Animal constructions such as termite mounds have received scrutiny by architects, structural engineers, soil scientists and behavioural ecologists but their basic building blocks remain uncharacterized and the criteria used for material selection unexplored. By conducting in-situ studies and controlled experiments on termite Odontotermes obesus, we characterize the building blocks of termite mounds and determine the key elements defining material choice and usage by these accomplished engineers. We have studied material selection, transport and assemblage during mound construction.
Termites were observed during repairs of experimentally induced breaches in mounds and were offered soil and glass beads of varying particle sizes in the lab which they used to fabricate spherical unitary structures called boluses. Boluses are analogus to bricks in human construction. These boluses were collected and their volume measured. Termites were also offered 24 different materials ranging from granular materials, fibrous and polymers in the presence of extraneous moisture, and their latency to bolus making (T) and the rate of bolus making were taken as measures of the ‘ease of handling’ of materials. The packing of boluses during mound construction was studied by video recording the process of breach repair and identification of boluses deposited by different castes.
Results/Conclusions
Only major and minor worker castes of termites were found to make boluses; not soldiers. Within a caste, bolus volume increased with increase in particle size while across caste, major worker boluses were significantly larger than minor worker boluses. Termites used 21 out of 24 materials offered to them. Granular, hydrophilic, osmotically inactive, non-hygroscopic materials with surface roughness, rigidity and containing organic matter were the easiest to handle and determine the material systems termites can employ for making boluses. Boluses deposited by both castes during breach repair were interspersed imparting tight packing and high strength to the mounds.
We conclude that bolus making is innate to the worker castes of termites, with clear caste-based differences and a strong preference for granular materials. The aforesaid material properties apart from favourable climatic conditions and availability of water are crucial determinants of mound construction. The soil stratum of any geographic region which satisfies these conditions is likely conducive for mound construction by termites. Our findings can therefore also help predict the geographical distribution of mound-building termites at local and regional scales, a matter of considerable interest, especially since termites are known to modulate soil–water ecosystem processes and thereby climatic effects.