Wednesday, August 4, 2010

Sometimes, self-organizing systems fail

New World army ants are known for their self-organized swarm raids accross the forest searching for food. They form a dense carpet of ants being able to attack much larger animals like larger insects and even lizards. In order to form the swarm, the ants orient themselves by tactual stimulation and by chemical trails laid by other ants. While this system is very effective, it has a potential mode of failure. Sometimes, these ants can get trapped in a circular movement, where the tactual stimulation and the chemical trails will lead to a positive feedback towards moving in a circle. Such behavior has been observed several times in natural environment, it is also relatively easy to reproduce the behavior under lab conditions. In a paper from 1944, T.C. Schneirla elaborates the initial conditions for circling army ants. Under heavy rainfall, these ants tend to move together in a small area. After the rainfall, ants at the margin of the huddle will tend to move around first. They will mostly follow the peripheral of the group due to tactual stimulation and thus create a circular trail of chemicals, which will be followed by the other ants. Army ants have been observed to be circling until they die of dehydration. This example shows that even systems which are evolved and hardened by billions of years of evolution (for organisms in general, ants came into existence about 130 million years ago when they split from the wasps) can be trapped in unwanted behavior.


  1. The ants self-organize through stigmergic communication (by following pheronomone trails placed in the environment). A trail enforces itself, given enough ants.
    Now, one would think too many ants in same area going about in the same route (be it a circle, an eight, whatever closed loop) should result in this same behaviour.
    Would it be correct to describe that circular looping route as an attractor in a "self-organizing stigmergic solution landscape" which the ants inhabit? (Solution in the sense when ant behaviour is viewed as route-optimization to beneficial things such as food)

  2. The circling behavior is a special case of nomadic army ants. These ants must organize in form of a dense carpet in order to be successful in food raids. I don't think the outcoming circle is a special attractor. To my opinion, any closed loop without crossing could potentially be established when painted with sufficient initial pheromone (but I have not tested this claim :-). The geometry of the observed circles is most likely based on the size of the huddle during the rainfall.