How Do Termites Build Their Mounds? Architecture Without an Architect

Termites are not ants; they are social cockroaches, an order of their own that evolved eusociality separately about 150 million years ago, with a king and queen who live for decades, a queen that can lay thirty thousand eggs a day and swell to the size of a finger, and castes of workers and soldiers that are both male and female, unlike the all-female workforces of ants and bees. The mound-building species of Africa and Australia, chiefly in the genera Macrotermes, Odontotermes and Amitermes, are those that farm: they cannot digest wood themselves and instead chew plant material, add it to combs inside the nest, and cultivate a fungus on it that breaks down the cellulose, then eat the fungus and the treated material. The fungus needs a narrow range of temperature and humidity, and the mound is the machine that provides it.

No termite knows what a mound looks like. The mechanism, described by the French biologist Pierre-Paul Grasse in 1959 and named stigmergy, is that each insect responds to the state of the work rather than to instructions, so that the structure itself coordinates the builders. A worker picks up a grain of soil, mixes it with saliva and faeces, and deposits it where the local cues suggest, and the deposit changes the cues for the next worker:

• •Pheromone in the cement attracts further deposits nearby, so a random lump becomes a pillar
• •Pillars that grow near each other are joined at the top into an arch, because the workers follow the gradient of scent and the curve of the surface
• •Air currents and humidity gradients guide where walls and openings form, so that the structure responds to the climate it is creating
• •Damage is repaired because a breach changes the airflow and the scent, which brings workers to it within minutes
• •The queen's own pheromones set the scale and position of the royal chamber

A million termites and a fungus garden generate as much heat and carbon dioxide as a large mammal, and the colony would suffocate without an exchange of air; how the mound achieves it was misunderstood for fifty years. The textbook account, that the mound works as a chimney drawing a steady current through the nest, was overturned by work published in 2015 by a team who instrumented mounds in India and found the real mechanism, which is daily. The thin outer walls of the mound's flutes warm faster than the core in the morning and cool faster in the evening, so the air inside them rises and falls on a twelve-hour cycle, and that slow oscillation pumps carbon dioxide out and oxygen in through the porous walls, using the sun rather than metabolism. The mound is not a chimney; it is a lung, and it breathes once a day.

The mound is mostly not living space. The nest proper sits at or below ground level, with the fungus combs in chambers around a central royal cell, the nursery nearby, and a network of foraging tunnels running out underground for fifty metres or more, from which the workers emerge under cover to collect grass and wood. Above them the mound is a structure of ventilation shafts, thick where the sun strikes and thin where the exchange happens, built of soil carried up grain by grain and cemented with saliva into a material as hard as weak concrete, which resists rain, fire and the claws of aardvarks. Some Australian species build blade-shaped mounds aligned north to south so precisely that they are called compass termites, which minimises the surface exposed at midday and catches the low sun morning and evening.

Termite mounds are engines of fertility. The insects concentrate nutrients, aerate and drain the soil, and the ground around a mound is measurably richer, so that from the air the savanna of Namibia, Tanzania and northern Australia shows a regular polka-dot pattern of green patches, each around a mound, spaced with a regularity that comes from neighbouring colonies competing underground. Studies in Kenya found that grass grew better, trees seeded better and animals fed more around mounds, and that a landscape with them recovers from drought faster than one without; the fairy circles of the Namib and the murundus of Brazil, one of which was estimated in 2018 to cover an area the size of Britain and to have been built over four thousand years, are the largest structures of biological origin on land. The mounds also interest engineers: the ventilation of the Eastgate Centre in Harare was designed after termite mounds and uses a fraction of the energy of a conventional building, and roboticists have built machines that construct structures by stigmergy, responding to the work rather than to a plan.

Termite mounds are built by millimetre-long insects with no plan, through stigmergy, in which each worker responds to the state of the structure and the scent in the cement, so that pillars grow, arches close and the whole shape emerges from local rules. The mound is a climate machine for the fungus the colony farms, ventilated not by a chimney draught but by a daily oscillation as thin outer walls warm and cool faster than the core, and the mounds fertilise the landscapes they stand in and have been copied in low-energy buildings and robots.