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.