The question of whether slime moulds possess intelligence has sparked a bitter scientific debate that could fundamentally reshape our understanding of the natural world and humanity's place within it. At the heart of the controversy is Physarum polycephalum, a single-celled organism that can solve mazes, remember routes, and even improve upon human-designed transport networks, despite lacking a brain or nervous system.
From 'simple protoplasm' to maze-solving marvel
Forty years ago, Toshiyuki Nakagaki, then an undergraduate in Hokkaido, encountered a lemon-yellow stain in a petri dish. That meeting transformed him into a leading slime mould researcher and ignited a storm of biological and philosophical debate about the meanings of life and intelligence that continues today. Before Nakagaki's work, no one would have called a slime mould "intelligent"; the popular adjective was "simple". An 1895 description called it "a simple mass of protoplasm", while another termed it "among the most simple of organisms".
Nakagaki recalled his first impression: "I was just impressed with how fast it moved." Physarum polycephalum, the species in that dish, has a top speed of 1cm an hour—gold-medal standard for amoebic organisms. At first glance, Physarum appears to spread as irregularly as ink on blotting paper, but it actually senses its meals of bacteria or other microbes and reaches for them with a long, thin finger of protoplasm that not only digests food but then forages further or pulls back into the rest of the mould. Watching a slime mould over many days makes it impossible not to sense some deliberation at play.
The Tokyo railway experiment and its implications
In 2000, Nakagaki's astonishing research began to surface in the media. "Slime mould solves maze puzzle," one headline ran, summarising an experiment in which Nakagaki induced a Physarum to find the shortest route through a maze in search of food. In 2010, the New York Times reported on Nakagaki's most famous experiment—the canonical study scientists offhandedly call "the Tokyo railway one". Nakagaki placed oat flakes to resemble the map of Tokyo and 35 satellite towns, then let the Physarum loose from the centre of "Tokyo". The mould spread outwards, stretching tubules towards the suburban oat flakes, linking up with itself and displaying such efficiency that it nearly retraced the route map of the greater Tokyo area's railway system. Nakagaki noted that, free of human bias, Physarum may have improved the design of the network.
Nakagaki twice won the Ig Nobel prize—for research that is both amusing and thought-provoking. The first award cited his maze paper in the field of "cognitive science", while the second hailed the Tokyo railway study as a breakthrough in "transportation planning". After the maze experiment, Nakagaki said: "I thought we'd have to rewrite the dictionary."
The neuropurist versus biogenic divide
For more than a century, scientists assumed cognition and intelligence required neurons. But since the 1980s, a growing number of researchers argue that traits such as cognition are intrinsic to all life—a stance philosopher Pamela Lyon of Adelaide University calls "biogenic". Some scientists claim plants connect underground through fungal networks (nicknamed the "wood wide web"), allowing communication and resource sharing. Others suggest any individual cell, even a bacterium, can make memories—and that slime moulds are intelligent.
Neuropurists, as they are sometimes called, dismiss these claims. Lincoln Taiz, a veteran plant physiologist, accused biogenic theorists of having "brain envy". In 2007, a few dozen neuropurists signed a declaration arguing plants weren't intelligent and that "plant neurobiology" was merely a catchphrase. One signatory called the letter "the last serious confrontation between the scientific community and the nuthouse on these issues".
The battle is fraught, sometimes dirty. Michael Levin, a Tufts University professor and polarising figure among the biogenic group, believes every individual cell is intelligent and cognitive in its own way; he told me he regularly receives hate mail, including one letter calling him "dangerously insane". When Lyon co-edited a 2021 special issue on cognition for the Royal Society's Philosophical Transactions, she recalled: "One of the reviewers told us, in effect: 'This is the greatest thing since sliced bread'; another said: 'This looks interesting, but I've got my qualms about this, this and this'; and the third said: 'This is dangerous. You must not dignify this idea in a journal of this stature.'"
The nature of slime moulds and their abilities
Slime moulds are neither plants nor animals nor fungi, despite their name. Carl Linnaeus lumped them into a genus called "chaos". In 1866, Ernst Haeckel cast the 900-odd species into a bracket he called "protist". Anne Pringle, a professor of biology at the University of Wisconsin-Madison, noted that one slime mould kind can be hugely different from another: "It's like if someone picked birds and manatees and decided they have something in common, like eyes, and therefore belong in the same group."
The first slime moulds emerged 600 million years ago, compared with 300,000 or so years for Homo sapiens. Some species live in the Sonoran desert soil crust, on decaying dung in the Sahara, or in Antarctic moss cushions. Their folk names include wolf's milk, scrambled egg slime, tree-bark flower, demon droppings, and witches' butter. Some books speculate that The Blob, the 1958 film, was inspired by Physarum. Audrey Dussutour, a scientist in Toulouse working on cognition, has written books titled Everything You Wanted to Know About the Blob But Were Afraid to Ask and Moi le Blob.
Google Scholar returns more than 10,000 results for publications about Physarum in the last quarter-century. Dussutour spoke fondly of Physarums past, saying each specimen can surprise you: "If you cut one slime mould into four pieces, those pieces are all the same—they're clones of each other, so you'd expect them to behave the same way. But they act differently. In an experiment, one piece will pick option A, and another will pick option B." Chris Reid, a researcher at Macquarie University, added: "It's good to have a mould with a strong will to live. You go away for the night, come back the next morning, open the drawer, and find that it has spilled out of its petri dish."
Physarum uses its slime as an aide-memoire to recognise paths it has travelled. Reid's team harvested slime from a mould, coated one route in a maze with it, and introduced the mould; it nearly always chose the dry, un-slimed areas. More impressively, the Physarum can distinguish between its own slime and that of other species. In 2008, Nakagaki and colleagues subjected a Physarum to cold, dry air at regular hourly intervals, prompting it to slow its locomotion. After three gusts, the mould spontaneously slowed at the top of the fourth hour—even when the mistral didn't blow. No one can figure out how the slime mould keeps time.
The semantics of cognition and the future of the debate
Alex Kacelnik, a zoologist at the University of Oxford, argued that stretching "cognition" to brainless beings "misleads the public. It hollows out the word itself." He pointed out that "even an oak tree 'decides'—in inverted commas—how many acorns it will produce". Pringle cautioned against anthropomorphism: "Why should we be so arrogant as to assume that our behaviour and our natural history is the lens through which to understand Earth?"
David Glanzman, a UCLA professor, has suggested that practically any cell can "remember". In one study, he induced neurons from a sea slug to form strong new synapses, then "erased" the memory with a chemical. When a mild serotonin dose was administered, strong synapses grew again, as though the memory had been hiding in the cell all along. In 2018, he extracted RNA from sea slugs that had been shocked and injected it into untrained slugs; a day later, the untrained slugs showed the same lack of alarm. Glanzman said: "That paper destroyed my ability to get NIH funding, because people just didn't buy it. They wouldn't even discuss my applications."
Levin argues for a continuum: "Nature is telling us that there's a continuum—there are no bright lines that appear to say: 'This side cognition, that side not.'" He added: "If Newton, worried that people wouldn't understand, had decided to call the force that makes an apple fall from a tree 'gravity' and the force that keeps the moon going around the Earth 'shmavity', maybe it would've been simpler. But he'd have missed out on a great conceptual unification."
Lyon offered a stark warning: "People are just afraid of acknowledging that this thing we call intelligence which we value in ourselves—all living things have a share of that, in whatever attenuated sense. If we do acknowledge that, we might have to start paying attention to the shocking and horrible things we're doing to the rest of the living world."
