Mound-building termites comprise a diverse group of termite species that construct mounds from a composite material of soil, termite saliva, and dung. These species inhabit regions across Africa, Australia, and South America. Some mounds can attain diameters of up to 30 meters (98 ft). A majority of these structures are situated in well-drained environments. Typically, termite mounds persist longer than the colonies that construct them. Exposure of the nest's internal tunnels generally indicates the colony's demise. Following the extinction of the original inhabitants, mounds may be subsequently occupied by other colonies, either conspecific or heterospecific.
Mound Structure
Mound structures exhibit considerable complexity. Internally, mounds feature an extensive network of tunnels and conduits, functioning as a sophisticated ventilation system for the subterranean nest. To facilitate effective ventilation, termites construct multiple shafts that descend to a cellar situated below the nest. The mound itself is erected above this subterranean nest. The nest proper is a spheroidal formation composed of numerous gallery chambers. These structures display significant morphological diversity. For instance, species such as Odontotermes construct mounds with open chimneys or vent holes, whereas others, like Macrotermes, build entirely enclosed structures. The mounds of Amitermes (commonly known as Magnetic termites) are characteristically tall, slender, and wedge-shaped, typically exhibiting a north-south orientation.
Mound Ventilation
The intricate network of tunnels and conduits within mounds has long been recognized for its role in internal climate regulation. Termite mounds effectively regulate temperature, humidity, and the distribution of respiratory gases. An initial hypothesis posited a thermosiphon mechanism. Metabolic heat generated by the termites confers sufficient buoyancy to the nest air, propelling it upwards into the mound and ultimately to its porous surface, where heat and gases exchange with the external atmosphere via the porous walls. Subsequently, the air density near the surface increases due to heat dissipation, causing it to descend below the nest and recirculate through it. This specific model was proposed for mounds featuring capped chimneys and lacking large vents, as constructed by the species Macrotermes natalensis. Conversely, a comparable model, predicated on the stack effect, was suggested for mounds possessing open chimneys. Tall chimneys experience elevated wind velocities compared to ground-level openings, attributable to surface boundary layer conditions. Consequently, a Venturi flow mechanism draws fresh air into the mound via ground-level apertures, which then circulates through the nest and exits through the chimney. Unlike the circulatory flow characteristic of the thermosiphon model, the stack effect model exhibits unidirectional airflow.
In Odontotermes transvaalensis mounds, internal temperature regulation is not primarily achieved through ventilation. Instead, their tall chimneys primarily facilitate ventilation by inducing airflow via the Venturi effect. Investigations into Macrotermes michaelseni mounds have revealed that their principal function is the exchange of respiratory gases. The intricate interplay between the mound structure and the kinetic energy of turbulent winds constitutes the primary impetus for the colony's gas exchange. However, more recent studies on Macrotermes michaelseni mounds, employing an advanced custom airflow sensor, indicate that internal air movement is predominantly driven by convective flows resulting from diurnal oscillations in external temperature. A secondary thermal gradient arises from the differential solar exposure of the mound's eastern side in the morning and western side in the afternoon. The enhanced reliability of the sensor data suggests that wind's contribution to ventilation is secondary to the prevailing thermal mechanism. While wind augments gas exchange near the mound walls, it does not generate substantial average or transient airflow within the structure. Collectively, a comparable ventilation and thermoregulation mechanism has been observed in both Macrotermes michaelseni and Odontotermes obesus mounds.
Social Castes
The worker caste, characterized by its diminutive size, constitutes the most populous segment of the colony. These individuals are universally anophthalmic, apterous, and sexually undeveloped. Their primary responsibilities include provisioning and grooming all other dependent castes. Furthermore, they undertake tasks such as tunnel excavation, foraging for sustenance and water, maintaining atmospheric homeostasis within the colony, and constructing and repairing the nest.
The primary function of the soldiers is to safeguard the colony against intruders. During an attack, these large soldiers expel a corrosive, brown salivary liquid that disperses between their open mandibles. Upon biting, this liquid is transferred to the adversary. This secretion is frequently described as toxic or as possessing air-coagulating properties that render it adhesive.
The final caste comprises the reproductives, specifically the king and queen, whose role is procreation.
While the queen may attain lengths of up to six centimeters, individuals of the lower castes typically measure less than one centimeter.
Biotic Communities on Termite Mounds
The flora found on termite mounds typically exhibits significant divergence from the surrounding vegetation. In African savannas, for instance, Macrotermes mounds establish distinct "islands" characterized by elevated tree densities. This phenomenon is commonly ascribed to the enhanced fertility of mound soils, resulting from termite excavation and the decomposition of organic matter. Furthermore, mound soils have demonstrated higher water retention capabilities compared to adjacent areas, providing a distinct ecological advantage for plant proliferation in savanna environments. The elevated tree densities on these mounds, in turn, attract substantial populations of browsing herbivores, a consequence of the nutrient-rich foliage produced by mound-dwelling trees, or potentially due to the abundant food and refuge available within these structures.
Termite Mounds in the Brazilian Caatinga
The Caatinga ecoregion in northeastern Brazil contains approximately 200 million termite mounds, distributed across an area comparable to Great Britain. Individual mounds can reach heights of 3 meters (10 feet) and widths of 10 meters (33 feet), with an average spacing of about 20 meters (66 feet). Beneath these structures lie extensive tunnel networks, the construction of which necessitated the excavation of an estimated 10 cubic kilometers (2.4 cubic miles) of soil.
Researchers conducted radiometric dating on eleven of these mounds. The youngest mound was determined to be 690 years old, whereas the oldest was at least 3,820 years old, potentially exceeding twice that age. These structures were constructed by Syntermes dirus termites, which measure approximately half an inch in length. Regional deforestation facilitated the discovery and assessment of the mounds' full extent by the scientific community. A researcher commented that these mounds seemingly constitute "the world's most extensive bio-engineering effort by a single insect species."
Amitermes meridionalis, known for the magnetic termite mounds found in northern Australia.
- Amitermes meridionalis, magnetic termite mounds of northern Australia
- Eusociality