Priority rules govern the organization of traffic on foraging trails under crowding conditions in the leaf-cutting ant Atta colombica.

Foraging in leaf-cutting ants is generally organized along well-defined recruitment trails supporting a bi-directional flow of outbound and nestbound individuals. This study attempts to reveal the priority rules governing the organization of traffic on these trails. Ants were forced to move on a narrow trail, allowing the passage of only two individuals at a time. In this condition, a desynchronization of inbound and outbound traffic was observed, involving the formation of alternating clusters of inbound and outbound ants. Most clusters of inbound ants were headed by laden ants followed by unladen ants. This occurred because inbound unladen ants did not attempt to overtake the laden ants in front of them. As unladen ants move on average faster than laden ants, these ants were thus forced to decrease their speed. By contrast, this decrease was counterbalanced by the fact that, by staying in a cluster instead of moving in isolation, inbound unladen ants limit the number of head-on encounters with outbound ants. Our analysis shows that the delay induced by these head-on encounters would actually be twice as high as the delay induced by the forced decrease in speed incurred by ants staying in a cluster. The cluster organization also promotes information transfer about the level of food availability by increasing the number of contacts between outbound and inbound laden ants, which could possibly stimulate these former to cut and retrieve leaf fragments when reaching the end of the trail.