How army ants build city-like nests with their own bodies

Eciton army ants are so named for their military-like foraging tactics in which large numbers simultaneously swarm over an area the size of a football field, relentlessly devouring their prey, mainly other ant species and wasps. The thoroughness of these “raids” prevents army ant colonies from returning to the same area. So the colonies – some 200,000 individuals – must be in constant motion.

That raises the problem of where to nest and army ants have come up with a unique solution. They use their own bodies to make complex nests called bivouacs, large conical structures that hang from trees or sit in rotten logs. Every 20 days or so they move to a new location, reforming their nest when they get there.

Bivouac is an extraordinary example of self-assembly. Inside, the colony protects its queen, breeds eggs and hatches larvae. This requires specific temperatures and humidity, which the ants control by opening and closing vents. These complex structures are like living, breathing cities.

But the structure of these bivouacs, and how ants form them, is poorly understood, not least because army ants react fiercely to any external disturbance.

That’s why the work of Thomas Bochynek at Northwestern University in Illinois and colleagues is so exciting. This group has created an artificial environment in which army ants can build bivouacs, while an X-ray scanner records the internal structure of the resulting nest. The result is a fascinating visualization of army ant nests and a unique insight into the behavior behind such a complex self-assembly.

The team’s experimental room is a Plexiglas cylinder sealed at the top with a bulletin board for the ants to hang from. The perspex cylinder sits in an X-ray scanner that takes a series of images of the nest from different angles to build a three-dimensional picture of the bivouac structure.

All this was set up in a field lab in a tropical forest on the island of Barro Colorado, which is located in the Panama Canal in Central America. Here, Eciton army ants are relatively easy to find.

Bochynek and co begin their experiment by collecting some 50,000 ants from the nearby forest using a modified vacuum cleaner. They then release the ants into the experimental chamber and record what happens.

When the bivouac is fully formed and the shooting is over, the team removes the ants using the vacuum cleaner and releases them back into the room where the nest building process starts all over again. At the end of the experiment, the ants are released back into the forest.

In total, the team collected five different colonies and recorded the formation of 25 different bivouacs.

This gave them an unprecedented insight into the way bivouacs form. “We show that bivouacs are heterogeneous structures, maintaining a thick shell as they grow around a less dense interior that contains empty spaces resembling nesting chambers,” say Bochynek and co.

All this is happening with the bivouac in a state of constant change with individuals moving in and out of the structure. Indeed, Bochynek and co consider it a superorganism themselves.

By knowing the 3D structure, the team was able to calculate the stresses that each ant in the bivouac must experience. “We find that ants in the bivouac do not carry more than about eight times their weight, regardless of the size of the structure or their position in it,” they say.

In addition, it is easy to imagine that ants near the top of the structure are dealing with greater forces than those elsewhere in the bivouac; but not so. The team says these tensions are more or less evenly distributed among the individuals, regardless of their position within the structure.

Learn to assemble yourself

Bochynek and co hypothesize that if this type of division is maintained as the nest grows, the size of bivouac structures has no theoretical upper limit.

How each ant knows exactly what to do to maintain this structure is not yet clear. But Bochynek and co are confident that their approach will allow for a more detailed study of ant behavior, which should break the ground rules of self-assembly.

With that knowledge, the ability could emerge to create robotic systems that can artificially assemble themselves. These types of synthetic swarms could one day be able to repair roads and bridges, for example, or perform more sophisticated activities.

At present, however, there is much more to be learned from Panamanian army ants using this fascinating new method of visualizing the structures they create.


Ref: Anatomy of a superorganism – structure and growth dynamics of army ant bivouacs: arxiv.org/abs/2110.09017

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