Termites build mounds by transporting soil from deep within the ground to the surface. This outward soil conveyor is prodigious: the dark patches on the photograph at the left represent a single night's deposition of new soil onto the mound surface. By some estimates, a single Macrotermes colony can move as much as a cubic meter of soil annually up into the mound, making these termites major agents of soil turnover and soil modification in arid savannas.
This soil conveyor has been harnessed to social homeostasis of the nest atmosphere, by co-opting mechanisms for repair and regeneration of the mound. The mound architecture is dynamic, changing through time as termites transport soil up into the mound and erosion removes soil and spreads it onto the surface. As mound architecture varies, so too does its ability to capture wind for ventilation.
By coupling soil transport to the mound's physiological function as a wind-driven ventilator, the regulation of nest atmosphere emerges. In a profound sense, social homeostasis is really structural homeostasis of the mound.

J S Turner. 2000. Architecture and morphogenesis in the mound of Macrotermes michaelseni (Sjöstedt) (Isoptera: Termitidae, Macrotermitinae) in northern Namibia. Cimbebasia 16: 143-175.

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Soil transport and mound repair

Damage to a mound elicits a repair process that unfolds over the course of a day or so. Within about five minutes, small scout parties of workers appear at the breach to begin repairs. In the figure at the left, a few termites have begun daubing little grains of soil onto the exposed tunnel surface, gluing it into place with a sticky secretion from their salivary glands.

Contained within these daubs of soil is a pheromone that strongly attracts other termites to also lay down daubs of soil. Thus, the presence of one daub soon results in the deposition of other daubs on top of it. This phenomenon is known as stigmergy (literally, 'driven by the mark').

As worker termites are attracted to the site of the breach, they are attracted to pheromone plumes emanating from the freshly deposited soil, and are most strongly attracted to sites where the daubs are the densest. Consequently, the building takes on the form of pillars and walls that eventually close off the tunnels leading into the breach. Click here for a video clip of stigmergic building in action. (warning: 10 MB)

This repair process is not limited to the area of the breach itself, but actually is widely distributed throughout the mound. It is also a positive-feedback driven process. More repair work elicits more attractive pheromone, which elicits still further repair, and so on. Consequently, repair continues long after the immediate breach is sealed

The ongoing repair eventually produces a kind of spongy tunnel network. At the left is an end view of a 110 mm PVC pipe that was inserted into a hole that had been drilled into a mound. The termites engaged in repair activity in the pipe as they would have in the mound. The pipe was removed after about a day. The extensive deposition of soil and the complicated tunnel network is evident. The spongy tunnel network is also evident in the plaster cast depicted to the right.

Repair of the mound therefore elicits a prodigious burst of soil transport into the mound. Termites repairing even a small breach will transport several kilograms of soil into the mound. Top

Mound repair and gas exchange

Mound repair mechanisms are readily co-opted into regulating nest atmosphere. Mound architecture and nest atmosphere are coupled by the mound's capture of wind energy. Any perturbation of mound architecture will alter how the mound captures wind energy, which will perturb nest atmosphere. The perturbation will be widespread through the mound and nest.

If the atmospheric perturbation elicits recruitment of workers for repair, any repair work that is done will alter how the mound interacts with wind, feeding back ultimately to alter nest atmosphere.

Thus, repair, nest environment and soil transport for nest repair are coupled into a loop. Social homeostasis is a fairly simple outcome of adjusting the responsiveness of the various steps in the loop.

This is illustrated in the more detailed flow diagram to the right. Nest atmosphere can be perturbed either by damage to the mound, or by colony metabolism. Both can elicit recruitment of workers for repair work, which could involve either excavation of deposition of new surface. This repair work then feeds back onto nest atmosphere, until the perturbation is ameliorated, easing recruitment.

Any mismatch in ventilation and metabolism then elicits a structural modification of the mound, which restores the two fluxes to balance. Social homeostasis!