A termite mound in the Boatle region of Botswana. Termite mounds are made of a combination of soil, saliva and dung. Credit: Oratile Leipego/ CC BY-SA 4.0.
A study found evidence that termites, when building their nests, are guided mainly by water evaporation that allows them to identify the regions of the structure with the largest curvature1.
Termites nests, or mounds, can reach up to several metres in height, but so far scientists do not agree on how these tiny insects manage to do this. “For a long time, scientists believed that termites marked with pheromones the points in the structure where new pellets should be deposed, similarly to ants,” says Andrea Perna from IMT School of Advanced Studies in Lucca, who led the research. “But so far these chemicals haven’t been found by experiments”.
Other researchers suggested that curvature and elevation could play a role. In 2020 Perna and his collaborators developed a model with curvature as the only factor driving deposition2. “The model successfully reproduced typical components of termite nests, such as pillars and walls,” says Giulio Facchini, researcher at the CNRS Laboratory Matière Systèmes Complexes of Paris, and first author of the study.
In the new study the authors wanted to test the model in experiments and understand how termites can sense curvature. They took 50 individuals of Coptotermes gestroi, a termite species native to Asia, from a captive colony of tens of thousands at the Sorbonne Paris Nord University. The individuals were placed on a clay disk on top of a perforated Petri dish. The clay was kept at constant humidity by a wet cotton pad below the Petri dish.
At the centre of the clay disk, researchers had previously placed pellets to form two short pillars. Other pellets were evenly scattered across the disk surface. A camera recorded from above.
After 36 hours, termites had collected and deposed all the pellets and the authors saw the two pillars growing in height. Model simulations were in good agreement with experimental data across nearly 60 repetitions of the experiment. To check that the structure height didn’t play any role, they prompted the termites with a short wall at the centre of the disk. Termites still built two pillars starting at the wall corners.
Due to the marked difference between the room humidity and that of the clay disk, the researcher speculated that there ought to be a relevant evaporation flux near the disk surface, and models predict that the flux should increase with curvature. If that was the case, termites could perceive curvature through evaporation, since they are extremely sensitive to humidity due to their soft and thin shell. To check whether evaporation increased with surface curvature in the experimental set up, the authors imbibed the building substrate with a solution of water and bicarbonate. As the water evaporated, white crystals of salt accumulated on top of the pillars and on the two corners of the wall, marking the regions with the highest curvature.
“We believe this is a very general mechanism, but nests structure varies a lot across termite species, and it should be tested whether our simple model is flexible enough to reproduce all this variation,” Perna concludes.
