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From bits to boluses: How do termites build strong mounds?

Termites, Architecture, Construction, Boluses, Materials, IISc

It is hardly news that nature has created some of the world’s best architects and builders. We marvel at the bees efficiently using space with the hexagonal building units of their hives, the intricacy with which a weaver bird builds its nest by weaving together grass, and the beavers strategically building dams and manipulating waterways. Now, a recent study from the Indian Institute of Science, Bengaluru, is all set to add another spectacular ‘architect’ to this list —termites. The study details how Odontotermes obesus, a species of termites, builds its colossal mounds using mud.

Termites are eusocial insects, that live in colonies characterised by cooperative brood care, different generations of adults living at the same time, and strict division of labour between various classes of members of the family. A termite colony consists of a fertile ‘king’ and ‘queen’ that are responsible for reproduction. The colony has ‘soldiers’ with specialised sabre-like mouthparts, responsible for protection of the colony. Most of the labour in the colony, including building mounds, is carried out by the ‘workers’ that are divided into two types based on their head sizes.

In this study Ms. Nikita Zachariah and her team based in the lab of Renee Borges at IISc focused on how these ‘workers’ make ‘boluses’ or  building blocks of the colony, much like the bricks in our buildings, or the stone slabs of the pyramids. Though researchers have studied aspects of the architecture, collective building behaviour and regulation of the internal environment of termite nests, not much is known about the building blocks of these structures.

“This study was part of a larger project aimed at understanding the physical, chemical and behavioural aspects of mound construction by termites. In the first part of this study, we had set out to explore the behavioural aspect of mound construction by characterising the building blocks of construction and understanding the criteria used for material selection. We wanted to draw parallels between termite and human construction by understanding three stages of termite mound construction — material selection, transport and assemblage”, says Ms. Zachariah.

Through their work, the researchers have studied how the two different types of workers make boluses to build mounds. The study reveals interesting aspects on how these workers made the boluses using their saliva as a bio-cement and sheds light on the key factors the termites depended on, while selecting a material to make boluses.

“Currently we are studying the strength characteristics of termite mounds and its chemical basis. We all have seen termite mounds standing in rain and heat for many decades and not dissolving even though they are made up of soil. Previous work from Prof. Tejas Murthy's (Associate Professor at the Department of Civil Engineering, IISc) lab has shown that termite manipulation of soil imparts a ten-fold increase to its strength. This happens without termites treating their boluses at high temperatures (unlike bricks made by humans that are baked at very high temperatures in a kiln for imparting high strength). We are trying to find out what is the cementing material in termite mounds that leads to such high strength”, adds Ms. Zachariah.

To study what materials the termites preferred to build their mounds, the researchers allowed the worker termites to choose from an array of materials found in natural environments, such as red soil, as well as materials that were new to the insects, such as glass beads, copper and brass, and materials that had different chemical properties such as agar and salt.

Through analyses of termite building behaviour, the researchers found that the termites were hardwired to make boluses. The insects did so irrespective of the location, the purpose for which the bolus was made and the shape of the particle provided. The team also found that the workers with larger heads made bigger boluses than those workers with smaller heads. The size of the bolus increased with the size of the particle provided, but only up to a point. With glass beads both castes could carry more than twice their respective body weights. While workers with larger heads could carry boluses up to twice their head volumes, it was surprising that the ones with smaller heads were far more efficient and carried boluses as large as five times their head volume.

Granular materials such as soil and minerals were transported the fastest, and used to the greatest extent by the workers. Moisture played a crucial role in material handling: give termites something mushy (like very soft agar) and they can’t walk on it; give them something hydrophobic (like wax) and they can’t mix it with their saliva; give them something osmotically active (like table salt) and they die. Thus, the right amount of moisture is necessary for bolus making. Remove the organic material from red soil, and the boluses are too fragile and collapse. The study goes further and shows that frictional forces too are important for the selection of materials by the workers. The termites did not use steel balls until the surface roughness was increased.

And the method of construction was found to be incredible too! Larger boluses were first assembled when the mound was damaged and the smaller boluses were used to fill the gaps between these, hence efficiently packing the break. But why so? Ms. Zachariah is quick to explain with an example — “imagine you have an empty glass jar. You can fill it with golf balls, but you are still left with some spaces/voids between the balls. You can fill these voids with some smaller spheres like marbles. This will help you achieve tight packing in the glass jar. This is similar to what we see during termite construction. Termites make boluses of two different sizes and pack them such that the large and small boluses are interspersed. This achieves high density of boluses in a mound wall.”

The study provides the first insight into the complexities involved in material selection by termites for building their mud castles. It also lays down some important aspects on what material and conditions are ideal for termites to make mounds, thus helping ecologists in predicting the bio-geographical distribution of termites at a local or regional level.