Local rules and geometric constraints enable robust leaf-nest construction in weaver ants

Weaver ants (Oecophylla smaragdina) construct their nests by gluing together leaves in tree canopies. They accomplish this by forming tool-like self-assemblages, which they use to bend the leaves into place. To study this, we developed a novel experimental approach that recreates nest building in the lab using artificial leaves and multi-viewpoint 3D reconstruction. In all tested conditions, the ants combine the leaves into viable, closed structures where all leaves bend either upward or downward. Leaf thickness and initial placement were major factors in determining the configuration adopted. We quantify the dynamics of ants' self-assembled tools and the resulting leaf manipulation, and we show that local ant-scale rules operating under geometric constraints explain why some nests bend downward while others bend upward. We also compute the transition angle. Finally, we use differential geometry to suggest why, under our experimental conditions, nests are constrained to be convex, architecturally stable, sphere-like surfaces. Our findings provide insights into the interplay of geometry, biomechanics, and local-ant-scale rules in complex collective construction.