Ants use soil physics to excavate metre-long tunnels that last decades

Ant colonies can reach several metres underground and house millions of insects. They also last decades without the use of machinery or reinforcing materials. Three-dimensional Xray imaging and computer simulations are helping to reveal the secrets behind these amazing architectural structures. They could also be used for developing robotic mining machines.
Jos Andrade, from the California Institute of Technology, and his collaborators set up miniature colonies of ants in a container that contained 500 millilitres of soil. Also included were 15 western harvester (Pogonomyrmex westerianais) ants. High-resolution Xray scans were then taken every 10 minutes for 20 hours to capture the position of each ant and each grain of soil.

Researchers were able to get exact information from X-rays about the tunnel's shape and which grains had been removed in order to make it. The scans were used to create a computer model that reflected the forces exerted on the tunnels. The model allowed for the creation of the size, shape, and orientation of each grain. It also allowed for the calculation of the direction and magnitude of the force exerted on each grain, including gravity, friction, and cohesion due to humidity. The scanner's 0.07 millimetre resolution allowed for accurate modeling.

These results show that soil forces tend to wrap around tunnel axis when ants excavate. This creates what the team calls arches in soil, which have a larger diameter than the tunnel. This decreases the load on soil particles in the arches where the ants are building their tunnel. These particles can be easily removed by the ants to make the tunnel longer without creating cave-ins. The tunnel is also stronger and more durable thanks to the arches.


Andrade says that we had mistakenly believed that the ants were playing Jenga. They were tapping on grains, or maybe even wiggling them. Andrade says that it is now evident that the ants don't seem to have any idea about forces or show any signs of decision-making. Instead, they follow a very simple behavioral pattern that has evolved over time.

The grain-to-grain forces of soil were randomly aligned before ants dug tunnels (left). But as the ants dig (right), these forces align themselves and wrap around Caltech's tunnel Caltech

Ants tend to dig straight tunnels that descend at an angle of repose where a granular material naturally forms mounds. This was approximately 40 degrees. They choose the correct grains to remove in order to make a protective arch.

Andrade says they found a method for digging that was not only in accord with the laws of physics but also extremely efficient.

The team believes that the behavioural algorithm could be further analysed and eventually replicated to be used in automated mining robots here on Earth as well as on other planets where the already dangerous business of mining is even more dangerous.

Journal reference: Proceedings of the National Academy of Sciences, DOI: 10.1073/pnas.2102267118