Just as altruism isn’t unique to humans, neither is taking advantage of another’s altruism. In any given group dynamic, there will be those who would try to game the system to their advantage—and they would be wise to do it. Social slime mold such as D. purpureum and D. discoideum are prized for their role as model organisms, and through them, the mechanics of cheating behavior have been studied extensively*.
Quick recap. You can skip this paragraph if you’re up to date. Cellular slime mold are microscopic amoeba that eat bacteria and reproduce via mitosis. When food runs out, they shift gears and release a chemical signal that makes them all gather together into a large, mobile “slug.” The slug moves to a good spot, takes root, and begins building a stalk. The stalk is composed of cells who kill themselves, effectively creating a tower of corpses that the rest of the group can climb to the top. Finally, the survivors form a fruiting body that will spread spores to more fertile ground, and the process begins anew.
Here’s where it gets complicated. Normally, when it comes time to form the stalkFor slime mold, it’s a matter of being in the right place at the right time. The cells in the front of the slug form the spores, while the cells in the back kick the bucket. In single isolate groups, this isn’t so bad. After all, everybody is related. But in chimeral (mixed) groups, where two or more unrelated colonies join together, things can get bloody. Like a microscopic Hunger Games, uneasy trusts quickly turn into mad dashes to be king of the hill.
You can think of it like two groups of people banding together to pick fruit from a tree by forming a human pyramid. The groups benefit from the extra manpower, but either group runs the inherent risk of the other group contributing no members to the pyramid, scrambling up to the top, and eating all the fruit. Similarly, in a mixed group of amoebae, one group may cheat by not providing any members to the stalk, ensuring that more of their group will reproduce as spores.
One would think that natural selection would lend to populations composed entirely of cheaters, but thankfully, thwarting cheaters is also selected for. In this series, I often make connections between humans and slime mold, to show we’re not so different after all. How do slime mold isolates protect themselves against cheaters? Just like humans: Policing and good ol’ xenophobia. Also just like human examples, the severity of these adaptations run the gamut—ranging from simple vigilance to make sure nobody steps out of line and everybody contributes, kin selection and favoring related members, to all out poisoning the other group into forming a stalk. Brutal, but survival’s the game here.
I’ll probably dip into some of the other examples of biological cheating in a future comic, but in the meantime, know this: If a system can be gamed, I guarantee that there are individuals who game it. Out of the ones mentioned in the comic, the Calopogon tuberosus var. tuberosus is my favorite. Not only does it not reciprocate in its relationship with pollinators, the flower makes sure that it gets pollinated. When an insect lands in the flower, it snaps shut. While not fatal, it is difficult to get free, and in the struggle the poor sap spreads pollen everywhere. Devious.
This post was brought to you by Tellurium (Te).
*For more information about all the stuff in this post, check out Altruism, Cheating, and Anticheater Adaptationsin Cellular Slime Molds, and Social Evolution: Kin Preference in a Social Microbe. The former is a difficult read, but I highly suggest it if you’re interested in the topic. If you can’t get past the paywall on the latter, Carl Zimmer has a good write-up about it.