Insects crossing: Ants foraging along an experimental trail set up in the laboratory.

Credit: Audrey Dussutour/University of Sydney

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DIJON, FRANCE: Biologists are learning that ants have an increasingly large number of inbuilt rules which govern their behaviour on foraging trails, and which could offer clues to better control human crowds.

In the human world, road signs and traffic lights coordinate the movement of vehicle and pedestrian traffic to prevent collisions. Ants, however, are able to manage the two-way movement of large numbers of individuals by following a few simple rules.

Stop and go

To find out what happens when happens when ant highways become too narrow for two-way traffic, Vincet Fourcassié, a biologist from Paul Sabatier University in Narbonne, France, set up experiments in the laboratory. The results were detailed at the European Conference on Behavioural Biology held in Dijon, France.

Fourcassié's team studied the behaviour of a European kind of common garden ant, Lasius niger which feeds on sweet secretions produced by the aphids that they farm and protect from predators. They also observed the behaviour of the South American leaf-cutting species Atta colombica, which uses cut leaves to cultivate fungi for food.

In the experiments, the ants had to cross a bridge to get from nest to foraging site and back. The researchers found that when the bridge width was reduced – so that there was only room for the ants to move in single file – a clear set of priority-based rules emerged.

In the black garden ants, the rule is not to enter the bridge if another ant is coming from the opposite direction, but if an ant in front of you is crossing the bridge you can follow it. In leaf-cutting ants, the rule is to give priority to ants carrying leaves back to the colony: if a leaf bearing ant is on the bridge, give way; and if he is in front of you, don't overtake.

Maximising efficiency

In both species these rules allow for orderly behaviour which results in groups of ants crossing the bridge together, in one direction, avoiding collisions and hence maximising foraging efficiency.

Fourcassié said that similar rules seem to regulate movement of humans in crowded situations in absence of signs or other imposed means of traffic control.

Nigel Franks, a biologist from the University of Bristol in England, said these findings were interesting and may contribute to understanding both ant and human behaviour in crowds. Franks presented research at the same conference, showing that ant societies have individuals who, like political leaders, make decisions for the entire colony.

Among humans, when a space becomes dangerously crowded – such as at a packed stadium or when people are trying to escape from a burning building – chaotic movement and 'stampedes' can result in injury and even death. Experts believe that studying how ants avert such disasters could give us useful clues to more effectively managing human crowds.