Stuff You Should Know – “Selects: Slime Mold: 0% Mold, 100% Amazing”

Host(s): Josh Clark & Charles W. ‘Chuck’ Bryant
Podcast: iHeartPodcasts
Release Date: September 20, 2025
Summary by: Podcast Summarizer

Episode Overview

This episode is a “select” from the SYSK archives (originally June 2021), where Josh and Chuck dive into the surprisingly fascinating world of slime mold. Despite its unappealing name and appearance, slime mold turns out to be one of the most mind-blowing biological phenomena, blurring the lines between intelligence, cooperation, and what it means to be an organism. The hosts unpack what exactly slime mold is, its crucial role in ecosystems, and the almost uncanny “intelligence” it displays without a brain. They also explore slime mold’s real-world applications in science and even technology.

Main Discussion Points & Insights

What Is Slime Mold?

• Slime mold is not a mold, not a fungus, and not an animal.
  • “It’s not an animal, it’s not even a fungus. … It’s a protist.” – Josh (03:30)
• Protist: Belongs to the kingdom Protista, along with amoeba and protozoans.
• It was once classified as a fungus due to its spore-producing habits and appearance, but advances in biology led to its reclassification.

Major Types of Slime Mold

1. Plasmodial Slime Mold
   • Shocking fact: Some can reach the size of a pizza (~12 inches across) but are technically a single cell with millions of nuclei.
   • "Some of those types of slime mold that are as big as a pizza are one giant cell." – Josh (09:37)
2. Cellular Slime Mold
   • Exist as individual cells but, when stressed (lack of food), join together into a larger structure to survive or reproduce.
   • Analogy: “Kind of like a swarm.” – Josh (12:34)

Evolution, Role, and Interaction

• Ancient Organisms: Around a billion years old, among the earliest life forms (06:17).
• Decomposition & the Food Web:
  • They engage in phagotrophy—engulfing, rather than externally digesting, food.
  • Vital for breaking down dead matter and recycling nutrients, serving as food for specialized beetles and larvae. (07:12)
• Human Interaction:
  • Harmless to humans—edible varieties are eaten in Mexico as “caca de luna” (moon poop). (08:10)

Appearance & Behavior

• Colors: Yellow, orange, red, white, maroon; rarely black, blue, or green. (15:12)
• Habitat: Found on the forest floor and can appear on lawns after rain during heat waves.
• Movement:
  • Move extremely slowly (typically 1mm/hour, but some can reach 1.5 inches/hour).
  • Movement is visible only in time-lapse but is purposeful: to find food or escape noxious stimuli. (23:08)

Extraordinary “Intelligence” in Slime Mold

Altruism & Group Behavior

• Self-Sacrifice:
  • “Slime molds … will sacrifice themselves to save others. And these are things without a brain or a central nervous system.” – Chuck (18:43)
  • Example: Engulfing toxins and separating from the colony to die, protecting the group.

Complex Decision-Making & Problem Solving

• Labyrinth Navigation (Maze Experiment):

  • Japanese researcher Toshiyuki Nakagaki created a maze with food at endpoints; slime mold repeatedly found the shortest path. (24:32)
  • “They basically learned to get to that food in the quickest, fastest way every single time.” – Chuck (25:21)

• Subway System Mimicry:

  • Atsushi Tero mapped oat flakes in the pattern of Tokyo’s neighborhoods; slime mold’s network almost exactly matched Tokyo’s real commuter rail system. (26:44)
  • “That’s. I mean, I had to reread that, like, five times to even believe that’s what happened.” – Chuck (26:52)
  • Also mimicked ancient Roman roads in the Balkans. (27:31)

• Implications:

  • Researchers propose slime mold algorithms for urban planning and infrastructure optimization. (28:41)

Learning & Memory Without Neurons

• Experiment: Slime mold crosses a bridge with a “noxious but harmless” substance (like salt); learns it’s safe.
• Social Transfer: If a “habituated” slime mold (used to salt) is fused with a “naive” one, the naive mold also crosses without hesitation (even after separation, if fused for 3+ hours). (45:57)
• “Slime Trail” as Spatial Memory: Their secreted trails help avoid previously searched areas. (46:11)
• Researchers still don’t fully know how this memory transfer works. (47:09)

Scientific & Technological Relevance

• Decentralized/Bottom-Up Decision-Making:

  • No pacemaker or “leader” cells; response is emergent, akin to flocking birds or swarm robots.
  • “Instead of like a hierarchy, it’s more like … how a flock of birds operates.” – Josh (36:41)

• Artificial Intelligence & Nanotechnology:

  • Insights from slime mold algorithms inspire developments in swarm robotics and decentralized AI.
  • “Anytime you have a huge amount of things that you’re trying to all get to do roughly the same thing…if you can figure out how to strip it down to a basic enough algorithm…you’ve got the key to the universe in your hand.” – Josh (39:49)

• Cosmology:

  • Modeling universe filaments: Sage Jenson used slime mold-inspired code to try mapping the structure of the universe by treating galaxies as food sources and “growing” a virtual slime mold among them. (49:07)
  • “If the slime mold recreated how the universe is invisibly linked together, the structure of it…I can’t even put a word on that.” – Josh (51:02)

Notable Quotes & Memorable Moments

• On the Myth of “Mold”:

  • “We definitely need to point out that slime mold is also not mold.” – Chuck (09:01)

• On Slime Mold as a Giant Cell:

  • “Put your sock garters on because I’m about to blow your socks right off your feet.” – Josh (09:37)

• On Slime Mold’s Efficient Problem-Solving:

  • “The Tokyo subway map, it was almost a perfect match.” – Chuck (26:44)
  • “This is crazy. Why not? … All you have to do is have some oat flakes and a petri dish, and you’re good.” – Josh (29:11)

• On Mind-Blowing Intelligence:

  • “My mind’s definitely been blown.” – Josh (41:41)

Timeline of Important Segments

| Timestamp | Topic / Insight | |---------------|-------------------------------------------------------------------------------------------------------------| | 03:30 | Slime mold’s reclassification as a protist, not a fungus | | 09:37 | Plasmodial slime mold as a giant single cell | | 12:34 | Cellular slime mold: swarming and collaboration | | 15:12 | How to identify slime mold outdoors | | 18:43 | Altruism in slime mold: self-sacrifice for the group | | 24:32 | Maze-solving and demonstrable learning in slime mold (Japanese experiment) | | 26:44 | Tokyo subway mapping experiment | | 27:31 | Ancient Roman roads recreated by slime mold | | 36:41 | Emergent, bottom-up decision making (not top-down/pacemaker) | | 39:49 | Implications for nanotechnology/algorithms and the fabric of the universe | | 45:57 | Memory transfer between slime molds (habituated and naive experiment) | | 49:07 | Using slime mold logic to help map the universe’s filament structure | | 51:02 | Real-world limitations: verifying whether slime mold universe maps are accurate |

Takeaway & Closing Thoughts

Slime mold challenges what we think we know about intelligence, decision making, and cooperation in living organisms. With no brain or nervous system, it nonetheless displays learning, memory, and problem-solving capacities that dazzle scientists and have practical implications for network design, AI, and more.

Final Mind-Blowers:

• Slime mold can learn, remember, and “teach” without neurons.
• Its problem-solving ability has inspired new approaches in tech, urban planning, and even cosmology.

“Slime mold is teaching us to open our horizons and hearts…to new ideas of what constitutes consciousness and intelligence.” – Josh (32:33)

For Listeners Who Missed It

This episode is equal parts mind-expanding science and quirky SYSK banter. If you’ve never cared about “goop in the woods,” Josh and Chuck will have you seeing slime mold as a rock star organism that might just know the best way to run the Tokyo subway, map the cosmos, or inspire the robots of tomorrow.

Further Resources & Links

• TED Talk on Slime Mold Intelligence
• Scientific articles on Physarum polycephalum and biological computing
• “The End of the World” podcast by Josh Clark (artificial intelligence discussions linked to this episode)

Prepared by: Podcast Summarizer, September 2025