At first glance, a towering termite mound might look like a bustling insect skyscraper, but the insect megalopolis that is home to hundreds of thousands of termites is buried in the ground beneath. For decades, termite mounds had been thought to regulate the climate of their colony; however, Scott Turner, from the State University of New York (SUNY), Syracuse, USA, had suggested that they worked like a lung driven by the wind. Inspired by this, Hunter King, from Harvard University, USA, Samuel Ocko, from the Massachusetts Institute of Technology, USA, and L. Mahadevan, also from Harvard, set out to investigate the airflows through Odontotermes obesus termite mounds in India, and discovered that the structure built by this species does function like a lung. However, the flow of air through the mound is driven by the sun moving across the surface of the mound, generating temperature gradients that drive air currents inside the structure, which, in turn, allow gases to move across the mound walls, removing carbon dioxide from the air and bringing in oxygen. Could termites that live in other environments benefit from a similar ventilation mechanism?
Together with collaborators Turner, Paul Bardunias (SUNY), Rupert Soar (Nottingham Trent University, UK) and David Andreen (Lund University, Sweden), Mahadevan, King and Ocko travelled to Otjiwarongo, Namibia, to measure the temperature gradients through Macrotermes michaelseni termite mounds and the air currents flowing through them. Ocko recalls that it did not take the soldier and worker termites long to attack the newly installed airflow sensors, smothering the probes with wet soil. ‘The field work turned into a real hit-and-run game against the termite colonies’, he laughs. In addition to visiting ∼30 mounds to collect over 120 internal airflow measurements at different times of day, while also measuring the external wind speed and direction, the team implanted iButton temperature loggers around the circumference of one mound to build an understanding of how the thermal gradients within the structure varied through the day.
Reconstructing the airflows through the Namibian mounds, the team realised that the circulation reversed twice a day, with air in the core rising to the top of the mound before descending through the flank channels during the night, with the flow switching in the opposite direction sometime before mid-day. And when they analysed the temperature gradients throughout the mound, they realised that the warm zone on the surface that was heated by the sun moved over the mound from dawn to dusk, generating shifting temperature gradients to drive the air currents. Instead of being driven by external breezes, the currents in the M. michaelseni mounds are solar powered, just like the Indian termite mounds.
The team also measured the carbon dioxide levels at two heights in one of the mounds and in the nest below, and found that the carbon dioxide concentrations within the nest and the mound remained consistently high at 5%, which was different from the Indian mounds, where the gas concentrations ebbed and flowed. ‘If the bulk flow slowed down, as one might expect at some points in the day, the nest would then stagnate and we'd see an elevated carbon dioxide level relative to the mound’, says King. And when the team measured the porosity of the mound walls, the structure was leaky enough to keep the nest well ventilated.
‘Termite mounds are like an inverted lung’, says King, who hopes that we can learn from the termites’ abilities to harness the power of the sun to keep their nests well ventilated. He says, ‘This passive termite solution will hopefully inspire human engineering and architecture, especially in coming up with less energy-costly replacements for conventional heating, ventilation and air-conditioning systems’.