The digging dynamics of ant tunnels: movement, encounters, and nest space

Underground ant nests are constructed by decentralised self-organisation wherein the ants respond to local stimuli and produce coordinated structures through globally regulated behaviours. One such regulation is the reduction in digging effort that occurs when available nest space has reached an adequate size. Tunnels have a distinct configuration relative to other nest elements and the processes regulating their excavation are poorly understood. We examined the relationship between digging effort and tunnel space by presenting groups of 10 Acromyrmex lundi workers with either short or long tunnel spaces and demonstrated that they will dig significantly less over time in a tunnel that is already long compared to one that is short. Additionally, we provided the same treatment to groups of 100 workers and found no significant effect of length, suggesting that group size has an important impact on tunnel excavation dynamics. Automated tracking was then used to examine tunnel digging in greater detail. Groups of 10 Atta colombica ants were tracked while excavating sand in a tunnel apparatus. There was a significant correlation between mean walking speed and excavation rate. Additionally, the ants would maintain a consistent level of proximity with each other over time. This suggests that as tunnel space expands, several factors combine to lower the chance of ants encountering the tunnel digging face and taking up excavation.