What Can Slime Molds Teach Psychologists About Learning?
Learning may be defined by behavior, not by brains.
Posted May 29, 2026 | Reviewed by Gary Drevitch
The yellow blob that you see in the picture at left is a slime mold, a strange one-cell organism that lives in damp, shadowy areas on the forest floor where it slowly navigates its environment, searching for decaying matter, fungi, and bacteria to feed on. When it finds a food source, it reorganizes itself, retracting from areas with less food.
It looks like a fungus, but it isn’t. It’s also not a plant, and despite its ability to navigate, it’s not an animal either. It belongs to a different group altogether: the protists.
What is really interesting from a psychologist’s perspective is that this single cell shows complex behaviors such as optimizing paths, solving mazes, and adapting to its environment in efficient and sometimes unexpected ways.
Scientists also examined whether slime mold can learn. At first, this seems very unlikely. Learning, we tend to think, happens in the brain. No brain, no learning. Because a slime mold consists of only one cell, it clearly does not have a brain so surely it cannot learn, right?
Research challenges that intuition .
Imagine the following experiment (shown in the picture below): A slime mold sits in a petri dish. Nearby, another dish contains food, connected by a narrow bridge. There’s just one problem: the bridge is covered in substance that is harmless but unpleasant for the slime mold. The slime mold therefore remains mostly on its side, with little expansion onto the bridge. But eventually, it extends across and reaches the food.
Now the situation is repeated multiple times. Something changes. Over time, the slime mold reaches the bridge and crosses it faster each time. It seems that what was initially avoided gradually becomes tolerated.
This pattern has a name: habituation, a simple form of learning in which a response decreases after repeated exposure to a stimulus.
And it’s not just a loss of sensitivity. If a different unpleasant substance is introduced, the slime mold once again avoids the bridge. In other words, its behavior changes as a function of experience with that stimulus. Its behavior resembles learning but it doesn’t have a brain. So what should we make of this?
Part of the paradox comes from how we define learning. If learning means a change in the brain, then the answer is straightforward: Without a brain, there can be no learning.
But the experiment exemplifies a different way to look at it. Instead of focusing on what the system is made of, we can focus on what it does. From this perspective, learning can be understood as a change in behavior as a function of regularities in the environment . Under this definition, whether slime molds learn does not pose a paradox.
This shift has consequences. Once learning is defined functionally in terms of behavior rather than mechanisms, it is no longer limited to organisms with brains. It can be studied in plants, in artificial systems such as AI , and even in groups of individuals (systems in which no single brain coordinates the whole).
The question is no longer whether a system has neurons. Whether a system learns can be determined only by looking at its behavior changes systematically with experience. Using a single definition of learning that can be applied to all kinds of different systems, scientists can develop a general science of learning that cuts across systems. Surely slime mold do not learn in the same way as humans or other systems but only now can we start examining what the similarities and differences are.
Alejandro Macías is a postdoctoral researcher at the Learning and Implicit Processes Lab at Ghent University. The author thanks Jan De Houwer for his feedback on this text.
Boisseau, R. P., Vogel, D., & Dussutour, A. (2016). Habituation in non-neural organisms: evidence from slime moulds. Proceedings of the Royal Society B: Biological Sciences, 283(1829). 10.1098/rspb.2016.0446
De Houwer, J., & Hughes, S. J. (2023). Learning in individual organisms, genes, machines, and groups: A new way of defining and relating learning in different systems. Perspectives on Psychological Science, 18(3), 649–663. https://doi.org/10.1177/17456916221114886
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The Learning and Implicit Processes Lab at Ghent University examines when and how people learn and evaluate. In doing so, the lab focuses on the role of implicit processes, or processes that occur spontaneously.
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