Tunnel remodeling in fire ant (S. invicta) collectives

Many collective systems rely on excavation, transport and deposition of material to create structures necessary for survival. For example, social insect species such as fire ants (S. invicta) create complex nest structures composed of subsurface tunnels and a porous mound above. Strategies by which ants excavate in confined spaces have been studied; however, principles governing where ants choose to deposit excavated material are less understood. In this study, we monitor material flow by creating quasi-2D “ant farms” with 6 layers of colored sand (particle size 75-400 μm, saturated with water 10% by mass). The ~300 ants in each trial formed 41 tunnel structures reaching >50% of the depth of the containers over 60 hrs. Ants bring material up tunnels to the surface, creating proto-mounds with color bands ordered opposite to the original substrate (3 trials). In 92% of tunnels after 60 hours, material from deeper layers lined the walls of previously dug tunnels; after 45 hours one tunnel diameter at its widest location was 3.25 mm, shrinking to 1.83 mm after 60 hours. We posit this represents intentional remodeling of tunnel walls by the collective, possibly to enable alternative locomotion control strategies [Gravish et al, PNAS 2013] or to decrease energy expenditure during early nest construction. These results demonstrate ways in which organisms and multi-agent systems can modify their environments to benefit the collective performance.