Scale is an interesting thing, especially when related to human perception of scale. This happens a lot with numbers. For most people’s frame of mind, the difference between a million and a billion is negligible. They’re just “big numbers.” But the difference is incredible. To put it one way, a million seconds is almost 12 days. A billion seconds is 31.6 years. There’s nuance in those scales.

Similarly, we tend to do the same to very small amounts. For example, the general assumption is that all microorganisms are the same size. Which is patently false. Just like in our macro world, the micro world is full of creatures of all shapes and sizes, the large preying on the small (and sometimes vice versa*). Physarum polycephalum, today’s featured slime mold, is visible without the aid of a microscope—appearing as a squishy yellow blob. It’s what’s known as a plasmodial slime mold, or myxogastria. It is an amoeba, meaning it traverses by spreading itself out with gooey extensions of itself. Like many amoeba, P. polycephalum is pretty free-wheeling with its personal space. That large, yellow blob is actually formed from many amoeba coming together. This is different from D. purpureum (who we met last week) or most multicellular life forms in that they don’t keep their cellular boundaries, instead becoming one large cell with many nuclei inside. You may be saying, “That’s cheating!” but hey, it’s technically a single cell.

There are advantages of being a giant amoeba. When looking for food, P. polycephalum casts a wide net, extending tendrils in every direction, similar to a plant. Once food is found, however, the similarity ends. Our clever yellow slime can retract any unused tendrils or strengthen the connections of those that have found a food source. It creates a network that becomes increasingly more efficient as time goes on. That’s how we arrive at the 2010 Hokkaido University experiment, led by Prof. Atsushi Tero.

Published in the journal Science, the experiment showcased P. polycephalum’s ability to create vast, efficient networks within hours. Of course, Tero and his team used the Tokyo rail system as the standard of efficiency**. What’s truly fascinating is not the result, which mirrored years of public planning, but the process in which the slime mold would adjust the network until maximum efficiency was reached. Similar experiments have been conducted in England and Spain. Check it out:

The fruits of their labors? Tero and his team won the 2010 IgNobel Prize in Transportation Planning. P. polycephalum had previously taken the stage in 2008 when it was shown it could find the most efficient path through a maze***. Among its other accolades include the ability to predict environmental patterns and, perhaps the creepiest, its applications in electronics integration. Bravo!

This post was brought to you by Barium (Ba)

For the first part of this series, check out The Altruism of Slime

*Radiolab recently had a feature on the difference between microscopic plankton and the viruses that feed on them.
**Those of you who have had the chance to ride it can attest to how well-oiled a machine it is. Unless you’re taking the last train out of Ginza on a Saturday night.
***It was allowed to grow throughout a maze, then food was placed at the beginning and end. The slime mold consolidated itself to the most direct path between the two food points.