When the ants go marching in

Fire ants (Solenopsis invicta), like most ants, work together to dig complex tunnel networks underground. Researchers use insect colonies as models for learning how hundreds of individuals avoid creating traffic jams while working in a confined, crowded environment. Now researchers at Georgia Tech have found that individual workers follow counterintuitive rules to excavate the collective’s early-stage tunnels.

Daniel Goldman , Jeffrey Aguilar, and colleagues filmed 30 fire ants, each painted a different color for identification, as the ants dug tunnels in a damp soil-like pack enclosed in a transparent container, as shown in the photo above. The researchers observed that only three to five ants dug at any given time; in fact, 30% of the ants did 70% of the work. When individuals encountered even a small traffic jam on their way to the tunnel, they retreated without collecting any soil.

The team used traffic flow models as a guide to devise simulations of ants excavating tunnels. In flow models used by traffic engineers, too many cars create a traffic jam, whereas too few cars cannot sustain a steady flow. An intermediate, critical number of cars maximizes traffic flow. Similarly, the ants moved at maximum efficiency, without jamming, when following an optimized division of labor similar to that observed in the biological experiments.

The simulations captured group behavior. To investigate the strategies of individual ants, the researchers built robots, shown below, designed to excavate a three-robot-wide channel filled with plastic spheres. Each capsule-shaped robot wheeled into the channel, scooped up a ball, and carried the ball back to the entrance. Goldman and colleagues found that up to three robots could dig simultaneously, but adding a fourth caused a standstill. By programming the robots to retreat when they encountered a delay, the researchers could mitigate the clogging effects and optimize the system for high traffic flow and low energy consumption. Even so, the relatively clumsy robots couldn’t match the ants’ performance.

The findings help to explain how confined active systems evolve when individual agents lack knowledge about the system’s overall structure and the positions of their fellow agents beyond their nearest neighbors. That understanding could help scientists develop robot swarms that perform tasks in confined spaces, such as carrying drugs through the bloodstream or searching through disaster rubble. (J. Aguilar et al., Science 361, 672, 2018 .)