Coordination Without Communication in Fire-Ant Excavation

Many social animals, such as ants, are millimeters in scale and collectively dig meter scale nests in diverse substrates. To discover principles by which fire ant [S. invicta] collectives self-organize to excavate their crowded narrow (body-length) self-generated tunnels, we recorded 48 hour videos of groups of 40-70 ants digging in a quasi-two-dimensional system 14×21×0.25 cm³ composed of slightly wet 700 um glass beads.  Excavation proceeded in three stages: an initial constant rate lasting approximately an hour followed by a rapid decay in rate, and finally a slower decay scaling in time as t^-1/2. To understand such scaling and motivate how such rate modulation emerges without global control avoiding deleterious traffic clogs, we use a cellular automata model with multiple tunnels. In the model ants can count collisions with other ants, but otherwise do not directly communicate; this minimal model reproduces the observed excavation stages. A scaling argument assuming a constant number of workers and no collisions rationalizes the final t^-1/2 scaling. This study furthers our understanding of how insect societies work together to regulate the early stages of nest construction in which getting below ground quickly protects against challenging environments and predation.