Slime is not something that’s normally thought of as particularly intelligent, but one particular type of primordial sludge has just learned a new trick, according to a study published this week in the Proceedings of the Royal Society B.

Researchers from Toulouse University “taught” a single-celled organism called Physarum polycephalum to adapt its behavior when exposed to certain substances – a remarkable achievement for a gooey mold that has no brain or neural system. Also known as “true slime mold,” P. polycephalum’s skills may pale in comparison to the mind-reading “mood slime” that appeared in Ghostbusters 2, but it has helped to shed light on the evolution of learning behaviors in simple organisms.

Like most unicellular beings, P. polycephalum is typically unconcerned with life’s larger questions and complexities, and tends to devote itself solely to fulfilling its need for food. When placed in a petri dish containing a food source, the slime can be relied on to migrate towards the nutrients.

Researchers, therefore, wanted to see how the mold would react when a barrier – made of unpleasant but ultimately harmless substances – was placed between it and the food. To investigate, they separated the slime from its bounty using a bridge made of either caffeine or quinine – two bitter substances that are known to repel P. polycephalum. Importantly, they made sure to use concentrations that, while distasteful, were not damaging to the organism.

On the first day, the slime showed a reluctance to cross the bridge, and took several hours to build up the courage to do so. However, by the sixth day, the mold had apparently learned that the substances in these bridges were not harmful, and therefore accessed the food in the same amount of time as they had when these repellents were absent.

^{Researchers say the slime's behavior is an example of a type of learning known as habituation. Audrey Dussutour (CNRS)}

According to the study authors, this type of adaptive behavior is an example of a form of learning known as “habituation,” whereby organisms learn to “ignore irrelevant stimuli and focus selectively on important stimuli that are key to their survival.” In other words, the slime stopped paying attention to the repellents once it realized that they were not harmful.

The fact that P. polycephalum has no brain raises a number of questions regarding the mechanisms behind this surprising learning capacity, although the researchers suggest that it may be controlled by epigenetics, whereby certain genes become either silenced or activated in response to environmental stimuli.

For instance, they propose that repeated exposure to caffeine and quinine may inhibit the genes that encode for the receptors responsible for detecting these substances. As such, the number of caffeine and quinine receptors is reduced, enabling the organism to ignore these chemicals.

Amazingly, this research is not the first to illustrate the surprising capabilities of P. polycephalum. Something of a superstar among slimes, the organism has previously displayed an ability to navigate mazes and avoid traps, although this latest study is the first to provide an explanation for this all-singing, all-dancing behavior, suggesting that habituation may play a key role in the adaptive skills of single-celled organisms.