There is a plain south of Darwin that baffles anyone arriving without warning. Over the dry grass rise hundreds of narrow grey slabs, the height of a person or more, driven into the ground and all oriented in exactly the same way.
They look like a cemetery and they are not. They are nests, and their orientation is the part that deserves explaining.
01 A plain full of headstones
The species is called Amitermes meridionalis and its nests are known as magnetic mounds, a misleading name because it has nothing to do with magnetism in the sense of magnets, but with the fact that they point consistently to the cardinal directions.
The shape is the key. They are not cones or towers: they are walls. A typical mound can be up to 4 metres tall and 2.5 wide, but only around 1 metre thick. Seen from the north it almost disappears; seen from the east it is a wall.
And the orientation is collective. In a field with hundreds of mounds, all present their edge in the same direction, with minimal deviation between neighbours. That regularity is what makes the place a rarity rather than a curious heap of mud.
02 The problem the orientation solves
The underlying problem is thermal. A termite colony needs the nest to stay within a narrow temperature range, and in the tropical north that is difficult: it is hot all year, with average highs between 31.1 and 33.8 degrees, and the midday sun is almost vertical.
The solution is geometric. At midday, with the sun high, the mound offers it only the narrow edge: the exposed surface is minimal and the nest does not overheat. First thing and last thing, when the sun is low and weak, it strikes the two broad faces full on.
In other words: the shape captures heat when it is useful and dodges it when there is too much, with no moving parts and no energy spent. It is an oriented building, put up by animals a few millimetres long working in the dark.
At midday the nest offers the sun only its edge; in the morning and the evening, both full faces.
03 The other solution: chimneys
In the same savanna, and sometimes a few metres away, there are completely different mounds: solid, rounded, pinnacled and up to 5 metres tall. They are built by another species, Nasutitermes triodiae, the so-called cathedral termite.
Its strategy for the same problem is different. Instead of orienting itself, it ventilates: the mound is threaded with hollow columns that let air rise from the cooler base towards the top. It is convection, not geometry.
Having both solutions in the same landscape is what makes the place instructive. Two species, the same climate, two incompatible architectures solving the same thing, and both work.
04 What they do when they are not building
It is worth looking away from the mound and at what the colony does to the ground, because that is why the savanna works. In the north more than 1,800 millimetres of rain fall concentrated in four months, the grass grows at speed and then spends eight months dead and standing.
Somebody has to process all that dead matter, and in this climate it is not the soil bacteria, which need constant moisture. It is the termites. Studies of these savannas attribute to them two thirds of the mass loss of dry wood and around a third of that of dry grass.
The scale is what that role would lead you to expect. In tropical soils, termites account for between 40 and 60 per cent of soil macrofauna biomass. They are not a detail of the ecosystem: they are the night shift that keeps it running.
05 And what they do to the soil
The second effect is on the ground itself, and it is the least reported. To build, termites bring earth up from well below and mix it with saliva and droppings until they obtain a material that sets and resists the rain.
That turns over the soil profile and concentrates nutrients in and around the mound, in a landscape where soils are characteristically poor. Around a large mound the vegetation is usually different from that of the surrounding flat.
And there is a third effect, hydrological. The galleries termites open underground work as infiltration routes: when the wet season arrives, water goes in through them instead of running off the surface. In these savannas, the regulation of water and of nutrients runs through the same animal.
06 Where and how to see it
The place where both architectures can be seen together and with easy access is Litchfield National Park, south of Darwin. There is a flat with a field of wedge mounds and, separately, isolated cathedral mounds several metres tall.
The check worth doing is the obvious one and almost nobody does it: take out the phone's compass and stand in front of a wedge mound. The edge points north with an accuracy that surprises when you measure it rather than read about it.
And it is worth looking at the scale of the whole rather than photographing a single one. The striking thing is not that one nest is oriented, but that all of them are: hundreds of independent constructions, raised by different colonies, solving the same calculation and arriving at the same result.
