Slime molds are among the world’s strangest organisms. Long mistaken for fungi, they are now classed as a type of amoeba. As single-celled organisms, they have neither neurons nor brains. Yet for about a decade, scientists have debated whether slime molds have the capacity to learn about their environments and adjust their behavior accordingly.

For Audrey Dussutour, a biologist at France’s National Center for Scientific Research and a team leader at the Research Center on Animal Cognition at Université Paul Sabatier in Toulouse, that debate is over. Her group not only taught slime molds to ignore noxious substances that they would normally avoid, but demonstrated that the organisms could remember this behavior after a year of physiologically disruptive enforced sleep. But do these results prove that slime molds — and perhaps a wide range of other organisms that lack brains — can exhibit a form of primitive cognition?

Slime molds are relatively easy to study, as protozoa go. They are macroscopic organisms that can be easily manipulated and observed. There are more than 900 species of slime mold; some live as single-celled organisms most of the time, but come together in a swarm to forage and procreate when food is short. Others, so-called plasmodial slime molds, always live as one huge cell containing thousands of nuclei. Most importantly, slime molds can be taught new tricks; depending on the species, they may not like caffeine, salt or strong light, but they can learn that no-go areas marked with these are not as bad as they seem, a process known as habituation.

“By classical definitions of habituation, this primitive unicellular organism is learning, just as animals with brains do,” said Chris Reid, a behavioral biologist at Macquarie University in Australia. “As slime molds don’t have any neurons, the mechanisms of the learning process must be completely different; however, the outcome and functional significance are the same.” ­­­­­­­

For Dussutour, “that such organisms have the capacity to learn has considerable implications beyond recognizing learning in nonneural systems.” She believes that slime molds may help scientists to understand when and where in the tree of life the earliest manifestations of learning evolved.

Even more intriguingly, and perhaps controversially, research by Dussutour and others suggests that slime molds can transfer their acquired memories from cell to cell, said František Baluška, a plant cell biologist at the University of Bonn. “This is extremely exciting for our understanding of much larger organisms such as animals, humans and plants.”

A History of Habituation

Studies of the behavior of primitive organisms go all the way back to the late 1800s, when Charles Darwin and his son Francis proposed that in plants, the very tips of their roots (a small region called the root apex) could act as their brains. Herbert Spencer Jennings, an influential zoologist and early geneticist, made the same argument in his seminal 1906 book Behavior of the Lower Organisms.

However, the notion that single-celled organisms can learn something and retain their memory of it at the cellular level is new and controversial. Traditionally, scientists have directly linked the phenomenon of learning to the existence of a nervous system. A number of people, Dussutour said, thought that her research “was a terrible waste of time and that I would reach a dead end.”