Brainless, Footless Slime Molds Are Weirdly Intelligent and Mobile

By: Jesslyn Shields  | 

slime mold
Physarum polycephalum, literally the "many-headed slime," is a slime mold that is sensitive to light and inhabits shady, cool, moist areas, such as decaying leaves and logs. Bernard Spragg/Flickr

For a long time, nobody understood slime molds. To be clear, nobody really understands them now either, but scientists now know that these pulsating piles of jelly found on rotten logs in the forest are not fungi, but are, in fact, more closely related to amoebas. And though there's nary a neuron in a slime mold's entire gelatinous body, they seem to be able to solve relatively complex problems.

There are over 900 species of slime molds (phylum Myxomycetes) living in the soils, leaf litter and rotten logs of this planet. Researchers have found a slime mold cast in amber that remains entirely unchanged from what you could find in a modern forest, dating back at least 100 million years. Slime molds in general, though, have probably been squishing their way around Earth for around a billion years. In fact, it's possible they're one of the first multicellular-ish organisms created by single cells joining together.

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Slime Molds Are a Diverse Group

Slime molds are a really diverse group. Some, called cellular slime molds, live as a single cell for most of their life, but collect with others in a swarm in response to chemical signals like, "Food shortage!" or "Gotta procreate NOW!" Others, called plasmodial slime molds, spend their entire lives as one humongous organism enclosed in a single membrane, containing thousands of nuclei. These are created when thousands of single, flagellated cells meet up and fuse together.

The only thing all slime molds have in common is their life cycle, loosely resembling that of a fungus, which is why taxonomists lumped them in the fungi kingdom for so long. Basically, when they've vacuumed as much food out of their environs as they can, they turn their bodies into clusters of spore packets, usually on stalks and sometimes wildly colored, called sporangia. These fruiting bodies disperse a fine mist of spores into the air, which germinate wherever they fall. The single-celled organisms that spring from these spores start the slime mold life cycle over again.

slime mold
The fruiting bodies, or sporangia, of carnival candy slime mold (Arcyria denudata) feed on the bacteria, yeast and other fungi that are commonly found in dead plant matter, contributing to the decomposition of dead vegetation.
Ed Reschke/Getty Images

"We still know very little about the ecology of 'wild' slime molds," says Tanya Latty, who studies slime molds in the School of Life and Environmental Sciences at the University of Sydney, in an email. "For example, how they interact with other organisms and what role they play in ecosystems is still somewhat mysterious."

Latty studies cognition in both insects and slime molds, and though we don't give insects much credit for their intelligence, with slime molds, the tricky concept of cognition gets even weirder.

"Slime molds and social insects are both 'decentralised' systems where there is no 'leader' in charge of decision making," says Latty. "However, in the case of insects, each individual operates both at the individual level — they have brains — and at a collective level. In slime molds it's much harder to even define what an individual is."

slime mold
The fruiting bodies of one of the most common slime molds, Ceratiomyxa fruticulosa, are only few millimeters tall and live on rotted logs in humid conditions, a common slime mold habitat.
Ed Reschke/Getty Images

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How Slime Molds, Octopuses and Humans Learn

We humans rely on our brains for cognition, but other animals have the ability to reason, learn, plan, solve complex problems, etc. without such a giant brain as ours. Take, for instance, the octopus — a cephalopod closely related to clams and snails. It has a brain, but most of its neurons are spread throughout its squishy body — mostly its arms. Still, an octopus has an undeniable intelligence: the kind that can tell the difference between humans that are dressed identically or can even make an escape from its tank, out a drainpipe and back into the ocean. But this impressive cognitive functioning bears no physiological relationship to ours — the neural processing equipment of an octopus evolved completely separately from ours, because our evolutionary lineages separated over 460 million years ago.

But slime molds don't have brains or even anything that resembles a neuron. Still, though, scientists can press plasmodial slime molds into solving mazes. So, while the process of learning is completely different in each case, the outcome for a slime mold, an octopus and a human can look basically the same.

One type of learning slime molds are capable of is habituation. You do this too — you can get used to the temperature of a cold lake after a few minutes, or the initially unpleasant buzzing sound of a fluorescent light in a room — your brain helps you ignore the annoying sensation of cold or noise. But the unicellular slime mold Physarum polycephalum can habituate to environments and chemicals they don't love — acidic, dusty, dry, salty places or chemicals like caffeine or quinine — if it means they're rewarded for putting up with it.

Not only can slime molds habituate to less-than-ideal circumstances if it means they'll be rewarded, they also seem to be capable of memory. Physarum polycephalum — the same, oft-studied species from the habituation study — seems to be able to remember things. An experiment involving slime molds that were intentionally habituated to salt, a known repellent, before going into a dormant period, showed that they remembered how to become habituated to living in a very salty environment after a year of lying dormant. They also seem to be able to decide which direction to travel based on the food they've encountered there before.

Just wait — in a few years the slime mold will score a 1,200 on the SAT and scientists will really have some explaining to do.

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