Moisture alone is sufficient to impart strength but not weathering resistance to termite mound soil

Abstract

Soil is used for the construction of structures by many animals, at times admixed with endogenous secretions. These additives, along with soil components, are suggested to have a role in biocementation. However, the relative contribution of endogenous and exogenous materials to soil strength has not been adequately established. Termite mounds are earthen structures with exceptional strength and durability including weathering resistance to wind and rain. With in situ and laboratory-based experiments, we demonstrate that the fungus-farming termite Odontotermes obesus which builds soil nest mounds, when given a choice, prefers soil close to its liquid limit for construction. At this moisture content, the soil–water mixture alone even in the absence of termite handling undergoes self-weight consolidation and upon drying attains a monolithic, densely packed structure with compressive strength comparable to the in situ strength of the mound soil; however, the soil–water mixture alone has lower resistance to water erosion than the in situ mound samples, suggesting that termite secretions impart weathering resistance and thereby long-term stability to the mound. Therefore, weathering resistance and compressive strength are conferred by different aspects of termite soil manipulation. Our work provides novel insights into termite mound construction and strength correlates for earthen structures built by animals.

Submillimetre mechanistic designs of termite-built structures

“…electronic supplementary material, figure S5. It consists of inorganic and organic materials admixed with endogenous secretions made of lignin and cellulose, originally from the plant cell walls [1,10,13,20,22] and which show up at wavelengths of paraffins-distinctly related to terpenes and resins (cf. figure 2b).…”

“…For comparison, the global compressive strength of load supporting walls at foraging sites of Coptotermes acinaciformis were estimated to be 0.22 MPa, much lower than concrete, indicating that there might be a difference between the nest where highest safety is required and the foraging site where the termites invest just as much energy as necessary [10,20]. However, the strength of soil depends on mineral composition, grain size distribution and the ratio of clay/silt and other particles/materials, degree of saturation (water in interparticle junctions and related water surface tension), stress history [21,22]. Zachariah et al [23] showed for O. obesus that construction particle choice being close to its liquid limit attained high compressive strength similar to that of mounds found in the field when dry, by forming a monolithic, densely packed structure.…”

“…Clay, especially if exposed to both temperature and humidity extremes, has a higher risk of crack formations due to tension [35,[51][52][53][54], resulting in lower stability. Cracks in the outer shell, however, may be 'designed' for the purpose of supporting porous peripheries and permeability for cooling or for water retention as recently evidenced in nests of the fungus growing termite O. obesus [22]; or are simply an effect of weathering. Often, mounds have a rough surface which gets smoothed by rain and wind and which may also lead to different chemical reactions and segregation of particles.…”

“…Some termites, such as Coptotermes acinaciformis, have shown the ability to use their clay-built structures as a foraging tool to support load and to unlock otherwise inaccessible food resources [10,20]. The mound and the constructions built on their foraging sites have been shown to be macroscopically structurally different: Odontotermes obesus mounds to take up to 1.5 MPa compressive loads [21][22][23]; the clay walls designed to keep loads at a foraging site in C. acinaciformis had a pressure load capacity of 0.22 MPa, which maximize foraging efficiency by maintaining compressive loads from tree weight [20]. Perna et al [24] introduced computed tomography (CT) to visualize the architecture of Cubitermes sp.…”

“…Apart from those macroscopic mechanical properties, which have received some attention in recent years contemporary research has been mainly concerned with the topology of tunnel systems, aspects of self-organization or swarm behaviour, often only computationally [1,8,18,19,24] and no study yet is focussed on the microscale mechanical or microgeometrical properties of real-life termite nests [1]. While Zachariah et al and Oberst et al studied the moisture content, stability and also weather resistance to moisture and conducted compression tests on soil samples [10,21,22], including termitecontacted timbers, there are few detailed analyses of other influential factors for many termite species, especially particle size and soil organic matter have been conducted. Zachariah et al [23] studied the building blocks (boluses) of O. obesus, which are ca 1 mm-sized to fit into termite worker jawsand described them as key elements of spherical unitary shape made of granular hydrophilic, osmotically inactive, non-hydroscopic materials with surface roughness, rigidity and organic matter.…”