Fire ants perpetually rebuild sinking towers

Abstract

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.

Attraction, Dynamics, and Phase Transitions in Fire Ant Tower-Building

“…As we are not considering eects of stability, we will ignore rule (iv) in the present study. We simulate the rules (i)-(iii) from Mlot et al [2012] and Phonekeo et al [2017] with a naive approach to what constitutes an available space adjacent to non-moving agents, assuming no direct knowledge of the agents about where they are relative to the rest of the tower. At each time step, each individual agent randomly chooses an adjacent square to move into, performing a random walk and fullling rule (ii).…”

“…With the three modications mentioned above, we modify the rst three rules of Phonekeo et al [2017] and Mlot et al [2011] into four new local rules:…”

“…The work of Phonekeo et al [2017] shows an agreement between the resulting tower shapes in the longtimescale limit; however, it does not explore the time dynamics and parameter space systematically. This is what the present work aims to do, since local rules such as these provide a systematic way of analyzing collective behavior through agent-based modeling, and importantly, they are directly implementable in swarm robotic systems.…”

“…When the rafts approach vegetation emerging from the surface, they may attach to the vegetation and form towers on top of their oating rafts. In a recent study, Phonekeo et al [2017] described an experimental assay of the tower-building process in re ants. In their analysis, the authors propose four rules which allow ants to build towers:…”

“…Agents move horizontally and climb if they the voxel they intended to move into is occupied by a locked agent. Note that the local rules described above, from Phonekeo et al [2017], refer to agents linking with one another, while in the present work we will refer to an agent that stops to support tower building as locked. Each pixel along the horizontal plane has an associated height equal to the number of locked agents on top of each other in that location.…”