The structure of above-ground macrotermite mounds facilitates gas exchange in the below-ground nest using internal air currents driven by solar heat.

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Mound-building macrotermites construct vertical mounds out of soil, saliva, and dung, with some mounds in Africa measuring up to several meters high. The mounds generally resemble chimneys, but different species ventilate their mounds in different ways. Some species may create ‘open’ mounds with chimneys or vent holes, while others build ‘closed’ mounds that lack large openings but have porous walls. Inside both of these mounds, worker termites can dig a complex array of tunnels of various sizes. The termites themselves live in nests below ground in colonies that can contain up to a million individuals.

The most recent published research on termite mounds suggests that they function much like mammalian lungs and act as accessory organs for gas exchange in the underground nests. It was previously thought that termite mounds functioned to continuously maintain the nest’s internal temperature within a narrow range in the face of extreme outside temperature fluctuations, but research on mound-building termites like Macrotermes michaelseni, which construct closed mounds, is expanding our understanding of how these mounds function. During the day, changes in internal nest temperature are less extreme than changes in outside temperature, but over the course of a year, nest temperature does vary and closely follows the temperature of the surrounding soil. The soil has a large thermal capacity, meaning it can absorb or lose large amounts of heat energy before experiencing any changes in temperature. In a way, the soil around the termite nest acts as a “buffer” against daily changes in outside temperature.

Researchers are actively studying mounds to understand precisely how mound structure facilitates gas exchange in the underground colony. It appears that the main mechanism is through internal air currents driven by solar heat. As outside temperatures change throughout the day and the sun strikes different surfaces on the mound, temperature gradients develop between the mound periphery and center. These temperature gradients create currents of rising and falling air inside the mound. The direction of these currents varies as temperature gradients change throughout the day. Wind energy from unsteady airflows outside the mound may also play a secondary role in ventilation. The internal airflows llikely promote mixing between air in the mound and air in the nest, ultimately facilitating gas exchange in the nest. This growing understanding of macrotermite mound structure and function could inspire new biomimetic technologies in energy-saving climate control systems.

For more detailed information on how a termite mound can function like a lung, check out this video.

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“Recent experimental evidence in the mounds of a single species, the south Asian termite Odontotermes obesus, suggests that the daily oscillations of radiant heating associated with diurnal insolation patterns drive convective flow within them. How general this mechanism is remains unknown. To probe this, we consider the mounds of the African termite Macrotermes michaelseni, which thrives in a very different environment. By directly measuring air velocities and temperatures within the mound, we see that the overall mechanisms and patterns involved are similar to that in the south Asian species.” (Ocko et al. 2017:3260)

Journal article
Solar-powered ventilation of African termite moundsJournal of Experimental Biology, 220: 3260-3269September 15, 2017
Ocko SA, King H, Andreen D, Bardunias P, Turner JS, Soar R, and Mahadevan L

“As it is in lungs, the colony’s respiratory function is dominated by a mixed-phase regime that is sandwiched between the subterranean structures (where natural convection dominates), and the upper parts and peripheral air spaces of the mound (where wind-driven forced convection dominates). By our best estimates, this mixed natural/forced convection regime occupies the lower parts of the chimney and the deeper parts of the mound reticulum [15].” (Turner and Soar 2008:222)

“In most building designs, walls are erected as barriers to isolate spaces: internal spaces from the outside world,internal spaces from one another and so forth. Yet spaces, if they are to be occupied and used, cannot be isolated. Resolving this paradox is what forces building designs to include infrastructure—windows, fans, ducts, air conditioning, heating etc—all essentially to undo what the erection of the walls did in the first place. In short, the paradox forces building design toward what we call the “building-as-machine” paradigm (BAM).

Living systems, which also are avid space-creators, resolve the paradox in a different way: by erecting walls that are not barriers but adaptive interfaces, where fluxes of matter and energy across the wall are not blocked but are managed by the wall itself [28, 29]. This is illustrated dramatically in the complex architecture of the interface that termites build—the mound—to manage the environment in their collectively constructed space—the nest [30].” (Turner and Soar 2008:225)

Journal article
Beyond biomimicry: What termites can tell us about realizing the living building.First International Conference on Industrialized, Intelligent Construction (I3CON)May 14, 2008
Turner JS, Soar RC

Journal article
On the Mound of Macrotermes michaelseni as an Organ of Respiratory Gas ExchangePhysiological and Biochemical ZoologyJuly 26, 2002
J. Scott Turner

Web page
Extended Physiology of an Insect-Built StructureAmerican EntomologistJ. Scott Turner

Journal article
Architecture and morphogenesis in the mound of Macrotermes michaelseni (Sjöstedt) (Isoptera: Termitidae, Macrotermitinae) in northern NamibiaCimbebasiaAugust 1, 2000
Turner JS

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