The architecture of termite mounds: a result of a trade-off between thermoregulation and gas exchange?

Abstract

We examined the influence of gas exchange on the architecture of termite mounds. In Comoe National Park (Cote d’Ivoire), Macrotermes bellicosus builds, as an adaptation to ambient temperature conditions, differently shaped mounds in the shrub savanna and the gallery forest. Previous studies suggested that there might be a constraint that limits the degree of thermal insulation of the interior (i.e., nest) of the mounds in environments with relatively low ambient temperatures. This factor causes, in proximate terms, suboptimal low nest temperatures and ultimately leads to reduced reproductive success in the gallery forest. In this study, we examined whether the necessity for gas exchange might constrain mound architecture. We measured CO2 concentrations in the air channels of mounds in different habitats and under manipulated temperature regimes. During both the dry and the rainy season we found higher CO2 concentrations in mounds of the gallery forest than in mounds of the savanna. Additional measurements in forest mounds, architecturally resembling those of the savanna due to an experimental increase in ambient temperatures, revealed lower CO2 concentrations than unmanipulated mounds in this habitat. Generally, concentrations were higher during the rainy season compared to the dry season and lower during day than during night. Summarizing these results we present a model that illustrates this trade-off between thermoregulation and gas exchange under different temperature regimes. Both factors together result in different mound architectures under different environmental temperatures and may finally limit the distribution of this species.