Mound building by Macrotermes bellicosus: Thermoregulation and other architectual aspects

Scriptie Orientatiefasecursus Populatiebiologie 1998

Anna den Held and Thijs van der Velden


Introduction

We were asked to write a literature study about a subject that interested us during the course population biology. An amazing phenomenon in nature is the ingenius moundbuilding by certain termites. We decided to focus on the Macrotermes bellicosus, which belongs to the subfamily Macrotermitinae. These are fungus-growing termites which occur in parts Africa and Asia. First we will give some general information about this so-called social insect, and then go to to our main point, the thermoregulation in the nest of the M. bellicosus by different architecture.

The founding of a nest starts by the royal couple who make a hole in the soil, called copularium. In here the queen lays her first eggs which develop into stunted workers. They accomplish the further developement of the nest and after a while receive help from newborn soldiers. Just after four years alate reproductives are produced.

As said before the M.bellicosus is fungus-growing. They build fungus combs which

occupy special gardens in the center of the nest in large compartements. The gardens are organized into complex sponge-like structures with numerous convoluted ridges and tunnels, evidently designed to give the maximum surface for growth. The substrate of the fungi is finly chewed wood provided by the termites themselves.

At first there was doubt about the exact function of these gardens. Some thought they were somehow part of the ventilation system of the big nests by providing some of the heat needed for convection. Nevertheless convincing evidence has been collected that the fungi are an important nutritive symbiont for some of the Macrotermine species. Microscopic examination of macrotermes workers gut contents indicated that the fungi serve to degrade the lignin and to expose the minute fragments of cellulose for quicker digestion by the intestinal bacterial flora.

Even more remarkable than the phenomenon of fungus-growing is the size and complexity of the nests constructed by the macrotermites. An important aspect of this complexity is the thermoregulation concerning the nests. Several studies have been made about the ability of M. bellicosus to regalate the internal temperature of their nests and the influence that the environmental temperature has on the architecture of the mounds.

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Termitenests and their composition.

The nests of termites are generally regarded as a future of social organisation. The inside of a adult nest is always build according to a certain plan, containing one or more breeding centres from which radiate a network of galleries and runways to special chambers which store food, water and all kind of soilparticles.

In general we can classify the nests into six categories, gathered into two main groups. The first group contains three categories of areal nests, i.e. in the aerial parts of plants. The second group contains three categories of terranian nests; one under the soil surface, one half on it, and one above the soil surface, called mound. It is in this kind of nests that our M.bellicosus lives. Their mounds are impressive feautures characterising whole landscapes. They reach heights up to 8 meters, and appear sometimes in densities of 83 mounds per hectare.

The mounds are constructed of soil particles, (like coarse sand, fine sand, silt, clay and organic carbon), extracta and saliva, these in varying proportions. The M.bellicosus uses re-packed, orally transported soil particles, cemented with saliva into walls.

It is thought that mound structure of termites is determined by three important factors: the species, the soil composition and the microclimatic conditions (like rain and temperature). This last item is supposed to be very important because it can change on large scale in the same area.

That there is correlation between moundshape and thermoregulation within the nest, has often been suggested. Stable temperature inside the nest is very important for an optimum in fungus production, and so the termites use special archtitectual technics to construct a mound with certain thermoregulatory qualities.

Investegation on this subject has been done in Comoé-National Park (Ivory Coast) by Korb and Linsenmair (1997). In this park we find the M.bellicosus in two very different areas. First, they are found in the very warm shrub savanna. Here, the mounds have rather thin walls with numerous ridges and complex structures. The ridges on the outside of the mounds are made to increase the surface. The result is that the temperature inside will not increase above the critical level during the day. Inside the nest there are several ventilationchannels, essential for the gasexchange. These channels are also involved in the thermoregulation. During the cold savanna night, the diameter is reduced by constructing obstacles in the channels so ventilation decreases and heatloss is regulated. When temperature increases during the day these obstacles are removed so that ventilation takes place and heat is lost.

Second, they appear in the moisty gallery forest. This habitat has a relative cool, but stable climate. So in this environment it is important for M.bellicosus to reduce the loss of heat. Therefore the mounds are dome-shaped with massive walls and hardly any portraiding structures. The reason for this is that in this type of habitats it is not nescessary to have an extended channelsystem for this would only increase heatloss.

It can be concluded that the differences in moundarchitecture performed by termites of the same species is caused by the diffrences in temperature between the two habitats. The

M. bellicosus apparently is capable of influincing its own microclimate By building mounds with special structures they can actually achieve thermoregulation.


References

Wilson, Edward.O.,1971, ‘The Insect Societies’, Harvard University Press.

Brian,M.V. (ed) 1978, Production ecology of ants and termites, Cambridge University Press.

Korb,J., Linsenmair, K.E., 1997, ‘The effect on …… savanna’. Insectes Sociaux 45:1-112.

Korb,J., Linsenmair, K.E., 1997, ‘Experimental heathing ….architecture’. Insectes Sociaux 45:.235-347

Thanks to H.Velthuis for his important contribution.

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