Bi-layered architecture facilitates high strength and ventilation in nest mounds of fungus-farming termites

Abstract

Mass-energy transfer across the boundaries of living systems is crucial for the maintenance of homeostasis; however, it is scarcely known how structural strength and integrity is maintained in extended phenotypes while also achieving optimum heat-mass exchange. Here we present data on strength, stability, porosity and permeability of termite mounds of a fungus-farming species, Odontotermes obesus. We demonstrate that the termite mound is a bi-layered structure with a dense, strong core and a porous shell that is constantly remodelled. its safety factor is extraordinarily high and is orders of magnitude higher than those of human constructions. the porous peripheries are analogous to the mulch layer used in agriculture and help in moisture retention crucial for the survival of fungus gardens, while also allowing adequate wind-induced ventilation of the mounds. We suggest that the architectural solutions offered by these termites have wider implications for natural and industrial building technologies. Mass-energy transfer across the boundaries of living systems is crucial for the maintenance of homeostasis 1. These boundaries can be that of an individual (human skin) 2 or a colony of individuals (swarm cluster of honeybees) 3 or that of a constructed extended phenotype (termite mounds) 4,5. The boundary conditions in these systems determine the mass and energy fluxes; therefore, the boundary must be able to respond to ambient changes. Homeostasis can be achieved, for example, by regulation of blood flow to skin 2 , or the movement of individuals between the periphery and core of a honeybee swarm 3 ; however, little is known about this transfer when the boundary consists of non-living materials, e.g. soil, as in the walls of termite mounds. Regulation becomes more challenging when the construction crew is subterranean as in fungus-farming termites with their fungus gardens 6-8. Moreover, while the external boundaries of these earthen structures must primarily be designed to respond to changes in the external environment, the internal regions need to maintain structural strength in order to prevent collapse. Termite mounds are excellent examples of biocementation and collective construction 9. Mounds can be three orders of magnitude larger than individual termites 9 and can maintain structural integrity for decades to centuries 10. Termites collect small, irregular spheres of soil, which they use for construction 9. These are analogous to bricks used in human construction, are termed 'boluses' (singular: bolus), and are made by mixing their secretions with moist soil 11. Once deposited at the construction site, boluses merge and form an almost monolithic structure resembling construction using amorphous materials such as foam. This material nature allows mound construction on irregular surfaces 9. Soil manipulation by termites imparts a tenfold increase to its strength 11. Compressive strength and density of mound soil increases from the top to the bottom of the mound due to compaction under self-weight over time 11...

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

“…The results from the previous experiment suggested that the peak compressive strength of soil without termite manipulation was similar to the peak compressive strength of in situ termite mound soil as reported by Kandasami et al [32] and by Zachariah et al [39]. Therefore, we tested the role of selfweight consolidation (densification of soil under its own weight without application of any external force [40]) in the strength of mound soil.…”

“…In order to understand the relative contribution of soil organic matter and termite secretions towards compressive strength (referred to as 'strength' henceforth) to termite mound soil, we offered red soil and burnt soil (red soil combusted at high temperature to remove organic matter [38]) to termites in the laboratory, collected boluses, packed them in cylindrical moulds (2 cm height × 1 cm diameter), allowed them to dry under room conditions followed by drying overnight at 80°C and then tested their strength following ASTM protocol [38]. The samples were placed in a Universal Testing Machine and were subjected to unconfined compression at 1 mm min −1 until sample failure [39]. Stress-strain graphs were plotted for each sample.…”

“…Therefore, all further experiments with soil were conducted with red soil (soil containing organic matter). We previously established that soil samples of 2 cm height × 1 cm diameter show the same behaviour under stress as soil samples of 6 cm height × 3 cm diameter (recommended by ASTM [38]) and that no scaling of strength occurs [39]. Still, preparing cylindrical moulds with boluses required obtaining a sufficient number of boluses from termites at a very high rate.…”

“…7: 200485 2.8. Total soil suction measurement Because termite mound soil is known to contain in situ moisture [39], and soil alone after self-weight consolidation attains strength comparable to the in situ mound strength, therefore, the role of soil suction in imparting strength to the soil was investigated. Samples were obtained from live mounds and their dry density and moisture contents were determined.…”

Submillimetre mechanistic designs of termite-built structures

“…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.…”