Although an individual ant is quite small (measuring only 2.2 to 2.6 mm in length) and wanders quite aimlessly in isolation, a group of many ants exhibits extraordinarily intelligent behavior, recognizeable to humans as meaningful pathways to food sources. In laboratory experimentsi, over time, Argentine ants will not only find food, but create the shortest path to it. In my own informal experiments, they will find their way into my second-floor apartment, through cabinets, under the refridgerator, across the floor, and around the corner to garbage container. No matter how many obstacles in all dimensions, no matter how many ant hotels I put by the door, no matter how vigilant I am about erasing their paths, they circumvent it all to find a solution.
The explanation for this emergent intelligence was once a mystery, but now we can summarize the process in pseudocode:
1. At the outset of the foraging process, the ants move more or less randomly – this “random” movement is actually executed such that a considerable amount of surface area is covered, emanating outward from the nest.
2. If it is not carrying food, the ant “deposits” a nest pheromone and will prefer to walk in the direction of sensed food pheromone.
3. If it is carrying food,the ant deposits a food pheromone and will prefer to walk in the direction of sensed nest pheromone.
4. The ant will transport food from the source to the nest.
By applying these rules, which know nothing about “other ants,” linearity, or distance, the nest can develop the shortest path to the closest food source. These rules, however, aren't enough to describe how this path actually comes into being; to understand that we need to examine the rules-in-action. This is only another way of saying that the ant collective, as a non-linear system, does not obey the principle of superposition – we can analyze the individual ants forever and not understand the nest because, to develop an understanding of the nest, we have to look towards the space in between the individuals and see what happens over time.
Pierre-Paul Grasse wrote about this between-space almost half a century ago:
Self-Organization in social insects often requires interactions among insects: such interactions can be direct or indirect. Direct interactions are the "obvious" interactions: antennation, trophallaxis (food or liquid exchange), mandibular contact, visual contact, chemical contact (the odor of nearby nestmates), etc. Indirect interactions are more subtle: two individuals interact indirectly when one of them modifies the environment and the other responds to the new environment at a later time. Such an interaction is an example of stigmergy.ii
Writing about termites, he offered a more general definition of stigmergy - “the stimulation of the workers by the very performances they have achieved.”iv - that articulated the role of positive feedback in social insect behavior. As a pheromone “trail” becomes stronger, the more ants follow it, leaving more pheromone along the way, which makes more ants follow it, and so on. This autocatalytic mechanism explains how trails in general become articulated through the environment, but it fails to explain how it is that ants develop the shortest possible trail. Quite simply, better paths (that is, shorter ones) are established sooner (by virtue of being shorter) and consequently, are followed sooner and by more ants. In other words, a coupling of the autocatalytic mechanism to the implicit evaluationv of solutions produces the optimal solution.
