“Classic” model of metabolism-driven circulation within a Macrotermes mound.

The mound is a respiratory organ for the nest, which has a collective metabolic rate in the range of 50-210 watts. The colony’s metabolism is supported by a respiration rate roughly equivalent to that of a goat (at the lower end) or to a cow (at the upper end). For many years, nest ventilation was thought to be driven solely by the colony’s production of metabolic heat. In this conception, heat and water vapour generated by the nest’s metabolism imparts buoyancy to the nest air, lofting it into the chimney. This force then purportedly drives spent nest air downward through the surface conduits, wherein there is an exchange of heat and water vapour with the atmosphere. The refreshed air then sinks down into the cellar, positioned to begin another circuit through the mound.

We now know this classical model of nest ventilation is, at best, incomplete. Rather than a circulatory flow, the nest is tidally ventilated, driven by dynamic pressures generated by the chaotic fluctuations of wind speed and direction which are common in the turbulent outdoor environment. As in the tidally-ventilated lung, respiratory gases are exchanged in three phases, each corresponding to a different locale within the mound. Exchange in the surface conduits and chimney cap occurs in a forced convection regime, with flows there strongly driven by wind, analogous to air-flows in the upper bronchi. Exchange in the lower part of the chimney, meanwhile, occurs largely in a natural convection regime.  Flows in this region are driven by density variations imparted to the air by nest metabolism: this is roughly analogous to the diffusion-mediated exchanges in the lung’s alveolus. Gas exchange in the lateral connectives and middle chimney occurs in a mixed forced/natural convection regime.

Updated model of wind-driven ventilation, showing zones corresponding to three phase gas exchange mode respiratory gas exchange. After Turner 2001.

Thus, the Macrotermes mound does not isolate the colony from its natural environment, as the old conceptions of nest ventilation implied. Rather, the mound intimately ties the colony and environment together, integrating and combining energy from two sources (i.e. metabolism-induced buoyancy and wind-driven pressure) to effect a colony-level function (i.e. ventilation).

Furthermore, the mound is an adaptive interface between these multiple sources of energy for ventilation. Colony function requires not simply ventilation, but regulated ventilation: homeostasis, in a word. As in the homeostasis of the body (a term coined by the American physiologist Walter Bradford Cannon), the overall health of the colony requires levels of oxygen, carbon dioxide and water vapour concentrations to be regulated within narrow limits. This the colony does by matching the rate of wind-induced ventilation exchange to the colony’s rate of respiratory gas exchange, brought about through adaptive modification of mound architecture. Thus, an increase in colony respiration which comes from growth, for example, is matched by an upward extension of the mound into stiffer winds that ventilate the mound more vigorously. The Macrotermes mound, in other words, is a quintessential example of an adaptive, or “smart” structure, one whose function can adapt to varying demands made upon it.