Hunter King, a postdoctoral fellow in applied mathematics, and Samuel A. Ocko, an associate in applied mathematics and a graduate student in the MIT physics department, discovered the key to ventilation in the mounds of an Indian termite species.
Their research, which was conducted under the supervision of Applied Mathematics professor Lakshminarayan Mahadevan, revealed that the termites’ mounds drive a convection current, which helps the release of stale air from the underground nest to the surface of the mound.
The visible portion of the termite mound sits above the termites’ underground nest and is made up of a network of peripheral buttresses—called flutes—surrounding a central cavity, called the chimney. Tubes facilitating airflow connect the central flutes to the peripheral chimney.
The difference in the rate of heat absorption between the chimney’s tubes and the peripheral flutes is crucial to the functioning of the convection current, Ocko explained.
“During the daytime, the peripheral flutes heat up quickly as their thin skin of clay makes them more exposed to the ambient temperature, while the less exposed chimney stays cool as it’s got a much larger thermal mass,” he said. “During the nighttime the buttresses are cooler than the center because they cool down faster, making the center warmer.”
According to King, the temperature gradient causes a convection current through the loop of tubes.
“The colder air sinks and the hotter air rises, leading to a circular movement of air through the mound both during the day and during the night that allows the gas from below to become released from the mound,” he said.
He added that the discovery debunked two previously formed hypotheses about termite mound ventilation, which pointed to external wind-powered changes in pressure and internal heating in the nest, respectively, as the mechanisms displacing the stale air.
“To test these hypotheses, we placed small probes inside the tubes, which measured the velocity of the air at three centimeters per second and also contained sensors to measure carbon dioxide concentrations,” King said. “The twice-daily shift in velocity and concentrations helped support the idea behind the convection current.”
As he sees it, the biophysical air-flushing mechanism solved one of the most puzzling mysteries about termites.
“We now know that termites build such a massive, empty structure above their underground nest in order to control their local environment,” King said.
Mahadevan explained that the termite mounds exemplify the interdependence of physical and biological systems.
“The organisms live in the environment and change the environment, but it is the environment which in turn changes the organism’s behavior,” he said.
According to Ocko, however, the impact of the discovery could extend beyond physics and biology.
“Now that we know what termite mounds are doing, engineers and architects are thinking of applying these principles of passive climate control to create energy-efficient buildings,” he said.