Variety of termite mounds

From Tropical Topics newsletter, No. 64 December 2000, produced by the Environmental Protection Agency. To read more about termites, click on the Continuing Pages above; you can also download a PDF of the entire issue from this page.

Magnetic mounds to regulate temperature

These wedge-shaped mounds are all aligned in a north-south direction. This appears to be a response to particular environmental constraints. The termites which build them feed on grass roots and other plant debris found in plains which are seasonally flooded. They are therefore unable to retreat from the extremes of the summer heat into underground chambers as many other termites do, but are forced to remain above the water, in the mound. However, in winter, opposite problems occur as cool air is trapped in these low-lying depressions — temperatures as low as 4ºC have been recorded. The mound therefore functions as a clever temperature regulator. The eastern side is warmed by the morning sun, and the western side by the evening sun, but in the middle of the day only the thin upper edge of the mound is exposed. At this time air temperature alone allows the mound temperature to remain stable. Experimental rotations of mounds, putting them in an east-west position, has caused an overall increase in internal temperature of 6ºC.

Wind and shade also affect the mound temperature but because of the wedge-shape the termites are able to adapt to different conditions by building mounds with different orientations. By orientating the mound slightly more to the north-west the termites can increase the amount of heat received on the eastern side, to compensate for the cooling effects of wind and shade.

Magnetic mounds are built by a number of termite species. One, Amitermes meridionalis , is found south of Darwin and across the western part of the Top End. Another, A. laurensis which is named after the Queensland town of Laura, is found in Cape York Peninsula and eastern Arnhem Land. South of Laura, mounds of this species are more rounded in shape, rather then ‘magnetic’. This is thought to be a response to drier conditions, where the seasonal flooding which dictates the ‘magnetic’ shape is not a factor.

Dome mounds

Tree-piping termites (Coptotermes acinaciformis ) construct mounds at the base of trees, particularly eucalypts. The termites enter a tree from below ground level and, with the help of soldiers which produce wood-solvent chemicals, create hollow pipes through the trunk and branches, filling the gaps with soil. The tree is weakened but not killed. These termites, which occur throughout Australia, are unpopular with the timber industry, but their activities create hollows for other animals to nest in — and benefit didjeridoo makers.

Tree mounds

A number of termite species build their mounds high in trees. Most of these are actually soil-nesting species, but after their colony has been flourishing underground for a while, some take to the trees and build a mound there, retaining a connection to the ground via covered runways. These mounds are more common in coastal areas. The same termite species, when living in drier, inland regions tend to restrict themselves to underground nests. The rounded mounds are created largely from semi-digested wood and organic matter but may have an earthen shell, depending on species. Some species prefer smooth-barked trees, others prefer rough-barked trees such as ironbarks and stringybarks where the tunnels connecting the nest to the ground can be hidden in the bark.

Bulbous mounds

Spinifex termites (Nasutitermes triodiae ) construct some of the biggest mounds in the world, containing in excess of a million termites. The largest, reaching more than 6 metres in height, are found in an area north of Pine Creek in the Northern Territory. However, mound shapes associated with this versatile termite vary considerably, from tall flanged forms to squat bulbous rounded ones. As the colony grows, the termites build on extensions in the form of bulging ‘ buds’. They are generally to be found around the edges of floodplains, on higher ground than the magnetic mounds. These termites are harvesters, feeding on grass which they carry back to the mound in 1cm lengths. They move along underground tunnels and build earthen structures around grass tussocks on which they are feeding.

Mound variations

Mound shapes vary greatly according to local conditions. It has been speculated that, since mound-building behaviour be largely genetically inherited, different species, with similar appearances, may be involved. Possibly evolution in termites has expressed itself in various mound shapes rather than variations in body form.