Building mud castles: a perspective from brick-laying termites

Abstract

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.

Technical note: Rapid image-based field methods improve the quantification of termite mound structures and greenhouse-gas fluxes

“…V E : epigeal volume; A E ; epigeal surface area; A B ; basal area (intersection of the TM with the soil surface); r B : radius of a hypothetical circle with a circumference equal to A B ; h: epigeal height; V Cy , V Hs , V Co : volume of a hypothetical cylinder, hemi-spheroid and cone, respectively, used to approximate V E based on r B and h; V µ : volume of micro-pores in the wall material; V W : volume of the wall material; A W : area of wall material in a cross section; V M : volume of macro-pores (all cavities large enough for termites; generally referred to as "chambers"); A M : area of macro-pores in a cross section; V F : full mound volume including all pores and hypogeal parts; A F : area of the full mound in a cross section; m F : mass of the full mound. Turner, 2001;Zachariah et al, 2017). Yet, due to their opaque and complex nature, the morphology and structure of TMs are inherently difficult to quantify, and there is a lack of methods to adequately determine even basic physical parameters such as epigeal volume V E and surface area A E (Fig.…”

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

“…Jouquet et al (2002) explored the preference of worker termites to soils, observing complex decisions when selecting aggregates and clays for differing structures, noting "the kinetics of water retention reveals the ability of each soil to hold and to exchange water with the outside environment", inferring that termites are not selecting construction materials based on a specific property, beyond how granular materials behave in the presence of water. Murthy et al (Kandasami et al 2016;Zachariah et al 2017) extended studies of termite soil properties in India, observing that workers modulate the water content of a mud bolus between the liquid and plastic Atterberg limits, i.e., the point of optimum 'workability' or 'buildability'.…”

Modern termites inherited the potential of collective construction from their common ancestor

“…Finally, the change of building materials might be important in the evolution of shelter tube construction. Multiple‐piece or separate‐piece nesters, that build shelter tubes in natural conditions, use specifically soils and wood carton for construction (Emerson, 1938; Oberst, Lai, & Evans, 2016; Zachariah, Das, Murthy, & Borges, 2017), while one‐piece nesters use random materials for construction, including wood frass, solid feces and any soil or dust that may be available (Mizumoto, 2018; Morgan, 1959). This is associated with changes of both environmental parameters (availability of materials) and behavioral parameters (preference of materials).…”