Ant chain mail: Investigating bivouac construction in two dimensions

How do ants decide when and where to build, and to what extent can ant architecture withstand perturbation? Some species of army ants form temporary structures–bivouacs–where they shelter their brood and queen inside a linked mass of living workers, rather than digging subterranean nests. To understand the connections between individual ants and anchoring surfaces that compose bivouacs, we employed the artificial constraint of a very narrow building area. By allowing Eciton army ants to build hanging networks of living workers inside this quasi-two-dimensional space, we were able to clearly visualize connection points in these networks. We quantified the preferences of groups of ants for building in small vs. large arenas as well as previously used vs. clean and new arenas. Ants were equally likely to build under each of these conditions, potentially reflecting their natural tendency to form multiple bivouacs simultaneously rather than investing in only one structure. Additionally, using a motor-powered moveable piston built into a two-dimensional arena, we exerted compression and then expansion forces on the networks built by ants. By tracing the connection points used by ants before and after bouts of compression and expansion, we were able to determine the relative stability of their networks. Notably, ants could often better maintain their original connections when perturbed by a slow-moving piston, as compared to perturbation by a fast-moving piston. Characterizing the properties of this “ant chain mail” demonstrates its capacity for memory and can serve as bioinspiration for material science applications.