In my search for fundamental behaviors of sociality, I found examples of dominance, sharing, and privacy in microbes, specifically slime molds.  Slime molds are not actually molds. They are single-cell amoebae, but interestingly for parts of their life cycle, they join together to act like a single larger organism. Scientists have been studying slime models for a while. One species of slime mold, called Dictyostelium discoideum is particularly interesting because of its life cycle.  Be sure to check out John Bonner’s slime mold movies to get an interesting visual of its behavior.

The following is a simplified description of the life cycle of Dictyostelium discoideum:


In the part of its life cycle called unicellular it lives as many individual amoebae spreading out and searching for food.  It’s food source is bacteria. Eventual they run low on bacteria and start to starve.  Then they grab the remaining bacteria they can find but don’t eat it.  They save a the bacteria for later and start to join together into clumps of amoebae called aggregates.  


During the aggregation stage the aggregates eventually form into slimy slug like shapes. Within these slugs a hierarchy forms with different genetic strains of the slime mold forming a stratum from the front of the slug to the back end.  If there are more than one strain in one slug they will organize in a strict hierarchical order with the strains always aligning in the same order from front to back   The slug then moves around like a real slug does (except much slower) sometimes merging with other slugs reorganizing its hierarchy of strains from back to front again.  


Then comes the culmination stage when the slug selects a location to stop and sprout a stock.  The front of the slug anchors to the ground and back lifts up to form the stock.  The stock grows upward pushing the strains of amoebae that were on the back of the slug up to the top.   The strain that is on the top of the stock is also at the top of the hierarchy.  The food that was saved previously is sent up to these elite amoebae.  The elite amoebae form into spores that have a chance to be taken by wind or animal to a new location that will hopefully have more food. The food given to them will hopefully give them a good start in their new location. Only the amoebae that become spores have a chance of survival.  The amoebae that form the stalk and the base sacrifice themselves to help the spores have a chance of survival.  Once in the new location, the amoeba spread out as individuals in search of food again as already described.


The three main stages of this life cycle, unicellular, aggregation, and culmination correspond to the three principles that Jack Hirshleifer talked about regarding resolving conflict.  These principles are dominance, sharing, and privacy. See, Dominance, Sharing, and Privacy, The Three Principles of Sociality.  In the unicellular stage, it acts like a rugged individualist and goes out on its own and takes the food it finds and uses it for itself corresponding to privacy.  When it starts to starve it begins to act like a sharing communist, gathering the food it can find to bring back and share with the collective.  Once it starts the aggregation stage it forms a cast system with strict linear dominance hierarchy.  Each strain of the species has a rank in relation to all the other strains and it always has the same rank in relation to all the other strains.  When the highest ranking strain is not present the next in line is on the top of this hierarchy, and so on. During culmination, the lower ranking strains altruistically sacrifice themselves to provide the base and the sock that lifts the highest ranking strain high so the elite can form spores and be carried to a new location.

I don’t think Hirshleifer wrote about slime molds but this species of slime mold is a great example of these three principles being used under different conditions to enhance survival.  It also hints as to the conditions under which each principle is likely to be successful.


John Bonner’s slime mold movies

Altruism and social cheating in the social amoeba Dictyostelium discoideum, by Joan E. Strassmann, Yong Zhu & David C. Queller

A linear dominance hierarchy among clones in chimeras of the social amoeba Dictyostelium discoideum by A. Fortunato, D. C. Queller, J. E. Strassmann