What ants can teach us about the Eiffel Tower

With a recent study from Georgia Institute of Technology on how fire ants form towers, the architectural world has a new potential take away from nature. Done simply through trial-and-error, the fire ants create bell shaped structures by climbing on top of each other until they find an empty spot, at which point they stop. Each ant supports only three others above them, creating a structure where each ant bears the same load and the results look a lot like the famed Eiffel Tower.1

Why the ants constantly rebuild the tower, researchers aren’t entirely sure—but they almost missed this crucial piece of information. They had planned to record ants building for two hours, but the camera rolled for three. Speeding through what they thought was an extraneous hour, the team was startled to get a different view of what was actually going on.

“We didn’t expect to see anything interesting in that extra hour, so we sped up the video to 10 times real speed,” said Craig Tovey, also a co-author of the study and professor at the Stewart School of Industrial & Systems Engineering at Georgia Tech.2

In the aftermath of a flood, fire ants, Solenopsis invicta, cluster into temporary encampments. The encampments can contain hundreds of thousands of ants and reach over 30 ants high. How do ants build such tall structures without being crushed? In this combined experimental and theoretical study, we investigate the shape and rate of construction of ant towers around a central support. The towers are bell shaped, consistent with towers of constant strength such as the Eiffel tower, where each element bears an equal load. However, unlike the Eiffel tower, the ant tower is built through a process of trial and error, whereby failed portions avalanche until the final shape emerges. High-speed and novel X-ray videography reveal that the tower constantly sinks and is rebuilt, reminiscent of large multicellular systems such as human skin. We combine the behavioural rules that produce rafts on water with measurements of adhesion and attachment strength to model the rate of growth of the tower. The model correctly predicts that the growth rate decreases as the support diameter increases. This work may inspire the design of synthetic swarms capable of building in vertical layers.