It's a familiar scene for thousands of commuters every morning; bumper to bumper, going nowhere! Traffic control is a major issue for human societies with large numbers of people wanting to get to, and from, the same place at the same time. But it's not a problem we often associate with other creatures. Animals such as wildebeests, herrings or locusts often converge in large numbers, but their traffic tends to be along a one-way street – all the individuals move in the same direction at the same time. Foraging ants, on the other hand, have to cope with traffic control problems that closely parallel our own. An ant trail is not an unfamiliar sight, one ant following another to a food source along a specific path whilst other ants, already laden with food, return using the same route. These trails are formed when a scout ant finds food and returns to the nest, leaving an odour trail to direct other ants to the source of food. Each ant that returns to the nest adds to the odour trail, reinforcing the signal for subsequent ants to follow. This system leads to ants travelling to and fro, along the same route. Normally this system works well, but Audrey Dussutour and co-workers from Toulouse, Dresden and Brussels asked how ants cope when faced with congestion.
First, the team allowed black garden ants (Lasius niger) to forage for a tasty sugar solution food source, but to reach their goal they had to cross a diamond-shaped bridge, which gave them a choice of routes once they'd begun to cross. The team expected the ants to use only one branch for both their outward and return journeys, thanks to the odour trails left by previous foragers. And this is exactly what happened when the branches of the bridge were wide. However, as the team replaced the bridge with narrower and narrower branches, the ants began to use both branches of the bridge simultaneously,ensuring that the number of ants getting to and from the food remained about the same, despite the constriction.
How do the ants achieve this traffic control? Odour laid down by ants returning to the nest with food serves as a guide for ants leaving the nest to find food, so when there's no congestion and the odour trail is strong, garden ants stick to that path when travelling to and from the food source. However,when monitoring the movements of ants on a congested bridge, Dussutour and co-workers found that two factors play important roles in optimizing ant traffic: the concentration of odour on a trail and interactions between ants moving in opposite directions on the same trail. An ant chooses a particular branch of the bridge because of the odour concentration on that branch. This ensures that only one branch is used, but as traffic becomes more congested,it takes longer for ants to battle their way through the crowd so the number of ants returning along the branch declines and the odour concentration drops. This causes ants to start making random choices between the two possible branches, so that they begin using both branches. Interactions between ants travelling in opposite directions along a narrow path also encourage the ants to explore alternative routes, by redirecting one of the ants down the other branch.
There may be lessons to be learnt from the strategies adopted by the ants to optimize their traffic flow but some of these strategies are already familiar. For example, traffic updates on the radio (just like the interactions between ants) warn commuters about congested routes and encourage them to find alternatives to avoid the rush hour.
