Guy Théraulaz

The organization of traffic in ant colonies

Many collective activities performed by social insects result from
self-organized processes. Nonlinear interactions among individuals can give
rise to highly structured collective behaviors and complex spatio-temporal
patterns despite a strong random component in individual behaviors. In ants, a
large number of these self-organized structures are networks of foraging trails
that drive a huge number of workers to the food sources and then back to their
nest. The organization of ant traffic along these trails involves both cohesive
and dispersive interactions. Trail formation ensures the cohesion of ant
displacements and results from a positive feed-back: the more ants use a given
trail, the more attractive that trail becomes to other ants; but it appears
that the dispersive forces such as head-on encounters may be at the heart of
efficient traffic routing. For instance in army ants these local interactions
among foragers amplify to segregate counterflows along raiding trails.
Moreover, simple experiments in which colonies of ants are presented with
bifurcating bridges between the nest and the food source have shown that thanks
to these local competitive interactions ants were able to re-organize the whole
traffic and reach global maximal flows. Finally some recent experimental
results revealed the geometry of trail networks is a source of directional
information that is used by ants in an adaptive way to re-orientate themselves
if they walk in the wrong direction. An interesting consequence of this ability
of ants to make use of the cues given by bifurcation angles is that the
efficiency of path selection toward a food source strongly depends on the
geometry of the trail network.