Tech

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

Ant colonies can descend several metres underground, house millions of insects and last for decades, despite being made without the benefit of machinery and reinforcing material. The secrets of these impressive architectural structures are being revealed by three-dimensional X-ray imaging and computer simulations, and could be used to develop robotic mining machines.

José Andrade at the California Institute of Technology and his colleagues set up miniature ant colonies in a container holding 500 millilitres of soil and 15 western harvester ants (Pogonomyrmex occidentalis). The position of every ant and every grain of soil was then captured by high-resolution X-ray scans every 10 minutes for 20 hours.

The X-ray results gave researchers exact details about the shape of each tunnel and which grains were being removed to create it. The team then created a computer model using those scans to understand the forces acting upon the tunnels. The size, shape and orientation of every grain was recreated in the model and the direction and size of force on each grain could be calculated, including gravity, friction and cohesion caused by humidity. The model was accurate to the 0.07 millimetre resolution of the scanner.

The results suggest that forces within the soil tend to wrap around the tunnel axis as ants excavate, forming what the team call “arches” in the soil that have a greater diameter than the tunnel itself. This reduces the load acting on the soil particles within the arches, where the ants are constructing their tunnel. As a result, the ants can easily remove these particles to extend the tunnel without causing cave-ins. The arches also make the tunnel stronger and more durable.

“We had naively thought that ants perhaps were playing Jenga, that they were tapping, maybe they were wiggling grains, maybe they were even grabbing the grains of least resistance,” says Andrade.  He says it is now clear that the ants appear to know nothing about forces and show no signs of decision-making, but instead follow a very simple behavioural algorithm that has evolved over time.

Before ants dig tunnels, the grain-to-grain forces in soil are randomly aligned (left), but as the ants dig (right), the forces align and wrap around the tunnel

Caltech

The ants tend to dig relatively straight tunnels that descend at the angle of repose – the slope at which a granular material naturally forms mounds – which was around 40 degrees in this case. They also pick exactly the right grains to remove to create a protective arch above.

“In a remarkable way – in a rather, you know, serendipitous way – they’ve stumbled upon a technique for digging that is in line with the laws of physics, but incredibly efficient,” says Andrade.

The team believes that if the behavioural algorithm can be further analysed and ultimately replicated, then it may find application in automated mining robots, either here on Earth or on other planetary bodies where the already risky business of mining would be even more dangerous for humans.

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

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