Ants: marching soldiers or disciplined drivers?

Leading ants drop a tiny amount of a signal chemical (called a pheromone) as they move forward. Others pick up the signal and follow the trail. File photo: K. Ananthan   | Photo Credit: K. Ananthan

At the recently held annual meeting of the Indian Academy of Sciences, Professor Debashish Chowdhury of IIT Kanpur spoke on his work on non equilibrium statistical mechanics, which was inspired by the way ants move about in groups. His lecture, titled above, is summarized here.

The Panchantantra, Jataka Tales and Aesop’s Fables all admire the industry and orderliness of the lowly ant – and how we can learn from then. More recently, the Harvard biologist E.O. Wilson wrote in admiration about them thus: “They marched in columns several tens of meters long…. Their disciplined legions resembling heavy traffic on an intercity freeway as seen from a low-flying airplane!”

It is this collective movement of ants that interested Professor Chowdhury. He has studied them with the aims of (a) learning the traffic rules they follow as they march from their homes to find food, and return laden with it, and (b) developing models based on the communications and interaction between them.

That is, how in the first place they collectively agree to, and evolve the network of trails: and once a trail is created what traffic rules they follow – outwards from their nests and inwards as they return with food. Clearly there is a moral here on how we humans can conduct ourselves as we move in queues or drive on the roads.

Ready answer

How they make a trail has a ready answer. Leading ants drop a tiny amount of a signal chemical (called a pheromone) as they move forward. Others pick up the signal and follow the trail. The other major question is whether there is a traffic jam in ant traffic on a trail. It appears that unlike us, they are able to avoid a jam even at high densities (or numbers per spot). No congestion of traffic ever occurs!

In any traffic jam, as the density increases, the speed of movement drops, finally to a halt. This is true in our highways, but why not in an ant highway? Chowdhury and his group studied ant movements in a natural trail, and analysed the distribution of speed as a function of crowding.

In any trail, some ants are slow (2 body lengths per second), some faster (6 bl/s) and some extra fast (10 bl/s). But as the number in the traffic increases, the speed distribution becomes narrower – the slow poachers are hurried, the superfast slowed down, and the average speed attains a steady value (4.7 bl/s). In other words, speed control becomes automatic. Ants form platoons, marching in step adjusting their speed at a pace desired for maintaining collective flow sufficiently high.

Chowdhury described an experiment devised by Dussutour and colleagues four years ago (J. Exp. Biol. 208, 2903, 2005), where they created a bottleneck in the ant trail.

The otherwise wide trail was narrowed into a thin lane in the middle. They expected a jam at the bottle neck, between the outbound ants and the one coming back (inbound) to the nest after foraging. What they found was remarkable.

Ants arriving at the entrance of the bottleneck gave way to those coming from the opposite direction, and hence queued up. The queue of the waiting ants became longer in time; at this point, the ants on the opposite side stopped and lined up for the earlier queue to clear. In other words, alternating clusters of inbound and outbound ants crossed the bottleneck!

They then did a second experiment (J. Exp. Biol. 212, 499, 2009), this time with the species of ants that cut leaves and carry them home on their bodies (species called atta colombica). Note here that the foraging ants that carry leaf home are laden with baggage and hence heavier, may be slower, than outbound ones who leave the nest for foraging. What happens when you create a bottleneck in their trail?

Astonishing is the word for the result. Unlike the earlier situation with the black ants (lasius niger) where the right of way was symmetric – once for inbound, next for outbound- with the leaf-cutters, the right of way was asymmetric. The incoming cargo-laden ants always had the right of way. Talk of courtesy and compassion!

In yet another experiment, the question was: what happens in a trail with traffic flowing in both directions and with heavy traffic? The experiment was done with army ants (eciton burchellis; see Couzin and Franks, Proc. Roy. Soc. London B 270, 139, 2003).

As the traffic density increased, it was found that the ants organized themselves into not two, but three lanes. The outbound ants took the outer lanes and the inbound returned in the central lane. Why?

The authors suggest that the outer lane ants protect the food brought home by the inbound returnees, and of course, also minimize collisions.

How do ants do this? Is there a commandant ant that forces the soldiers to march in step at a constant pace? Or do they become self-disciplined as a group of law abiding drivers? Is there a penalty for bad behaviour, for the slow one who stalls traffic or the speedster who can cause accidents?

The answer seems to raise the larger issue of collective behaviour in a society, or sociobiology. The collective good supercedes the individual’s. Each one gives a little so that the community gains – a phenomenon that sociobiologists have termed as reciprocal altruism. We see it not just in insects, but in birds, and even elephants. Darwin wondered about it 150 years ago, and these experiments validate it.

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Printable version | Feb 26, 2021 3:40:45 AM |

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