Moisture gradients, form a vapor cycle within the viscous boundary layer as an organizing principle to worker termites

Studies of termite mound building have considered the mud they prepare, its properties and its composition. Here we consider the behaviors of the mound building termites Macrotermes michaelseni, (Sjostedt), in the presence of the viscous boundary layer (VBL), which spontaneously forms over any surface that air passes over. We looked how soil moisture and air vapor are coupled to form a feedback loop and a spatiotemporal precursor to worker termites in the presence of mound material. We explored residency and activities of workers when presented with a VBL and either varying substrate temperature gradients or a soil moisture transition within the soil substrate. We report the emergence of a ‘vapor conveyor’, which forms around a neutral evaporative equilibrium point (NEEP) at the soil/air interface, where the soil-borne moisture temperature (along the gradient) and the 100% saturated air-borne vapor temperature coincide within the VBL, forming a bubble of neutral mass transfer which, we propose, worker termites are sensitive to as viscosity changes within. We found, on average, that 67% (std. dev 27%) of behavioral events (clustering, excavation, and deposition) occurred within 10C either side of the NEEP. We found negative correlation (-0.78) between the substrate temperature gradient (0.1-0.9 0C mm-1) and the extents of behavioral activity, suggesting coupling between soil-borne moisture and air-borne vapor advection within the VBL. We recorded unique behaviors relating to interaction with the viscosity of vapor saturated air at this scale. We speculate that workers may exploit the VBL to overcome a classic trade-off, i.e. how to push activities forward into potentially desiccating environments, while conserving moisture in both the termites and the soil they build with

Differential construction response to humidity by related species of mound-building termites

Macrotermes michaelseni and M. natalensis are two morphologically similar termite species occupying the same habitat across southern Africa. Both build large mounds and tend mutualistic fungal symbionts for nutrients, but despite these behavioural and physiological similarities, the mound superstructures they create differ markedly. The behavioural differences behind this discrepancy remain elusive, and are the subject of ongoing investigations. Here, we show that the two species demonstrate distinctive building activity in a laboratory-controlled environment consisting of still air with low ambient humidity. In these conditions, M. michaelseni transports less soil from a central reservoir, deposits this soil over a smaller area, and creates structures with a smaller volumetric envelope than M. natalensis. In high humidity, no such systematic difference is observed. This result suggests a differential behavioural threshold or sensitivity to airborne moisture that may relate to the distinct macro-scale structures observed in the African bushland

Surface curvature guides early construction activity in mound-building termites

Termite colonies construct towering, complex mounds, in a classic example of distributed agents coordinating their activity via interaction with a shared environment. The traditional explanation for how this coordination occurs focuses on the idea of a 'cement pheromone', a chemical signal left with deposited soil that triggers further deposition. Recent research has called this idea into question, pointing to a more complicated behavioural response to cues perceived with multiple senses. In this work, we explored the role of topological cues in affecting early construction activity in Macotermes. We created artificial surfaces with a known range of curvatures, coated them with nest soil, placed groups of major workers on them and evaluated soil displacement as a function of location at the end of 1 h. Each point on the surface has a given curvature, inclination and absolute height; to disambiguate these factors, we conducted experiments with the surface in different orientations. Soil displacement activity is consistently correlated with surface curvature, and not with inclination nor height. Early exploration activity is also correlated with curvature, to a lesser degree. Topographical cues provide a long-term physical memory of building activity in a manner that ephemeral pheromone labelling cannot. Elucidating the roles of these and other cues for group coordination may help provide organizing principles for swarm robotics and other artificial systems. This article is part of the theme issue 'Liquid brains, solid brains: How distributed cognitive architectures process information'

Solar-powered ventilation of African termite mounds

How termite mounds function to facilitate climate control is still only partially understood. Recent experimental evidence in the mounds of a single species, the south Asian termite Odontotermes obesus, suggests that the daily oscillations of radiant heating associated with diurnal insolation patterns drive convective flow within them. How general this mechanism is remains unknown. To probe this, we consider the mounds of the African termite Macrotermes michaelseni, which thrives in a very different environment. By directly measuring air velocities and temperatures within the mound, we see that the overall mechanisms and patterns involved are similar to that in the south Asian species. However, there are also some notable differences between the physiology of these mounds associated with the temporal variations in radiant heating patterns and CO2 dynamics. Because of the difference between direct radiant heating driven by the position of the sun in African conditions, and the more shaded south Asian environments, we see changes in the convective flows in the two types of mounds. Furthermore, we also see that the south Asian mounds showa significant overturning of stratified gases, once a day, while the African mounds have a relatively uniform concentration of CO2. Overall, our observations show that despite these differences, termite architectures can harness periodic solar heating to drive ventilation inside them in very different environments, functioning as an external lung, with clear implications for human engineering

Termite males enhance mating encounters by changing speed according to density

1.Search theory predicts that animals evolve efficient movement patterns to enhance encounter rates with specific targets. The optimal movements vary with the surrounding environments, which may explain the observation that animals often switch their movement patterns depending on conditions. However, the effectiveness of behavioural change during search is rarely evaluated because it is difficult to examine the actual encounter dynamics.2.Here we studied how partner‐seeking termites update their search strategies depending on the local densities of potential mates. After a dispersal flight, termites drop their wings and walk to search for a mate; when a female and a male meet, they form a female‐led tandem pair and search for a favourable nesting site. If a pair is separated, they have two search options—reunite with their stray partner, or seek a new partner. We hypothesized that the density of individuals affects separation–reunion dynamics and thus the optimal search strategy.3.We observed the searching process across different densities and found that termite pairs were often separated but obtained a new partner quickly at high mate density. After separation, while females consistently slowed down, males increased their speed according to the density. Under high mate density, separated males obtained a partner earlier than females, who do not change movement with density.4.Our data‐based simulations confirmed that the observed behavioural change by males contributes to enhancing encounters. Males at very low mate densities did best to move slowly and thereby reduce the risk of missing their stray partner, who is the only available mate. On the other hand, males that experienced high mate densities did better in mating encounters by moving fast because the risk of isolation is low, and they must compete with other males to find a partner.5.These results demonstrate that termite males adaptively update their search strategy depending on conditions. Understanding the encounter dynamics experienced by animals is key to connecting the empirical work to the idealized search processes of theoretical studies