IN THE heart of Africa’s savannah lies a city that is a model of sustainable development. Its buttressed towers are built entirely from natural, biodegradable materials. Its inhabitants live and work in quarters that are air-conditioned and humidity-regulated, without consuming a single watt of electricity. Water comes from wells that dip deep into the earth, and food is cultivated self-sufficiently in gardens within its walls. This metropolis is not just eco-friendly: with its curved walls and graceful arches, it is rather beautiful too.
This is no human city, of course. It is a termite mound.
Unlike termites and other nest-building insects, we humans pay little attention to making buildings fit for their environments. “We can develop absurd architectural ideas without the punishment of natural selection,” says architect of the Helsinki University of Technology in Finland. As we wake up to climate change and resource depletion, though, interest in how insects manage their built environments is reawakening. It appears we have a lot to learn.
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“The building mechanisms and the design principles that make the properties of insect nests possible aren’t well understood,” says of the CNRS Research Centre on Animal Cognition in Toulouse, France. That’s not for want of trying. Research into termite mounds kicked off in the 1960s, when Swiss entomologist Martin Lüscher made trailblazing studies of nests created by termites of the genus Macrotermes on the plains of southern Africa. It was he who suggested the chaotic-looking mounds were in fact exquisitely engineered eco-constructions.
Specifically, he proposed an intimate connection between how the mounds are built and what the termites eat. Macrotermes species live on cellulose, a constituent of plant matter that humans can’t digest. In fact, neither can termites. They get round this by cultivating gardens of fungi that turn wood into digestible nutrients.
These fungus gardens must be well ventilated, and their temperature and humidity closely controlled – no mean feat in the tropical climates in which termites live. In Lüscher’s picture, heat from the fungi’s metabolism and the termites’ bodies causes stagnant air laden with carbon dioxide to rise up a central chimney. From there it fans out through the porous walls of the mound, while new air is sucked in at the base.
Giant lungs
So simple and appealing was this idea that it spawned at least one artificial imitation: the in Harare, Zimbabwe, designed by architect . Opened in 1996, it boasts a termite-inspired ventilation and cooling system. Or at least it was thought to. It turns out, however, that few if any termite mounds work this way.
Keeping the temperature and humidity within termite mounds constant while at the same time getting rid of CO2 demands a very efficient process of gas exchange. A typical mound with about 2 million inhabitants needs to “breathe” about 1000 litres of fresh air each day. To investigate further what might drive such an exchange, , a termite expert at The State University of New York in Syracuse, and of Freeform Engineering in Nottingham, UK, looked into the design principles of Macrotermes mounds in Namibia. They found that the mounds’ walls are warmer than the central nest, which rules out the kind of buoyant outward flow of CO2-rich air proposed by Lüscher. Indeed, injecting a tracer gas into the mound showed little evidence of steady, convective air circulation.
Turner and Soar believe that termite mounds instead tap turbulence in the gusts of wind that hit them. A single breath of wind contains small eddies and currents that vary in speed and direction with different frequencies. The outer walls of the mounds are built to allow only eddies changing with low frequencies to penetrate deep within them. As the range of frequencies in the wind changes from gust to gust, the boundary between the stale air in the nest and the fresh air from outside moves about within the mounds’ walls, allowing the two bodies of air to be exchanged. In essence, the mound functions as .
This is very different to the way ventilation works in modern human buildings. Here, fresh air is blown in through vents to flush stale air out. Turner thinks there is something to be gleaned from the termites’ approach. “We could turn the whole idea of the wall on its head,” he says. We should not think of walls as barriers to stop the outside getting in, but rather design them as adaptive, porous interfaces that regulate the exchange of heat and air between the inside and outside. “Instead of opening a window to let fresh air in, it would be the wall that does it, but carefully filtered and managed the way termite mounds do it,” he says.
Turner’s ideas were among many discussed at a organised by Théraulaz in Venice, Italy, last year. It aimed to pool understanding from a range of disciplines, from experts in insect behaviour to practising architects. “Some real points of contact began to emerge,” says Turner. “There was a prevailing idea among the biologists that architects could learn much from us. I think the opposite is also true.”
Absorbent sponges
One theme was just how proficient termites are at adapting their buildings to local conditions (see “More than one way to make a mound”). Termites in very hot climates, for example, embed their mounds deep in the vast heat sink of the soil – a hugely effective way of regulating temperature. Other species maintain humidity by depositing a slurry of chewed wood and grass at the base of the mound. This acts like a giant sponge, which, with a capacity of up to 80 litres, can supply or absorb water to counteract any humidity fluctuations within the nest. Such a trick could be mimicked using water tanks positioned in the bowels of a building to restore humidity in hot, dry climates. “As we come to understand more, it opens up a vast universe of new bio-inspired design principles,” says Turner.
Tips might also be gleaned from the construction processes that insects employ. Some of the most thoroughly studied nest-building insects are the paper wasps, named after the fibrous material they use to make their combs. These consist of arrangements of tubular cells with hexagonal cross-sections, and while the designs are astonishingly diverse they are by no means random.
To find out how the combs are made, Théraulaz and his colleagues supplied different coloured paper to the wasps for each stage of nest building. This showed that the wasps observe general construction rules based on the configuration of neighbouring cells. “For example, they prefer to add cells to a corner area rather than starting a new row,” Théraulaz says. No individual wasp has any idea what the final structure will be, yet by following a simple set of rules – rules that evolution has determined maximise the insects’ chances for survival – the constructions they arrive at are sound.
Termites ensure a similarly successful outcome using chemical signals called pheromones. As the nest-builders chew soil pellets into a cement-like paste, their saliva adds a chemical which, for just a few minutes, can be “smelled” by other builders over a distance of a centimetre or so. This sets up a positive feedback: the more a pillar is augmented, the stronger a pheromone source it becomes, causing the termites to add even more material.
Such approaches are anathema to human ideas of design and control, in which a central blueprint is laid down in advance by an architect and rigidly stuck to. But Turner thinks we could find ourselves adopting a more insect-like approach as technological advances make it feasible. “There’s a huge opportunity for robotics to build systems of agents linked by a distributed intelligence that can remodel a building’s structure as conditions change,” he says. That might sound fanciful, but really it is just a return to the old human practices of organic building and settlement design, in which additions and alterations were made piecemeal over time in response to what went before.
“Insect architecture might lead us back to old human practices of organic settlement design”
Termites face many of the same challenges we do in our built environments, and meet them more efficiently, and sustainably. “A mound is in many ways as alive as the termites that build it,” says Turner. Human buildings could soon come to life too.
More than one way to make a mound
Termites of the African species Macrotermes bellicosus have developed two very different strategies to optimise mound ventilation to local weather conditions. On the hot, dry savannahs of east and west Africa, their mounds are many-spired “cathedrals”. According to biologist of the University of Osnabrück, Germany, this is one instance where heat gradients drive currents of air circulation that sink through the nest and rise in the walls during the day. This circulation gets more or less switched off at night when the temperature gradients disappear or reverse, avoiding heat loss and keeping the nest at a roughly constant temperature.
In the cooler forests of northern Ivory Coast, though, the same species builds simpler dome-shaped mounds in which buoyant warm air rises up through the nest and escapes through small holes in the walls. This design seems to trap more heat by limiting outward airflow, making sure that the fungus gardens that provide the termites’ food are kept at an optimal temperature.
Thousands of miles away, another species of termite has developed an innovative way of making sure it gets the most out of the sun. The magnetic termite of Australia uses Earth’s magnetic field to build mounds elongated in a north-south direction. The broad eastern and western faces soak up the weaker rays of the morning and evening sun, while a relatively narrow surface is subjected to the fierce glare of the midday sun – helping to keep the temperature relatively constant.
All termite mounds, Korb says, seem designed to produce homeostatic conditions in which the inner environment remains as constant as possible. The very different environments in which termites thrive show how successful they are.
