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In the dry forests of northeastern Brazil, an area of 230,000 square kilometres 鈥撀爈arger than Great Britain 鈥� is covered in 200 million regularly spaced mounds, each about 2.5 metres tall. These mounds, known to locals as murundus, are the waste earth dug out by termites to create a vast network of underground tunnels, and some of them are up to 4000 years old.

The termites have excavated over 10 cubic kilometres of earth to build the tunnels and mounds, making this the biggest engineering project by any animal besides humans, according to Stephen Martin from the University of Salford, UK.

Despite the enormous area covered by the mounds, they have hardly been studied until now. Martin came across them while researching honeybees in the Brazilian state of Bahia. 鈥淚 looked on Google Earth and realised they鈥檙e everywhere in this area, but I could find nothing about them online,鈥� he says.

The mounds are very conspicuous in areas where the forest has been cleared, but most of them are covered by caatinga forests, which consist of small, thorny trees that shed their leaves seasonally. These leaves are the only food for the termites, but they only fall once a year at most, and disappear quickly.

This sporadic food supply is the reason for the vast network of tunnels and the resulting mounds. 鈥淚t鈥檚 like if all the supermarkets were open for one day a year 鈥� the person with the fastest car would get the most food,鈥� says Martin. 鈥淵ou need a network of roads to get to the supermarket as quickly as you can because you鈥檙e in open competition with other colonies.鈥�

To estimate how old the mounds are, Martin and his collaborators used a technique called optically stimulated luminescence, which can be applied to quartz crystals in the soil to determine when they were last exposed to sunlight. Conservative estimates of the ages of the mounds they studied range from 690 to 3820 years old.

Their remarkable persistence is thanks in part to the dry, stable climate, with little rainfall to erode them. The soil is hard, acidic and lacking in nutrients, so the area has not been disturbed to create farmland.

Regular patterns

The researchers wondered whether competition between colonies might account for the regularly spaced pattern of the mounds. When they moved termites to neighbouring mounds, they did not show any aggression towards the locals, but when termites were moved into more remote mounds, they immediately fought with the termites they encountered. This suggests that individual colonies聽stretch over many mounds, so moving termites between adjacent mounds keeps them within the same colony. The team have not yet been able to determine exactly how big the colonies are.

Instead, Martin thinks the regular pattern of the mounds is simply the result of each colony having to dig vast, interconnected networks of tunnels to find food and minimise the energy costs of removing soil from the tunnels. The termites might use pheromones to help them find their way to the nearest waste mound.

Landscape modification of a similar magnitude has been seen in other termite species, particularly the Macrotermes, found in Africa and southeast Asia, says Scott Turner at SUNY College of Environmental Science & Forestry in New York. 鈥淭his demonstration that another, unrelated termite species is capable of similar feats underscores that large-scale landscape modification may not be an unusual feature of a few 鈥榗harismatic鈥� species, but may actually be the norm among termites,鈥� he says. 鈥淭he implications of this for ecology and evolution of landscapes are huge.鈥�

In most termite colonies, after mating the queen builds herself a chamber where she grows to an enormous size and produces huge quantities of eggs. Martin and his team were unable to find a royal chamber, so the structure of the colonies, as well as their size, remains unknown. It鈥檚 also unclear how the termites sustain themselves when their food supply is only available for such a short period. 鈥淲e don鈥檛 know any [termite] species that hibernate, but maybe they do,鈥� says Martin.