Temporal organization of bi-directional traffic in the ant Lasius niger (L.)

Foraging in ants is generally organized along well-defined trails supporting a bi-directional flow of outbound and nestbound individuals and one can hypothesize that this flow is maximized to ensure a high rate of food return to the nest. In this paper we examine the effect of bottlenecks on the temporal organization of ant flow. In our experiments ants had to cross a bridge to go from their nest to a food source. Two types of bridges were used: one with and one without bottlenecks. Traffic counts show that, in spite of the bottlenecks and the reduction of path width, the volume of traffic and the rate of food return were the same on both bridges. This was due to a change in the temporal organization of the flow: when path width decreases alternating clusters of inbound and outbound ants were observed crossing the bridge. This organization limits the number of head-on encounters and thus allows to maintain the same travel duration as on the wide bridge. A model is proposed to assess in various conditions the importance of the behavioural rules observed at the individual level for the regulation of traffic flow. It highlights how the interplay between the value of the flow and cooperative behaviours governs the formation and size of the clusters observed on the bridge.