- Termite mounds are incredibly strong earthen structures that are also surprisingly porous. This porosity is essential for ventilation of the termites’ underground nests without affecting the nest’s temperature and humidity levels.
- The secret of this balance of solid strength with porosity lies in a two-layered architecture. A dense core provides strength and stability, while a porous shell surrounding the core allows ventilation.
- Termite saliva does not contribute to mound strength but does render the mound weatherproof by increasing the mound’s resistance to erosion by rain.
Of the insect world’s many architects, the termites are undoubtedly the reigning monarchs. The earthen mounds that these tiny insects construct out of soil, water and their own saliva, are as intricate, if not more intricate, than human-made castles. The enigmatic structures have intrigued biologists, inspired architects, and even captivated robotics experts into studying and emulating them.
However, despite their fame in academic circles, relatively little was known about the architectural construction of termite mounds. An interdisciplinary project involving the Centre for Ecological Sciences and the Department of Civil Engineering at the Indian Institute of Science (IISc), Bengaluru, has only recently uncovered details of the mounds’ strength and stability, their weather-resistance, and what building materials termites prefer to use in their mounds.
The subjects of these recent studies are the mounds constructed by the termite Odontotermes obesus, found in India. These termites build underground nests that not only house their vast colonies of millions of workers, soldiers, and larvae, along with the kings and queens, but also the colony’s fungal gardens.
Maintaining a fairly constant temperature and high humidity levels within the nest is crucial for the proper growth of the fungus, which is the only food these termites eat. To ensure ideal living conditions for the fungus, as well as adequate ventilation within the nest, O. obesus build mounds above their nests.
These mounds, which can often be up to 6 feet in height, are incredibly strong and long-lasting structures. Despite their strength and stability, the mounds are also porous enough to ventilate the termites’ underground nests without affecting the nest’s temperature and humidity levels.
Biopsy of a termite mound
To uncover the termites’ architectural secrets, researchers have filled termite mounds with plaster, pumped them with propane before scanning them with lasers, and even used a saw to slice them into sections.
“Previously, when people wanted to study the core of a termite mound, they simply slashed through the mound and destroyed it just to obtain a sample,” said Nikita Zachariah, who, as a Ph.D. student at IISc, investigated the architectural properties of termite mounds. Zachariah thought this was fine for abandoned mounds but was loath to destroy any mounds over an occupied nest. “I designed and built my own drilling machine so that we could get samples of mound walls without destroying them,” she says proudly.
Zachariah’s drill allowed her to obtain small samples of termite mound walls from different parts of abandoned and occupied mounds. These samples were then placed in a micro universal testing machine, which resembles a nutcracker with a screw mechanism. By gradually increasing the pressure on a sample until it cracked, the strengths of different termite mound walls were calculated.
The results showed that the mounds’ core walls were 35–40% stronger than the mounds’ peripheral walls. Furthermore, CT scans and air permeability experiments revealed that the mounds’ peripheral walls are more porous than the core.
Together, these experiments indicate that termite mounds are bilayered constructs with dense, strong cores surrounded by porous shells.
“The conundrum of how termites can achieve two contradictory objectives – one, to achieve enough density to impart strength to the mound, and the two, to achieve enough porosity to ventilate the mound – has been solved. By building bilayered structures, the termites get the best of both worlds,” said Zachariah.
How stable are the termite mounds?
“We know that termite mounds are strong, but how mechanically safe is the mound structure? How likely are termite mounds to collapse under their own weights?” were the questions Tejas Murthy asked. Murthy is a collaborator on this project from the Department of Civil Engineering at IISc.
Using the strength and density measurements obtained from the mound samples, Saurabh Singh, a student at Murthy’s laboratory, carried out stability analyses. Singh modelled the mound as either a triangle or a trapezoid for these analyses. “These two geometries are extreme cases of what a termite mound’s shape can be; in reality, the mounds’ shapes are somewhere in between,” he said.
The stability analyses reveal that termite mounds are extremely stable, with safety factors of roughly 100 for the triangular shape and 50 for the trapezoidal shape.
“Usually, human constructions are built with safety factors between 1 and 2, or possibly 3. This means that such buildings will be stable under pressures 1 to 2, or 3 times their own weights; that they can withstand such pressures without collapsing,” said Murthy. “If a human building were to have a safety factor similar to that of a termite mound, I’d call it over-engineered! But our analysis only considers the effects of gravity, and it is possible that the high safety factor of these mounds may be required to withstand natural disturbances that we know nothing about,” he added.
The “bricks” in a termite mound
Most termite mounds are so hard that only a drill or hammer wielded with great force can break them. Some termite mounds are so resistant to weathering that they can survive intact for hundreds or even thousands of years.
These facts are even more surprising when you consider that this strength and weathering resistance comes from a biocement made of just soil, water, and termite saliva!
When termites build or repair their mounds, they produce their biocement as spherical bricks called ‘boluses.’
“Bolus-making behavior seems to be hard-wired in termites. Even if you leave termites in a petri dish with some filter paper and water, they will make boluses and deposit them at the edges of the dish,” said Renee Borges, Zachariah’s mentor and a professor at the Centre for Ecological Sciences, IISc. “Termites can make boluses out of just about any material – soil, agar, glass beads, paper, even metal powders!” she added.
But when given a range of substances, these insect architects can be quite choosy about what they use to make boluses, preferring building materials that contain organic matter and are granular, rough, and wettable. In other words, their favorite building material is usually just soil.
Pascal Jouquet, a soil ecologist from the Institute of Ecology and Environmental Sciences at Sorbonne Universités in France, describes the choice experiments with building materials as very novel. “Termite ecology and behaviour are often considered from a biological point of view, (for example, looking at the interactions between individuals or the role of pheromones) but rarely take into account the interactions between their [termites’] building strategies and the physical properties of the material they use to build their nest/construction,” he said.
While praising the multidisciplinary approach of this work, Jouquet added, “my one criticism is, perhaps, that all these experiments were either carried out in controlled laboratory conditions or on termite mounds within the IISc campus. It’s a pity that natural complexity (like different soil types) was not taken into account.”
The power of biocement: is it just soil and water or termite fevicol?
The termites’ biocement is made of an almost semi-liquid mixture of saliva-fortified soil and water (with a soil:water ratio of roughly 70:30) that is just plastic enough to be molded.
“In some of our experiments, where we gave termites glass beads as construction material, the only glue that held the beads together was the termite saliva,” said Zachariah. This observation got Zachariah, Borges, and Murthy thinking about the role of termite saliva in the biocement. Was the saliva a ‘termite fevicol’ that bound soil particles together, or did it have some other function?
To test their hypotheses, Zachariah recreated the termites’ soil–water mixture without termite saliva in the laboratory, dried it to form a block, and tested the block’s strength.
To the researchers’ surprise, the laboratory-made block was as strong as the termite mound wall samples!
It turns out that as the soil–water mixture (in the specific ratio that termites use) dries, the soil particles begin to settle under their own weights. As the soil particles settle, smaller sized particles fill up the spaces between larger particles to form densely packed consolidates. The consolidates become even stronger as the water between the soil particles dries and a cohesive force known as ‘soil suction’ builds up till the structure becomes as strong as the walls of a termite mound.
“It was a eureka moment for us, finding out that termites could make such strong structures with just the right ratios of soil and water,” said Zachariah.
Further experiments involving repeated cycles of wetting and drying revealed why termites use their saliva when building mounds. The saliva seems to protect the mound walls from weathering and erosion.
“Using a secretion like saliva as a strength-imparting cementing agent can use up a lot of energy, especially given the sizes of termite mounds,” said Borges and Murthy. “Our work suggests that soil moisture at the time of mound construction is enough to make the mound strong after drying. It’s a really efficient system from an energy point of view,” they added.
Banner image: Termites inside a termite mound. Photo by Nikhil More.