Substrate evaporation drives collective construction in termites

Reviewer #1 (Public Review):

In this manuscript the authors performed experiments and simulations which showed that substrate evaporation is the main driver of early construction in termites. Additionally, these experiments and simulations were designed taking into account several different works, with independent (and sometimes conflicting) hypotheses, so that the current results shine a light on how substrate evaporation is a sufficient descriptor of most of the results seen previously.

The authors managed through simulations and ingenious experiments to show how curvature is extremely correlated with evaporation, and therefore, how results coming from these 2 environmental factors can most of the time be explained through evaporation alone. The authors have continued to use their expertise of numerical simulations and a previously developed model for termite construction, to highlight and verify their findings. On my first pass of the manuscript I felt the authors were missing an experiment: an array of humidity probes to measure evaporation in the three spatial dimensions and over time. Technologically such an experiment is not out of reach, but the author's alternative (a substrate made with a saline solution and later measuring the salt deposits on the surface) was a very ingenious low tech solution to the problem.

One possible missing experiment (and possibly the explanation of the only inconsistency of their results to previous literature) is to perform similar topographical experiments in high humidity chambers, where no humidity, or very low humidity gradients are present. Previous experiments done by Calovi and collaborators in 2019 showed that termite construction activity (without distinguishing digging from deposition) was focused on high curvature (concave) regions, where here the authors have seen higher depositions on convex structures. Despite the difference of "activity" by Calovi 2019 (clearly acknowledged by the authors), another main difference is that the experiments of the 2019 manuscript were performed in a closed chamber with very high humidity, and smooth transitions between regions of positive and negative curvature. Therefore, it stands to reason that the only missing component of the current article, would have been to perform similar experiments with curvature (positive and negative) but under an environment where gradients are reduced to a minimum.

The results presented here are so far the best attempt on characterizing multiple cues that induce termite construction activity, and that also possibly unifies the different hypothesis presented in the last 8 years into a single factor. More importantly, even if these results come from different species of termites than some of the previous works, they are relatable and seem to be mostly consistent, improving the strength of the author's claims.

Reviewer #2 (Public Review):

This study investigates the drivers behind termite construction, with a particular focus on the environmental factors that drive pellet deposition. The authors performed experiments and computations in an attempt to disentangle the role of surface curvature, feature elevation, substrate evaporation, and a possible "cement" pheromone on the deposition of soil pellets.

In three different types of experiments, the authors present termites with pre-made, unmarked (pheromone-free) pellets, and they vary pre-existing topographic building cues: some experiments have two pillars, others have a wall, and a third type had no cues. In experiments with topographic cues, the authors find that deposition seems to occur preferentially at the locations of highest curvature (i.e., peaks of pillars and corners of the walls). Complementary experiments and simulations show that locations of highest curvature correspond to locations with highest evaporation rates, at least for pillars. Evaporation rates seem inconclusive in the wall geometry, yet the termites still deposit material at the high-curvature wall corners. The authors conclude that: (1) no "cement" pheromone is needed for construction, (2) that depositions preferentially occur at locations of high curvature (all experiments for pillars, 7 out of 11 experiments for walls), and (3) that evaporation (which is fastest at places of highest curvature, at least for pillars) drives deposition. The experiments and results seem sound and interesting, but some of the interpretations need more justification. For instance, why conclude that evaporation drives construction when there is not a measurable difference in evaporation rate across the wall geometry?

The authors also perform simulations (developed in a previous publication) that agree with their experimental observation that deposition occurs preferentially at locations of high curvature. However, there is not enough detail provided about the simulation to understand the degree to which simulation and experiment agree (e.g., is the agreement qualitative or quantitative?) as well as the significance of the agreement. The authors should provide additional details about the setup and mechanics of the simulation, the outputs and how they connect to experiments, and potential limitations of results/connections to the experimental system. Finally, more background about this termite species would be helpful in putting these results into context. For instance, what is known about the natural habitat and conditions, and natural nest locations and structures? What are (or might be, depending on what is known) the potential abilities/benefits for these animals to sense humidity gradients, and why building at these locations could benefit the animals?