`Magnetic' termite mounds: is their unique shape an adaptation to facilitate gas exchange and improve food storage?

Abstract

Social insects can build impressive nest mounds but the functional significance of their architecture is rarely studied in experiments. The `magnetic' termite mounds of monsoonal northern Australia built by Amitermes meridionalis are notable for their elongated wedge shape and north-south axial orientation. We tested whether the shape is an adaptation to facilitate gas exchange and the preservation of food stores by two experimental manipulations of mounds in situ covering all seasons. First, mounds were shaded to limit drying after rain and second, mound shape was amended from wedge to (approximate) sphere. Food storage, fungal contamination, and internal CO2 concentration were unaffected by manipulation, but showed a distinct seasonal dynamic, with storage peaking towards the onset of rains and fungal load towards the end of the rainy season. Internal CO2 concentrations were subject to a diurnal cycle, but also showed elevation during rains. We propose that one advantage of the wedge shape is the efficient use of building effort to achieve good passive ventilation for the food storage areas.