Animal constructions such as termite mounds have received scrutiny by architects, structural engineers, soil scientists and behavioural ecologists but their basic building blocks remain uncharacterized and the criteria used for material selection unexplored. By conducting controlled experiments on Odontotermes obesus termites, we characterize the building blocks of termite mounds and determine the key elements defining material choice and usage by these accomplished engineers. Using biocement and a self-organized process, termites fabricate, transport and assemble spherical unitary structures called boluses that have a bimodal size distribution, achieving an optimal packing solution for mound construction. Granular, hydrophilic, osmotically inactive, non-hygroscopic materials with surface roughness, rigidity and containing organic matter are the easiest to handle and are crucial determinants of mass transfer during mound construction. We suggest that these properties, along with optimal moisture availability, are important predictors of the global geographic distribution of termites.
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...
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.
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