Ants make efficient excavators

Soil is a ubiquitous example of a jammed system. Individual grains exert forces on their neighbors, and the resulting force network supports a stable, solid-like material. The stability of such disordered collections of grains is difficult to model, though. Even in a simplification of the complex material, each grain has far-reaching effects. Removing one particle could cause the whole network to fall apart, but existing models can’t predict which one.

Still, ants successfully build tunnels by excavating grains of soil one by one. They consistently select grains without destabilizing the surrounding soil and collapsing the entire structure. Now a study from Robert Buarque de Macedo and colleagues at Caltech, with collaborators at Grenoble Alpes University in France and Kansas State University, sheds light on the ants’ success.

The researchers used three-dimensional x-ray computed tomographic imaging to track tunnel building in an approximately 500 mL soil volume. The ants removed about 100 grains during the 10 minutes between each scan. Computer re-creations of the scans showed that the ants tended to dig in piecewise linear segments, each with a near-constant angle with respect to the ground. They also preferred removing 1–2 mm grains, comparable to their jaw size.

To understand the excavation’s stability, the researchers used the individual particle shapes and positions as inputs in a discrete-element simulation that reproduced the force network running through the jammed grains. The scan resolution, about 70 µm, enabled reconstruction of the shapes and locations of each of the millimeter-scale soil grains.

The simulations showed that the ants benefitted from soil arching—the redistribution of stress to stationary material when another area yields. The arching manifests as force chains in the soil’s bulk that wrap around the tunnel axis. An example is shown in the figure.

The soil arching reduced interparticle forces at the surface of the tunnel, making it easier for the ants to remove the grains. It also saved each ant from the fraught decision of which particle to remove: If none of the surface grains support large forces, then they can safely be removed without collapsing the tunnel. (R. Buarque de Macedo et al., Proc. Natl. Acad. Sci. USA 118, e2102267118, 2021 .)