Creep: that purple, fibrous, living mat that extends from zerg "buildings" in the computer game StarCraft. Its ominous presence indicates you're entering unsafe territory...unless you're playing as zerg, in which case it says "Welcome home!" But what exactly is it?

Well, let's think about it scientifically. It is produced by zerg buildings and spreads across any available surface. According to the Starcraft Wiki (which I will use as an authoritative source for all the minutiae of StarCraft trivia that I don't know) Creep has a cellular structure: it's not just mucus. The Wiki also says that creep can absorb sustenance from the underlying terrain, that it can be spread by "spores" or excreted by several units, and it provides nutrients to zerg buildings. It is averse to high temperatures but can grow in space and over water.


So, does any real-world living thing match this description? Surely not, right? Wrong. The closest analog that I know of are slime molds. You've probably seen slime mold before without knowing it. They are fungus-like organisms, often brightly colored and found growing on damp logs in the forest. They are also incredibly weird.

Some types of slime mold are made of multiple cells joined together to form a super-cell with a shared cytoplasm. Ok that's weird enough, but the other kind of slime mold starts off as a bunch of separate single-celled organisms, which can then coalesce into a multi-celled organism.

In terms of similarities with creep, slime molds are spread via spores but can also grow and multiply when they encounter nutrients. Slime molds can become quite large, and form branching networks of cytoplasm, allowing the leading edge of the slime mold to stream nutrients back to the rest of the "organism".


When the going gets tough for a slime mold and nutrients run out, it can transform and form structures called sporangia, which distribute spores. In some cases, separate cells will coalesce into a single "creature" in order to do this.

There are some definite similarities between slime molds and creep. They both come from spores but can also grow across nutrient-bearing ground. They both can transport nutrients to locations within them that need them, and are averse to hot, dry conditions.


There are some aspects of slime molds that would have been very interesting if they applied to creep. Most notably, slime molds have been reported to show some rudimentary intelligence. No, they don't sit there and ponder the meaning of life, but they have been able to choose the most nutritious food and can "solve" mazes to get to food sources. These aren't true intelligence, they are actually an example of something called ant-colony optimization, often used in computer programming.

In the diagram at the left, the slime mold starts out evenly spread through the maze, but when food sources are placed at two ends, the slime mold retracts from everywhere but the shortest path. The idea is that you don't know what the best set of steps to reach a certain goal is, so you test things out randomly. Some sets of steps don't give you the goal, but others do. The ones that do give good results are reinforced, while the ones that don't, are not. The analogy is that ants start off randomly searching for food, but when they find food, they emit pheromones encouraging other ants to follow the same path, so eventually you end up with the familiar narrow stream of ants going from the nest to the food and back. The exact same principle applies to slime molds.


That's nice, but weren't we talking about creep? Yes! My point is that creep could behave like this too. There's not necessarily a need for it to be spread evenly across the ground. It would make more sense for it to have thick branches connecting zerg structures (so that large amounts of nutrients could be provided) while narrower branches near the leading edge of the creep could do the work of absorbing nutrients.

Just to play devil's advocate though, I can see why it might spread across the ground evenly (other than because it makes the game more intuitive to be able to see a clear boundary to the creep). If it is able to suck nutrients out of any surface, then it wouldn't have to concentrate on certain areas. And by not coalescing into thick "veins", the creep is more robust: there's less chance of a building being cut off if there are many smaller veins feeding it.

Finally, all of this brings me to an interesting point: if creep can extract it's own nutrients, and if it is the way that zerg structures are fed, why do the zerg have to mine for minerals? They should just be able to engulf a mine in creep and let it do the work! That would certainly make for a different zerg strategy, especially if a "creeped" mine could not be used by other players!


Obviously the creep is still pretty science fictional. I mean, it can grow in space! There are actually some real-world spores that can survive in space, but I think the whole "zerg don't need spacesuits" issue needs to be tackled in a future post. But from now on, when you see creep, think "slime mold" and when you're out in the woods and you see a slime mold, be glad you don't have to watch out for zerglings!

This post originally appeared on the Science of Starcraft via Science In My Fiction.