The meridional termite mounds (Amitermes meridionalis) of tropical Northern Australia are a spectacle of insect architecture. Thousands of these unmistakable flattened structures permeate through the floodplain, resembling an eerie array of giant tombstones. And they can be over four meters tall. What is so striking and remarkable is that they are all parallel to each other, oriented more or less north-south, earning them the common name “magnetic” termites. Mound orientation varies between populations in Litchfield, Arnhem Land, and a closely related species in Cape York, though remains relatively conserved within populations. To investigate this property as an adaptation, scientists experimentally manipulated mound orientation and found that internal mound temperatures increased significantly when repositioned. Mound orientation is shaped by sun, shade, and wind conditions, varying geographically to maintain a stable internal microenvironment. As for the north-south axis, it is thought that the mounds receive more direct sunlight during the morning and evening (when ambient temperatures are lower), and receive less heat when the sun is at its peak in the sky. But why be above ground in the first place? In this seasonally inundated habitat, it would be difficult for the termites to maintain stable conditions within their nests. By constructing mounds above the ground, bacterial and fungal growth are kept in check, and there is adequate ventilation for food storage and developing larvae.
Cathedral termites (Nasutitermes triodiae) have a different strategy for keeping cool under the blistering hot sun. They are characterized by hollow columns that allow air to circulate from the cooler base to the top of the mound. Many animals take advantage of these gigantic structures as refuges, including goannas, snakes, agamids, small marsupials, and birds. Cathedral mounds are even taller than magnetic termite mounds, reaching heights of five meters— truly skyscrapers of the floodplain. To support themselves during perturbations during monsoon season, cathedral termites are more selective in their choice of soil, choosing locations where water drains more rapidly. This tendency was apparent in Litchfield where stretches of flat floodplain almost exclusively housed magnetic termites, along with adjacent clumps of cathedral termites.
Fun fact: Along roads in the NT and WA, people regularly dress up young cathedral mounds with t-shirts, sunglasses, and hats. I was lucky enough to spot a Santa termite mound during the holidays.
While wandering in the eucalypt forest at night I stumbled upon a giant black figure in the distance. As I walked closer the figure began to take form. It was a termite mound about eight feet high and five feet wide, blackened and charcoal-like to the touch, obviously long deceased. It was my first magnetic termite mound.
Photos below from Fogg Dam Conservation Reserve