Because they follow rules.
Individual ants in large colonies communicate with each other using pheromones – hormones secreted outside the body as chemical signals to trigger responses from other ants.
For army ants, Eciton burchellii, the trail formed by pheromones is especially useful: the ants are completely blind. Yet they march in distinct columns through jungles scouring for prey – other ant species, cockroaches, wasps…
Meticulous field work and observations by entomologists show that, in the first stage, army ants raid unidirectionally in a single lane: a few metres wide, one-fifth of a kilometre long collective of thousands of ants.
But, when the worker ants start returning to the nest with food, the flux of ants becomes bi-directional. In fact, for efficient flow, the traffic trifurcates to a three-lane highway.
“The ants form bi-directional columns of three, where ants on outer lanes go to the food site, and the inner middle chain carries food back to the nest”, explains Debashish Chowdhury, IIT Kanpur.
The formation has the obvious advantage of providing the food with protection from invaders. Previous studies also show that due to the extra weight, food-carrying ants cannot turn sharply to avoid collisions. So, the right of way is given to food-laden ants. Outbound ants turn either left or right to avoid collision. This leads to the favored three-lane traffic.
For Debashish Chowdhury and team at IIT Kanpur who are working in the interface between biology and physics, army ant behaviour provided a perfect opportunity to use statistical mechanics to connect subtle microscopic local interactions with macroscopic global properties.
The research group used cellular automata – a computational model which projects the inter-particle interactions of a large number of individual particles – to study the phenomenon. The model assumes that each ‘cell’ or entity is an autonomous system capable of taking decisions.
“In early 2000s, in collaboration with Japanese and German scientists, we had defined a set of logical rules for the ants in our model to follow, and ran our simulations with different parameters”, says Debashish Chowdhury, IIT Kanpur.
The researchers found that a two-lane traffic of ants would not be spatially stable.
So, now they extended the theoretical framework to model three-lane traffic where the flow of ants in the inner lane is opposite to that in the two outer lanes. The model also allowed lane-changing by some ants in the outer lanes which affects the traffic of the inner food lane.
“Then we got interested in the flux density in the inner lane of ants”, says Swayamshree Patra, IIT Kanpur. So in collaboration with researchers from Germany, the group derived flux-density relations of ant colonies to see the flow of traffic in the middle lane.
The movement of ants is dependent on pheromones. So, the probability of ants changing lanes is modified by the presence (or absence) of pheromones. Since pheromones are volatile, they evaporate – unless reinforced by other ants. By defining these rules for their model ants, the group ran their simulations.
For small evaporating rates of pheromones and low lane-changing probabilities, the flux of food-carrying ants increased due to the positive reinforcement of extra pheromones. This trend stayed true for mid-ranges also.
“But when we increased the lane changing probability of the ants, we were surprised”, says Smriti Pradhan, IIT Kanpur.
More ants moving into the inner lane clogged it. Even the extra pheromones from the lane changing ants in the inner lane cannot prevent the stagnation of the traffic of food-carrying ants.
“Our results show that army ants have just the right algorithm and fine tuned the parameters for self-organising their traffic”, chuckles Debashish Chowdhury, IIT Kanpur.
We are not so different from army ants. We also form similar lanes while walking mindlessly, contributing to a larger pattern. But put us behind the wheel with a destination, and madness happens.
Ants show that keeping things simple and following rules is the way to make traffic efficient. Though their mastery of collective movement is mesmerizing, the highly successful adaptation to collective foraging has a limitation. If they are separated from the main foraging group, army ants go around in circles to eventually die, exhausted.
This is when our mindfulness comes to our rescue. There seems to be more flexibility in the algorithms that we use.
Physica A Stat. Mech. Appl.,567 125664 (2021);
Manish Kumar Tekam
Holkar Science College, Indore
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