Ants are known for their efficient way of tracking in straight lines, but how do they manage traffic when their numbers increase to the thousands?

Along with humans, ants are one of the rare animals whose collective movements are two-directional. Yet unlike humans, ants are masters of traffic management. Maintaining a smooth flow of traffic along the trails linking food sources to the nest is no easy feat when colonies can number in the tens of thousands of ants. Yet they move with great efficiency, without passing each other or overtaking, to form a steady ow of traffic. Unlike humans, ants do not get stuck in traffic jams when a large number of individuals circulate in the same space.

To uncover the secret of how ants manage movement so effectively, scientists at the Université Toulouse in France, Arizona State University in the US and The University of Adelaide set up an experiment to find out whether ants can maintain their steady stream of traffic when their path to food gets more crowded. This involved manipulating the density of ants using a combination of different sized colonies (ranging from 400 to 25,600 Argentine ants) and changing the width of the bridge connecting the ants to the source of food. The experiment was repeated 170 times, and data was visually recorded on the density of traffic flow, speed of movement of the ants, and number of collisions as the ants commuted between their nest and the food source.

The findings, which were published in October 2019 in eLife, have demonstrated that ant colonies circulate easily, even in situations of extremely dense traffic. Ant colonies solve the main challenge of transportation by self-regulating their behaviour.

In human behaviour, the flow of movement – whether pedestrians on foot or car traffic – will slow down if occupancy levels (the amount of the road surface area covered by cars) reach over 40%. Whereas in ants, the flow of traffic showed no signs of declining even when bridge occupancy reached 80%.

When the density increases, ant flows (measured as the number of ants covering a given distance per unit of time) swell and then become constant. Human traffic, on the other hand, slows to zero flow and causes a jam. So how to ants do this?

The experiments revealed that ants do this simply by adjusting their behaviour to suit their circumstances. They accelerate until a maximum flow or capacity on the path is reached. So when the density of traffic is at a low level, the ants can speed up. When traffic becomes too dense and causes collisions between ants, they change tactics. Although the ants are protected by their exoskeleton and are not afraid of collisions, the ants will hang back, and avoid entering overcrowded trails to avoid collisions.

The reason why they do this is simple. Ant colonies share a common goal: foraging, which needs to be carried out effectively regardless of density. Furthermore, ants can keep the traffic flowing even with regular contact (for communications purposes) with other ants. However, keeping the interaction time short is vital to maintain traffic flow. In contrast, when driving, humans are responding to their own selfish requirements, even though advertising always asks for consideration of other road users!

The tactics ants employ to manage great volumes of traffic have long been studied by scientists. Looking to the future, ant behaviour will have relevance for scientists developing autonomous vehicles, which will need to mimic the cooperative and efficient behaviour of ants.

Further reading: Laure-Anne Poissonnier, Sebastien Motsch, Jacques Gautrais, Jerome Buhl, Audrey Dussutour. Experimental investigation of ant traffic under crowded conditions. eLife, 2019; 8 DOI: 10.7554/eLife.48945

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