Mound repair and mound structural homeostasis

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The mound is a dynamic structure, maintained by a balance between loss of soil by erosion, and transport of soil into the mound by termites. The mound is also a regulated structure: damage to the mound is met with a mobilzation of workers to repair the damage and restore the mound to its previous structure.

The mound's architecture is therefore the product of the rates and patterns of soil movement, whether mediated by physical factors such as erosion, or actively managed by termites.

This is the first of a series of pages that examine how the mound is maintained by the soil-transporting habit of the termites. Below you will find a brief outline of the course of mound repair, the mechanisms of soil transport, and the structures that result. Subsequent pages outline what we have learned recently about how these processes work in nature.

The mound is a regulated structure

The mound is a regulated structure

A homeostatic system responds to perturbation by doing work to offset the perturbation. A system for regulating body temperature will respond to a body cooling by generating excess heat to restore temperature to its "set" value.

Structures can also be homeostatic, at least if they are dynamic structures like the mound. In this instance, a perturbation to a structure's morphology will elicit a repair process that restores the morphology to its "regulated" state.

The mound is clearly a homeostatic structure. This is shown dramatically by a "surgical" procedure we call a complete moundectomy. Here, we use a front-end loader to scrape the entire mound away at ground level (right). Because the nest is underground, complete moundectomy leaves the nest unharmed, with its complement of workers free to rebuild the mound. This they do within a space of about 90 days.

To see this in action, watch the video below, which is a time-lapse sequence of photographs taken daily. After you click on the control, you can either play the video through or use the slider to move through the movie backwards and forward. Look for several things. First, you can see that the mound grows very quickly. Second, you can see that rainfalls erode the growing mound markedly. The erosion is quickly made up by new soil transport, so that the growing mound has the appearance of "heaving" upward. Third, note how the bursts of growth follow rainfall episodes. Finally, note how the mound develops a tilt as it grows, just as in the normal mound. The mound grows directly toward the sun.

The outcome is a restored mound with a structure very similar to that prior to the "disturbance": a homeostatic structure, in short.