Stigmergic foraging

In the locality of an ant colony, ants will lay down a trail of pheromones as they forage for food. They will have receptors to recognise the pheromones of other ants. There will be a decision function that activates appropriate ant behaviour when it encounters pheromone trails. The functions of the ants are genetic; the genes being supplied to each worker by the queen ant. It is this whole system of activity - including that of the queen ant - that can be emulated by stigmergic systems to enable people to help each other to locate sources of information.

In a paper, Social insects and self-organization , Jean-Philippe Rennard describes the foraging behavior of ants as follows:

At the beginning, a number of ants are walking, more or less randomly outside the nest. They are looking for food. All along their way, they deposit a light trail of pheromones. When an ant finds some food, it gets back hxme, deposing a stronger trail (the intensity of the trail possibly depends on the richness of the found resource). Since ants have trail-following behavior, a growing number of individuals will tend to follow it and to reach the food. When they get back, they reinforce the trail. A positive feedback (auto-amplification) therefore appears, more individuals reinforce the trail, attracting new individuals which will at their turns reinforce the trail...

In this example, the ants don't communicate directly. Information are exchanged through modifications of the environment (here local gradients of pheromones). This type of communication is known as stigmergy. This concept was proposed by P.P. Grasse in 1959. Studying the nest reconstruction in termites, Grasse showed that it doesn't rely on direct communications between individuals. The nest structure itself coordinates the workers tasks essentially through local pheromones concentrations. The state of the nest structure triggers some behaviors which then modify the nest structure and triggers new behaviors until the construction is over. The process is similar in ants foraging.

The ants tend to follow pheromones trails, but it is only an inclination. There is at any time a positive probability for the ant to abandon the trail and to move more or less randomly. It is then possible that the "lost" ant finds a new resource, eventually far richer than the one that was previously exploited. By constructing a new trail, this ant will attract new individuals and a new positive feedback loop will be set up.

Finally, when satiety occurs or when the resource is empty a negative feedback loop appears. For example, if pheromone decay is quick enough, when the resource is over, less and less ants will tend to follow the trail which will progressively disappear.

When translating this into human activity - where people use agents to share with each other sources of information - the most difficult concept to grasp is that such a system is not organised or controlled from a central source. Every individual acts independently.

Decentralised control

Stigmergy is a powerful way to coordinate activity over both time and space in a variety of different systems. It requires no planning, no leadership, no direct communication between participants. It is self-organising. This sets it apart from the conventional organisational methods usually associated with human cooperative activity.

The main features of a stigmergic system are:

Individuals do not rely upon instructions.

Each individual gathers information itself and decides for itself what it should do.

Information is gathered from a shared environment.

There is no centralised decision making.

Individuals need only a few elementary rules

You do not have to look hard to see how this process of stigmergy is already taking place on the Internet. News groups and discussion forums manifest in this same way; people find places of interest where they discover messages that are useful to them. The messages prompt other people to respond, by adding their own messages. People will move from one group to another, choosing to participate only in those where the messages are of specific interest them.

There has been no centralised, organising body to create the tens of thousands of news groups and discussion forums that have spontaneously birthed on the Internet. Organisations do not provide content. People do not collude with each other as to what messages will be sent. The whole system has evolved organically and is driven by people acting independently.

It is this strange phenomenon of stigmergy that is now becoming of increasing interest to e-businesses. It can be employed to create self-organizing databases that cost little to build and maintain, yet, provide a service and efficiency that far outstrips anything that can be created through rational design and control.

Two prime examples of stigmergic systems, growing and evolving without central control or organisation, are The Open Source Movement and Napster. Both of these phenomena appeared almost spontaneously; growing to such power and influence, they out compete the industry giants.

Even with its vast organisation and huge financial resources, Microsoft is no match against the cumulative efforts of tens of thousands of independent programmers in the battle for supremacy in the server market. Building upon each other's efforts, the unorganised, independent programmers have created a free and highly flexible system known as "Linux" which has consistently held Microsoft at bay in providing the server of choice for ISPs and maintaining UNIX as the preferred operating system.

Similarly with Napster, a simple system devised by students, evolved into a people space that created a distributed database spread over hundreds of thousands of private computers. The combined efforts of the whole of the music industry had to resort to legal action to stop the free distribution of its copyright protected, musical assets. Even so, the system adapted and evolved into new forms that had no central control for the legal injunctions to aim at.

This demonstrates the enormous potential of stigmergic systems. However, these systems have one vital omission that puts them a step behind the stigmergic systems of the insects: the ability to automatically update, to rid themselves of duplication, redundancy and out of date information. This essential feature is provided in the insect world by evaporation.


When an ant lays a trail of pheromones across a landscape, the pheromones last for only a short time before they evaporate. This ensures that recent pheromone trails are not confused with older pheromone trails, which might relate to circumstances that have changed. Stigmergic systems can be designed to have a similar feature - equivalent to evaporation - that arranges for paths to out of date or redundant information to be removed. This can prevent a system becoming less efficient over time, which is a major problem for most information management systems.