Termite mounds harness diurnal temperature oscillations for ventilation

Significance Termite mounds are meter-sized structures built by millimeter-sized insects. These structures provide climate-controlled microhabitats that buffer the organisms from strong environmental fluctuations and allow them to exchange energy, information, and matter with the outside world. By directly measuring the flow inside a mound, we show that diurnal ambient temperature oscillations drive cyclic flows that flush out CO2 from the nest and ventilate the mound. This swarm-built architecture demonstrates how work can be derived from the fluctuations of an intensive environmental parameter, and might serve as an inspiration and model for the design of passive, sustainable human architecture. Many species of millimetric fungus-harvesting termites collectively build uninhabited, massive mound structures enclosing a network of broad tunnels that protrude from the ground meters above their subterranean nests. It is widely accepted that the purpose of these mounds is to give the colony a controlled microclimate in which to raise fungus and brood by managing heat, humidity, and respiratory gas exchange. Although different hypotheses such as steady and fluctuating external wind and internal metabolic heating have been proposed for ventilating the mound, the absence of direct in situ measurement of internal air flows has precluded a definitive mechanism for this critical physiological function. By measuring diurnal variations in flow through the surface conduits of the mounds of the species Odontotermes obesus, we show that a simple combination of geometry, heterogeneous thermal mass, and porosity allows the mounds to use diurnal ambient temperature oscillations for ventilation. In particular, the thin outer flutelike conduits heat up rapidly during the day relative to the deeper chimneys, pushing air up the flutes and down the chimney in a closed convection cell, with the converse situation at night. These cyclic flows in the mound flush out CO2 from the nest and ventilate the colony, in an unusual example of deriving useful work from thermal oscillations.