Schematic cross-section through a mound of Macrotermes michaelseni, showing layout of tunnel networks, nest, and other structures. From Turner 2001.

Concrete casting of tunnel network of a Macrotermes bellicosis nest. From Ruelle 1962.

Structure of the Mounds of Macrotermes michaelseni

The key to realising this project’s objectives lies in mimicking and adapting the geometry found in the mound structures produced by colonies of the termite Macrotermes michaelseni. These termites have evolved a construction technique which extends the thermo-regulatory, digestive, respiratory and pulmonary systems found within all animals into the structures they inhabit. These structures respond and adapt to constantly changing internal conditions and external weather influences, to maintain an equilibrium in which the colony (which consists of both the termites and the symbiotic fungi essential to the colony’s health) can flourish.

The termites in a colony do not reside in the mound. The mound, rather, is a physiological infrastructure, built by the nearly one million worker termites residing in the subterranean nest, which contains the hive, nurseries, the royal chamber and the fungus gardens (Bonabeau et al, 2001). The mound is permeated by an extensive network of tunnels, which are differentiable into at least three distinct types:

1. The central chimney, which forms a large, vertically-oriented void above the nest. The chimney is not open to the outside, but is capped by a porous layer of soil.

2. The surface conduits, narrow channels approximately 20-30 mm below the mound’s external surface, and which run vertically along the complete height of the mound.

3. The lateral connectives, a highly reticulated network of tunnels which connect the chimney and the surface conduits.

Additionally, many termite species, including Macrotermes michaelseni, excavate an extensive underground space, the cellar, whereupon the excavated soil is transported upwards into the mound. When the cellar is present, its air spaces are continuous with those air spaces of the rest of the mound. The cellar may serve also in respiration, and is the site of some remarkable structures. For example, the cellar’s ceiling is actually a large base-plate on which the nest is built and which is supported by a solid pillar. As described by Collins (1979), the ‘underside of the base-plate bears a series of clay vanes, encircling the plate in a series of spirals. Three or four complete turns of the spiral are common before a break occurs and a new spiral begins. The vane is stalactitic in cross-section, up to 25mm thick at its attachment, 1mm thick and very fragile at the irregularly wavy edge’. These vanes are coated with a white layer of mineral salts, and are presumed by some to promote cooling of the nest (Bristow & Holt, 1987).

The mound can be an adaptive structure because it is being continually modified by the colony’s inhabitants. At the simplest, individual termites act as conveyors of soil from deep strata, upward onto the mound surface. These movements of soil can be massive, by some estimates about a cubic meter annually per hectare. These en masse movements of soil are directed partly by interactions between the myriad termites (and so are self-organised), and partly by large-scale concentration fields of respiratory gases within the mound. Most of the mound’s architectural features, for example, can be explained by a simple model in which termites’ self-organised soil transport is shaped by ‘gaseous templates’ laid down by metabolism-generated gradients in carbon-dioxide concentration within the mound. Homeostasis of the nest atmosphere then emerges from a simple “tuning” of soil transport to local variation of respiratory gas concentrations.

Thus, Macrotermes colonies provide a natural “model” system which encompasses many of the desiderata for structures that house and provide comfortable environments for people, whether for domiciles or workplaces:

1. They are built by simple and repetitive construction methods from locally available materials, namely assemblages of termites performing local transport and directed translocation of soil. 

2. They require little energy to build and maintain. The construction of a typical Macrotermes mound involves translocating 1200-1800 kg of soil per year upwards by about 5-10 meters. The work involved amounts to, at most, about 10% of the colony’s annual expenditures of energy.

3. They use readily available, natural and renewable sources of energy, namely kinetic energy in wind, to perform a vital function, ventilation.

4. They are compact, and largely self-contained, requiring only inputs of energy in the form of food, sunlight and wind, and without need for export of large quantities of waste. Most wastes are either incorporated into the structure itself, or are fed into an extended system of waste-processing which both extracts energy to support colony function, and culminates in the production of gaseous products which are vented by the same system which handles ventilation.

5. Their function is adapted to the dissipation of energy resulting from the inhabitants’ everyday activities.