this harmless substance. Am I the only one who is completely gobsmacked? But, wait. There is even more to this perplexing species!
In a third study, scientists repeated the salt bridge experiment with many slime moulds at a time. Keep in mind that some of them were trained for 5 days, and others were not. After being paired together and before facing the challenge, slime moulds fused into a single living entity. And…guess what? The merged entities that contained at least one trained individual showed less aversion while crossing the bridge! Plus, researchers also discovered that, just by fusing with trained individuals, untrained ones were able to cross the bridge on their own with a minimum display of aversive behaviours. I think it is amazing that these humble brainless creatures can learn, but it is absolutely bamboozling that they can also teach each other!
So, are Physarum polycephalum slime moulds smart? Well, I guess it is fair to say that they can challenge many species of this planet on an intellectual battle. Don’t you agree?
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