Extended Physiology of an Insect-Built Structure

Abstract

The savannas of southern Africa are dotted with the spectacular mounds of the fungusgrowing termites of the genus Macrotermes (Termitidae: Macrotermitinae). These mounds can reach several meters high and represent a colossal engineering project for the termites that build them (Fig. 1). The mound is a respiratory device, built to capture wind energy to ventilate the subterranean nest. The need for ventilation is acute. A typical Macrotermes nest contains roughly a million workers and a substantially larger biomass of the fungi they cultivate. Collectively, these organisms consume oxygen at rates similar to that of large mammals. By various estimates, a single Macrotermes colony is the metabolic equivalent of a goat or a cow. Macrotermes mounds are also external organs of homeostasis. The composition of the nest atmosphere is tightly regulated. Typically, oxygen concentrations in the nest are 2% lower than the surrounding air, carbon dioxide concentrations are commensurably higher, and nest humidities are very high. These conditions are maintained throughout the year, and in the face of considerable variation of metabolic demand from the colony. Such stability can only come about if the termites can match ventilation rate with the colony’s respiration rate. They do this by making the mound a “smart” structure, which means that it must also be a dynamic structure. Soil is continually eroded from the mound and is replaced by soil carried by termites out to the mound surface. Roughly a cubic meter of soil moves through the mound each year in this way. The mound’s architecture is therefore shaped by the relative rates and patterns of erosion from, and deposition to, the mound. For the mound to be an organ of homeostasis, these patterns of active soil movement must be coupled to the composition of the nest’s atmosphere. For example, excessive ventilation rates would produce patterns of soil transport that reduce the mound’s capture of wind energy. Insufficient ventilation would elicit soil transport patterns that enhance the capture of wind energy. How this coupling works is the focus of my research, which is underway in the southern African country of Namibia. The work is sponsored by the National Science Foundation and is aided by the National Museum of Namibia and the Namibian Ministry of Agriculture and Rural Development. The latter administers the Omatjenne Research Station near Otjiwarongo, where the work is based.