What slime molds can teach us about thinking

new thinking

The extraordinary learning, memory, and pattern-recognition behavior of some non-animals such as plants and slime molds show that it’s possible to have smarts without having brains.

The abilities of non-animals to remember events, recognize patterns, and solve problems are prompting scientists and philosophers to rethink what thinking is. In one experiment, shown here, researchers found that Physarium polycephalum incorporates ambient noise into decisionmaking.
Jerry Kirkhart/Flickr/PLOS

Visit this online directory of the nearly 200 faculty members at Hampshire College and you’ll find that, listed between a professor of communications and a visiting professor of video and film, is a petri dish of yellow schmutz.

The schmutz is a plasmodial slime mold, Physarum polycephalum, a glob of living cells that exhibits decidedly non-schmutzlike behavior, such as solving mazes and anticipating periodic events – so much so that in 2017 Hampshire, a private liberal arts school in Amherst, Mass., awarded it a position of “visiting non-human scholar.”

The abilities of non-animals to remember events, recognize patterns, and solve problems are prompting scientists and philosophers to rethink what thinking is. In the 20th century, science demolished the notion that humans are the only animals to exhibit complex thinking; in the 21st, biologists are beginning to see cognition in other biological kingdoms – not just slime molds, but also plants. This shift in thought could not only help scientists better understand cognition's workings and its origins, but it could also help in the search for intelligence beyond Earth.

“It seems like they can do some pretty smart things, things that we put in the intelligence column,” says Hampshire philosophy professor Laura Sizer of her gelatinous colleagues. “It's exciting because they certainly don't do it with any of the stuff that we use to do it. They don't have brains. They don't have nervous systems.”

Slime molds form their own branch on the tree of life alongside plants, animals, and fungi. They live on every continent, but, being neither useful for construction nor advisably edible, their presence has been largely overlooked by humans. A slime mold can live as a single-celled organism, or it can join with others into a plasmodium, a single giant cell a few inches in diameter, that contains many individual nuclei whose genomes are as unrelated to one another as are any two humans. 

“At least on this planet,” says Jonathon Keats, a conceptual artist and experimental philosopher who worked with Hampshire to create the non-human visiting faculty position and an interdisciplinary Plasmodium Symposium in March, “it seems that the stage that leads toward life is one of separation, and the one that leads toward intelligence is one of aggregation.”

E pluribus plasmodium

“The slime mold is really fascinating because it is simultaneously one and many,” says Megan Dobro, a Hampshire biology professor who participated in the symposium. “There are all these individuals who are committed to acting towards the best interest of the community.”

A plasmodium of Physarum nuclei moves by oscillating its cytoplasm back and forth every 50 seconds, shifting its center of gravity to move at a little more than a third of an inch an hour. When food is nearby, the plasmodium forms a network of slender tubules that branch out in search of it, eventually finding the optimal path.

Hampshire College awarded a plasmodial slime mold a post as 'visiting non-human scholar' in 2017. Physarum Polycephalum resides in an office at the Cole Science Center on the college's campus in Amherst, Mass.
Courtesy of Andrew Hart/Hampshire College

Mathematicians and computer scientists have taken interest in slime molds’ ability to solve optimization problems in geometry and information delivery. In a 2010 study, Japanese scientists arranged oat flakes in the pattern of cities near Tokyo around a Physarum plasmodium, and within 26 hours the slime mold had formed a network that was strikingly similar to the Tokyo rail system. Similar experiments have been conducted with the motorways of Britain, Canada, Spain, and ancient Rome.

Scientists have shown that slime molds also exhibit rudimentary learning behavior. Shock a plasmodium at regular intervals, and it will alter its behavior in anticipation of the next one. Expose a plasmodium to a repellent but harmless stimuli, and it will eventually ignore it. Some scientists suggest that slime mold behavior may even shed light on the origins of intelligence.  

Under some conditions, a plasmodium will produce a stalk that disperses cells to become new slime molds, but to do so, some individual cells within the stalk must sacrifice themselves for the good of future generations.

“This is behavior that humans don't seem to do,” Professor Dobro says. “And I mean not just the self-sacrifice for future generations, but even the thinking about future generations.”

Thinking fast and slow

Slime mold cognition has been overlooked for so many years in part because, on a human time scale, they appear inert, their behavior apparent only to those with time-lapse cameras or exceptional attention spans.

But there does exist at least one example of non-animal cognition that acts in what humans call “real” time: the Mimosa pudica plant, also known as the shameplant or touch-me-not.

Scientists have long known that Mimosa, whose leaves curl up in response to a touch, can become habituated to repeated stimuli. In 2014, Monica Gagliano, an evolutionary ecologist at the University of Western Australia in Perth, found that Mimosa plants can retain their habituation for at least a month. 

In 2016, Professor Gagliano and her colleagues demonstrated that pea plants display associative learning. By pairing air flow from a fan with light, the researchers “trained” the plants to grow toward the source of the air even when there was no light present.

“Of course that plant is not that stupid,” says Gagliano. “It wouldn't be surviving all the way to this time if it couldn't learn from its environment and associate different cues so that it can prepare itself for what's coming in the future.”

Some trees can live for thousands of years. Other plants, such as the Venus flytrap and the bunchberry dogwood, move at speeds far faster than any vertebrate.

“We seem to always take the human as the main golden standard which everything else gets compared to,” she says. “In general [plants] do things at such a different time scale that for us it's inconceivable. How conceivable it is for a human to think in hundreds of years? We don't.”

A space oddity?

These biases may pose a challenge to understanding – or even detecting – intelligence beyond Earth.

“Every case of life that we know on Earth is related,” says Susan Schneider, a professor of philosophy and cognitive science at the University of Connecticut in Storrs. “But for all we know, we could be an outlier case of life relative to the bulk of other forms of life out in the universe."

Similarly, she says, human intelligence could be an outlier. “Up until recently, the only form of intelligence which we knew was the brain and the nervous system.”

Plants and slime molds, while they don’t exhibit what Professor Schneider and other cognitive scientists call “domain-general” intelligence, may lead science toward a conception of thinking that does not does not place humans, Earthlings, or even biology at its center.

“The Copernican revolution never happened outside of science,” says Mr. Keats, who is currently at the University of North Carolina in Asheville assembling a “Copernican” orchestra to use sound, light, and gravitational waves to produce “music that is potentially accessible to any being anywhere in the universe.”

Keats, who has also produced gourmet cuisine and films for plants and designed a welcome mat for aliens, says he doesn’t know if aliens capable of contacting humans exist. But if there are, he says “we may have a deeper connection, even if they're non carbon-based, than we have with animals that are close at hand.”

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