Getting stuck in traffic is an everyday 21st century experience. But ants never seem to encounter the same traffic problems. Traipsing back and forth along a path while foraging, they always seem to solve problems before jams take hold. Vincent Fourcassié from the Université Paul Sabatier,France, explains that foraging army ants avoid getting into a jam by adhering to well-defined rules. They avoid head on collisions by staying in their lane. But what if you don't stick to your lane, like leaf-cutter ants? Audrey Dussutour wondered how these cargo-carrying ant juggernauts manage to avoid getting in a jam when they must be involved in head on collisions all the time. Travelling to the University of Illinois at Urbana-Champaign, USA, to work with Samuel Beshers' leaf-cutter ant colonies, Dussutour tried to get the ants jammed in a tight spot to see how they overcome the problem(p. 499).

Linking a well-stocked foraging site to the ants' nest with a wide bridge,Dussutour could see the ants scurrying to and fro, returning with pieces of leaf. But what happened when she replaced the wide bridge with a bridge that was too narrow for the ants to pass two abreast?

The traffic never ground to a halt, it always kept flowing. Unladen outbound ants heading to the foraging site always gave way to the cargo-carrying foragers as they returned. The outbound ants simply stepped down onto the side of the bridge and let the leaf-carting foragers go by. So,instead of flowing continually, the traffic broke down into clusters with groups of homebound ants following a cargo carrier, while groups of outbound ants stepped off onto the side of the bridge to let them go past.

Dussutour also noticed that the returning cargo-carrying ants were slower than the ants returning empty handed, but instead of jostling past, the unladen ants stayed patiently behind their burdened nestmates that made a path home through the outbound foragers. Fourcassié explains that this is like the clusters of cars that build up behind slow trucks on our own highways.

Calculating the amount of time that a fast, unladen nest-bound ant would waste in head on collisions with outbound foragers, Fourcassié explains that the insects could waste up to 64 s on a 300 cm bridge. However, by slowing down and following an unimpeded cargo-carrying ant, the empty-handed foragers would only be delayed by 32 s, returning faster than if they'd muscled past.

But what does all this mean for the nest's leaf supply? Surprisingly it was more efficient on the narrow bridge than on the spacious bridge. Fourcassié explains that the returning ants' patience is rewarded by speeding up the leaf delivery process. He also suspects that the fast-moving outbound ants encounter more head on collisions with cargo-carrying returners than with empty-handed returners, which could encourage the outbound insects to carve up more leaves at the foraging site, improving the nest's leaf supply.

Dussutour, A., Beshers, S., Deneubourg, J. L. and Fourcassié, V. (
). Priority rules govern the organization of traffic on foraging trails under crowding conditions in the leaf-cutting ant Atta colombica.
J. Exp. Biol.