Original Article: Termite mounds harness diurnal temperature oscillations for ventilation (Non-paywall version here.)
Disclosure: The first author of this paper, Hunter King, is a friend of the present writer (CPK).
Termites are among nature’s most spectacular builders, constructing mounds that can reach heights of several meters. Relative to the size of their bodies, these structures are considerably larger than the tallest skyscrapers constructed by humans [1]. Surprisingly, in many termite species, individual termites don’t spend much time in these mounds. Instead, they live in an underground network of tunnels and chambers that can be home to millions of individual insects. But, if not to live in them, why do termites build such intricate and gigantic above-ground structures [2]?
Scientists have suggested several possibilities: mounds might provide protection from predators, or guard against rain or dramatic changes in temperature. Recent research, however, has focused on the idea that a mound’s main purpose could be to provide ventilation. The problem of ventilation is particularly important for species such as Odontotermes obesus, native to the Indian subcontinent, that “farm” a species of fungus [3]. As human cultivators will no doubt be aware, indoor farming requires careful control of atmospheric conditions. According to this picture, the mound functions like a giant lung, enabling the colony to expel carbon dioxide and exchange it for atmospheric oxygen. But how exactly might this lung work?
Human lungs use a muscle, the diaphragm, to mechanically push out old (carbon-dioxide-rich) air, and suck in fresh (oxygen-rich) air. Obviously, termite mounds don’t have moving parts that would allow them to do this. So what is the physical mechanism that drives gas to flow around the ventilation shafts inside the mound? Over the years, researchers have proposed several ideas, including driving by thermal buoyancy (the tendency of hot air to rise upwards) or external wind. The details of these models are controversial: for instance, thermal-buoyancy-driven flows require temperature differences between different parts of the mound. Are these temperature gradients caused by external heating (that is, from the sun), or by heat generated by the bodies of the termites themselves [4]?
In today’s paper, Hunter King, Samuel Ocko and Lakshminarayanan Mahadevan describe a series of experiments that might help to answer some of these questions. To test the “mound-as-lung” model described above, King and co-workers designed and built directional airflow sensors tailored to the cramped environment and low airspeeds found in the ventilation shafts of mounds built by O. obesus. The mounds, shown in Fig 1A, look a bit like a half-folded umbrella, with ripple-like “flutes” decorating a roughly cone-shaped structure.
King and co-workers measure, as a function of time of day, the air flow velocity in the ventilation conduits near the base of the flutes. These measurements, as the authors put it, are “difficult for several reasons,” in particular the “hostile and dynamic” environment inside the mound — the tendency of termites to aggressively attack anything placed inside their nest, and cover it with “sticky construction material.” As well as measuring the air velocity, King and co-workers use temperature sensors to measure the temperature profile of the surface of the mound, and the carbon dioxide concentration in the nest, underneath the mound, and at the “chimney,” near the top of it. To test the role of heat generated by the bodies of the termites, the researchers also study a “dead” — that is, abandoned — mound.
The results of some of these experiments are shown above. In particular, King and co-workers observe similar flow and temperature patterns in the “living” and “dead” mounds and conclude that metabolic heating is not the central mechanism driving ventilation. Noting that the direction of the flow reverses during the night, King concludes that “diurnally driven temperature gradients” — that is, temperature differences caused by the day/night cycle — ventilate the nest. This process is facilitated by the most distinctive architectural feature of the mound, the flutes.
Like fins on a radiator, the flutes efficiently exchange heat with their environment. In the heat of the sun, the flutes heat up faster than the interior, as shown in the IR camera image above. This causes the air in the flutes to rise, thus creating circulation inside the mound. The resulting flow carries oxygen-rich air from the chimney down to the nest. During the night, the flutes cool down faster than the interior, causing the flow pattern to reverse. According to the model that King and his colleagues propose, the termite mound performs the unusual feat of extracting useful work from oscillations in an intensive (in the sense of thermodynamics) environmental parameter.
King and his co-workers speculate that this energy-efficient ventilation strategy, which has evolved over millions of years, might provide inspiration for human designers of environmentally friendly architecture.
Note: After this post was written (but before it was published), the same team published a second paper where they try to find out if the same model applies to mound built by another species of termite that lives on a different continent (spoiler: it does, but some of the details differ).
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Notes:
[1]
[2] http://www.bbc.com/earth/story/20151210-why-termites-build-such-enormous-skyscrapers
[3] The termites bring partially digested wood back to their nest, where the fungus extracts nutrients and energy from it. In return, the fungus produces fruiting bodies that the termites can eat. The relationship between termite and fungus can be referred to as “farming” or “symbiosis,” depending on your point of view.
[4] The latter mechanism is how honey-bees maintain a constant hive temperature. This ability to preserve “hive homeostasis” is one of the reasons that honeybees can survive in wildly varying climates.
