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Magnetic Termite Mounds of Australia
The ‘magnetic’ termite mounds of far northern Australia are imposing and spectacular, but what intrigues any observer is how these tall, thin mounds tend to align in the direction of north to south. Termites are found on all continents, but magnetic mounds are natural wonders of the tropical Australian landscape.
Two species of termites regularly build magnetic mounds. One of these is Amitermes laurensis, from Cape York Peninsula and eastern Arnhem Land. It is named after the township of Laura, near the southern limit of magnetic mounds in Queensland. Curiously, south of this area Amitermes laurensis does not build magnetic mounds, but erects simple conical mounds. The other species that builds magnetic mounds is Amitermes meridionalis from near Darwin in the Northern Territory.
On Cape York Peninsula, magnetic mounds are found in two different situations. The first is on so-called ’graveyard flats”, where a few to many hundreds of mounds occupy poorly drained, flat areas of one to three hectares. The other situation is on large, open grassy plains. These plains are usually located behind mangroves along stretches of low-lying coast. They may extend for several kilometers and there may be thousands of magnetic mounds. Both situations can be inundated for long periods during the wet season.
The mounds are built from soil brought from beneath the ground. The soil is cemented together with the insects' saliva and excreta. Magnetic mounds, besides being tall, thin and wedge-shaped with the longer axis orientated from north to south, differ from conventional mounds in several other ways:
Magnetic mounds are enlarged by the termites adding thin layers of galleries on the surface around a central, almost solid core. In other species (for example, the rounded mounds of Coptotermes), the mound is enlarged by concentrated building in one part of the mound. This produces a “budding'”effect. These have thick outer walls and the galleries are internal.
Why Build 'Magnetic' Mounds
The awesome spectacle of plains populated by giant thin mounds all lined up parallel to one another has caused considerable puzzlement. Whey do the termites build magnetic mounds? Many suggestions in the past have been largely speculative. Robert Logan Jack, an early Queensland Government geologist who led many expeditions into Cape York Peninsular late last century, thought the shape of the mounds might be to promote rapid drying during nest construction. Certainly the large flat faces of the mound directly face the morning and afternoon sun. Eric Mjöberg, a Swedish biologist and anthropologist who visited Cape York Peninsula in 1912, suggested the elongated shape of the mounds was to avoid damage from winds. Modern theories explain the phenomenon as a means to protect the colony inside the mounds from extremes of temperature induced by the strong tropical sun. Protection is particularly needed in the hot summer months that coincide with rains. Other species of mound-building termites can retreat into cool, insulated underground galleries when temperatures in the mound are high. But northern Australia has heavy monsoonrains, and the areas where the mounds occur are inundated with water during the “wet”, so the termites cannot retreat underground and must survive the summer heat in the mound itself. In the morning the sun shines full on the eastern face of a magnetic mound. At this stage the western face is not only in shade, but also insulated from the hot eastern face by the thick, solid core of the nest.
Temperature measurements show that there may be up to 8 °C difference in temperature between the two surfaces. In the afternoon the reverse happens and the western face becomes much hotter than the eastern. At midday, the hottest part of the day when no shade is cast, only the thin upper edge of the mound is presented to the sun so minimum heat is absorbed. It has been shown that more termites can be found in galleries on the western face in the morning and more on the eastern face in the afternoon - so there is obviously a pattern of migration to cooler parts of the nest during the course of the hot daylight hours.
Experimental rotation of mounds to an east-west orientation has been shown to upset the temperature pattern inside the mound causing an overall rise of up to 6 °C internal temperature. To build “magnetic'“mounds the termites must be able to sense the direction of the earth's magnetic field. It has been suggested that they do this by means of magnetite in their tissues, as has been shown in other animals, such as dolphins and pigeons, that orient themselves without access to visual guides. Another problem of the internal environment of a mound, perhaps solved by the flat shape of the structure, is that of 'breathing'. Just like a grazing mammal, the collective members of a termite colony breathe oxygen from the atmosphere and give off carbon dioxide. These gases must permeate the outer wall of the mound. During the wet season moisture reduces the permeability of the wall. The high ratio of surface area to volume of a flattened “magnetic” mound increases the area over which this essential exchange of gases can take place. In other words, the whole mound acts like a “terrestrial gill” for the colony. This idea is further supported by the frequent presence of thin side buttresses to the mounds of Amitermes laurensis. This further increases the surface through which gases can permeate.
ANDERSEN, A. N. & JACKLYN, P., 1993. Termites of the Top End. CSIRO, Australia.
GRIGG, G. C., 1973. Some consequences of the shape and orientation of ‘magnetic’ termite mounds. Australian Journal of Zoology, 21: 231-37.
Author: Geoff Monteith
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