A termite mound. Photo: David Davies / Wikimedia Commons, CC BY-SA
Of the many architects of the insect world, the termites are undoubtedly the ruling monarchs. The earthen mounds that these tiny insects build from earth, water, and their own saliva are just as intricate, if not more intricate, than man-made castles. The enigmatic structures have fascinated biologists, inspired architects, and even made robotics experts study and emulate them.
Despite their fame in academic circles, however, relatively little was known about the architectural construction of termite mounds. An interdisciplinary project involving the Center for Ecological Sciences and the Department of Civil Engineering of the Indian Institute of Science (IISc) in Bengaluru recently uncovered details about the strength and stability of the hills, their weather resistance and the termites of the building materials prefer to use in their mounds.
The subjects of these recent studies are the mounds built by the termite Odontotermes obesus found in India. These termites build underground nests that house not only their huge colonies of millions of workers, soldiers and larvae, as well as the kings and queens, but also the colony’s mushroom gardens.
Maintaining a fairly constant temperature and high humidity in the nest is critical to the proper growth of the fungus, which is the only food these termites eat. In order to ensure ideal living conditions for the fungus as well as adequate ventilation in the nest, O. obesus builds mounds over their nests.
A termite mound. Photo: Dinesh Valke / Wikimedia Commons.
These mounds, which can often be up to 6 feet tall, are incredibly strong and durable 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.
Termite mound biopsy
To uncover the architectural secrets of termites, researchers filled termite mounds with plaster of paris, pumped them with propane before being scanned with lasers, and even cut them into sections with a saw.
Also read: How termites build and repair their mounds
“Before, when people wanted to examine the core of a termite mound, they just cut it through and destroyed it to get a sample,” said Nikita Zachariah, a PhD student at IISc who studied the architectural properties of termite mounds. Zachariah thought this was okay for abandoned hills, but did not want to destroy hills over an occupied nest. “I designed and built my own drill press so we could get samples of hill walls without destroying them,” she says proudly.
Zachariah’s drill enabled her to obtain small samples of termite mound walls from various parts of abandoned and occupied mounds. These samples were then placed in a micro-universal testing machine similar to a nutcracker with a screw mechanism. By gradually increasing the pressure on a sample until it ruptured, the strengths of various termite mound walls were calculated.
The results showed that the core walls of the mounds were 35-40% thicker than the edge walls of the mounds. In addition, CT scans and air permeability experiments showed that the perimeter walls of the mounds are more porous than the core.
Together, these experiments show that termite mounds are two-layer constructs with dense, strong cores surrounded by porous shells.
“The mystery of how termites can accomplish two conflicting goals – one to achieve sufficient density to give strength to the mound and the other to achieve sufficient porosity to ventilate the mound – has been solved. By building two-layer structures, the termites get the best of both worlds, ”said Zachariah.
How stable are the termite mounds?
“We know termite mounds are strong, but how mechanically secure is the mound structure? How likely is it that termite mounds will collapse under their own weight? “Were the questions Tejas Murthy asked. Murthy works on this project in the Civil Engineering Department at IISc.
Saurabh Singh, a student in Murthy’s laboratory, performed stability analyzes using the strength and density measurements obtained from the mound samples. Singh modeled the hill as either a triangle or a trapezoid for these analyzes. “These two geometries are extreme cases of the shape of a termite mound. In reality, the shapes of the hills are somewhere in between, ”he said.
The stability analyzes show that termite mounds are extremely stable with safety factors of around 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 are stable under the pressure of 1 to 2 or 3 times their own weight. that they can withstand such loads without collapsing, ”said Murthy. “If a human building had a safety factor similar to that of a termite mound, I would call it overdeveloped! However, our analysis only takes into account the effects of gravity, and it is possible that the high safety factor of these mounds is required to withstand natural disturbances that we are unaware of, ”he added.
Also read: Termite mounds filter the methane that your termites produce
The “bricks” in a termite mound
Most termite mounds are so hard that only a powerful drill or hammer can break them. Some termite mounds are so weather resistant that they can survive intact for hundreds or even thousands of years.
These facts are all the more surprising when you consider that this strength and weather resistance comes from a bio-cement that consists only of earth, water and termite saliva!
When termites build or repair their mounds, they produce their biocement in the form of spherical stones called “boluses”.
“The behavior during the production of boluses seems to be hardwired in termites. Even if you leave termites in a petri dish with some filter paper and water, they will form boluses and deposit them on the edges of the dish, ”said Renee Borges, Zachariah’s mentor and professor at the Center for Ecological Sciences, IISc. “Termites can make boluses out of almost any material – soil, agar, glass beads, paper, even metal powder!” She added.
However, with a number of substances, these insect architects can be very picky about what they use to make boluses. They prefer building materials that contain organic matter and that are granular, rough and wettable. In other words, their favorite building material is usually just earth.
Pascal Jouquet, soil ecologist at the Institute for Ecology and Environmental Sciences at the Sorbonne Universités in France, describes the selection tests with building materials as very new. “Termite ecology and behavior are often viewed from a biological point of view (e.g. with regard to the interactions between individuals or the role of pheromones), but rarely take into account the interactions between them [termites’] Building strategies and the physical properties of the material they use to build their nest / construction, ”he said.
Jouquet praised the multidisciplinary approach to this work, adding, “My only criticism may be that all of these experiments were carried out either under controlled laboratory conditions or on termite mounds on the IISc campus. It’s a shame that the natural complexity (like different soil types) was not taken into account. “
The power of bio-cement: is it just soil and water or termite fevicol?
Termite biocement consists of an almost semi-liquid mixture of saliva-enriched soil and water (with a soil: water ratio of around 70:30) that is just plastic enough to be shaped.
Termites looking for a site. Photo: Wikimedia Commons, CC BY-SA
“In some of our experiments where we gave termite glass beads as a building material, the only glue that held the beads together was the termite saliva,” said Zachariah. This observation led Zachariah, Borges, and Murthy to reflect on the role of termite saliva in biocement. Was the saliva a termite fevicol that bound soil particles, or did it have some other function?
Also Read: Let’s Imitate Termite Nests To Keep Human Buildings Cool
To test her hypotheses, Zachariah simulated the termites’ soil-water mixture without termite saliva in the laboratory, dried it into a block and tested the strength of the block.
To the researchers’ surprise, the lab-made block was as strong as the termite mound wall samples!
It turns out that as the soil-water mixture dries (in the ratio used by termites), the soil particles begin to settle under their own weight. As the soil particles settle, smaller particles fill in the spaces between larger particles to form tightly packed consolidations. The consolidations become even stronger as the water dries between the soil particles and a cohesive force known as “soil suction” builds up until the structure becomes as strong as the walls of a termite mound.
“It was a eureka moment for us when we found out that with just the right proportions of soil and water, termites can form such strong structures,” said Zachariah.
Further experiments with repeated wetting and drying cycles showed why termites use their saliva in building mounds. The saliva seems to protect the hill walls from weathering and erosion.
“Using a secretion like saliva as a strength-imparting cementing agent can use a lot of energy, especially given the size of the termite mounds,” say Borges and Murthy. “Our work suggests that the soil moisture at the time the mound was built is enough to make the mound strong after it dries. It’s a really efficient system from an energetic point of view, ”they added.
This article was originally published on Mongabay.
