How Do Termites Build Mounds? Towers With Natural Ventilation

Which termites build mounds?

Only some termites build mounds. Of the roughly 3,000 known termite species, the great mound builders live mainly in the tropics and subtropics of Africa, Australia, South America and southern Asia. The most famous are the fungus-growing Macrotermes species of African savannas and the compass or magnetic termites of northern Australia.

Many other termites nest out of sight inside wood or underground. In North America, the eastern subterranean termite builds no mound at all, a point explained further below.

What a mound is made of

A mound is built from three main ingredients: soil particles, usually clay and sand dug from below ground; saliva, which binds them; and dung, which adds organic glue. Workers pick up a grain of soil in their mandibles, wet it with saliva and press it into a growing wall. When it dries, the mixture becomes hard, almost like sun-baked brick, and a large mound can be difficult to break even with tools.

Because workers bring soil up from deep layers, mounds are often a different color from the ground around them. In parts of Africa, they look red or orange on a gray or brown landscape.

How blind workers build without a plan

No termite knows what the finished mound should look like. Instead, building emerges from simple rules followed by thousands of individuals. In the 1950s the French biologist Pierre-Paul Grassé named this process stigmergy: each piece of work changes the surroundings, and the changed surroundings tell the next worker what to do.

Soil pellets carry a scent from the workers that placed them, so a fresh deposit attracts more deposits. Small piles grow into pillars, pillars bend toward each other and join into arches, and arches become walls and roofs. Workers also respond to gradients in humidity, carbon dioxide and airflow, building where air is too dry or too still and removing material where it gets in the way. If a section is damaged, the change in air and moisture brings workers to repair it within hours.

How a termite mound breathes

A big colony of Macrotermes may contain a million or more termites, plus a large fungus garden, and all of them use oxygen and give off carbon dioxide and heat. The mound’s job is to help the nest exchange gases with the outside air, a little like a lung, while keeping humidity high.

Scientists once thought that the colony’s own body heat drove air up through a central chimney, or that wind did most of the work. Later measurements showed something more subtle. A 2015 study of mounds of Odontotermes termites in India found that the daily temperature swing powers ventilation. During the day, the sun warms the thin outer ridges of the mound faster than the central chimney, so air rises in the ridges and sinks in the center. At night the ridges cool faster, and the flow reverses. Similar patterns have been measured in Macrotermes mounds in Namibia. Wind helps near the surface, but the daily heating and cooling does most of the work.

The porous walls let carbon dioxide seep out and oxygen seep in, and networks of tunnels connect the nest with these outer surfaces.

How tall and how old are termite mounds?

Most mounds are far smaller than the famous giants. Typical Macrotermes mounds in African savannas are a few meters high, and exceptional ones are reported at about 8–9 m (26–30 ft). A single report of a mound about 12.8 m (42 ft) high in the Democratic Republic of the Congo is often repeated, but it is hard to verify.

Compass termites build flat, wedge-shaped mounds lined up roughly north to south. Their broad faces warm quickly in the morning sun but present only a thin edge to the harsh midday sun, which helps keep the nest from overheating.

Mounds can also last far longer than a single colony. In northeastern Brazil, about 200 million cone-shaped mounds of Syntermes dirus cover an area about the size of Great Britain; dating of soil from some of them gave ages of roughly 700 to almost 4,000 years. These mounds are mostly spoil heaps, piled up as the termites dug vast tunnel networks beneath them.

In Namaqualand, South Africa, low mounds called heuweltjies, built by harvester termites, have been dated in recent studies to many thousands of years, possibly more than 30,000 years in some cases. Such figures come from dating the soil and are estimates.

Why North American subterranean termites stay underground

The eastern subterranean termite lives in a climate with cold winters, and it feeds on dead wood scattered through the soil rather than on a single food store. Its colonies do not grow fungus gardens and have no fixed central nest. Instead they spread through a network of tunnels and feeding sites, moving deeper in winter and closer to the surface in warm, moist weather.

To cross open ground, they build narrow mud shelter tubes instead of towers. These tubes keep the soft-bodied workers in the dark, humid air they need and protect them from ants. In this sense, the soil itself does the job that a mound does in Africa. Learn more about how these termites feed in what do termites eat?

Mounds as homes for other animals

Termite mounds are important features of savanna landscapes. Their soil is often richer in nutrients and water than the surrounding ground, so trees and shrubs cluster on them, attracting browsing animals. Aardvarks dig into mounds to feed and leave burrows that warthogs, porcupines, hyenas and many other animals later use. Monitor lizards lay eggs in mounds, and some kingfishers and parrots dig nest holes in them. The royal chamber deep inside, where the king and queen live, is explained in do termites have a queen?

Myths and facts

Myth: termite mounds are giant ant hills. They are built by termites, which are a social branch of the cockroaches, not by ants.

Myth: a building in Zimbabwe copies how termite mounds stay cool. The Eastgate Centre in Harare, opened in 1996, was inspired by older ideas about mound ventilation. Its design works well, but later research showed that real termite mounds are ventilated in a more complex way than those early models suggested.