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Architects and collectivists.

Termites are social insects. A nest of termites is founded by a single pair of winged reproductives (alates) whose sterile offspring (male and female) cooperate to collect and distribute food, care for the eggs and young, and defend the nest. Some of the offspring develop into workers or soldiers so specialized in defense that they cannot feed; the workers have to put food in the soldier's mouth. Termites can reasonably be compared to architects with collectivist tendencies.

Primitive termites live inside pieces of dead wood, which they eat and digest with the aid of their symbiotic intestinal protists (see p. 114). To maintain a healthy intestinal flora, they are constantly exchanging their stomach contents, either from mouth to mouth or by eating fresh droppings, so that the nest in fact has a "communal stomach." From this basic type, termites have diversified to eat all sorts of plant products. A few species eat live plants; many more eat recently dead or decaying plants; others eat leaf litter and humus. The original dependence on their intestinal protists has been reduced or lost in the higher termites (Termitidae), which are dependent on symbiotic bacteria. The African subfamily Macrotermitinae has also evolved a symbiosis with a basidiomycete fungus (Termitomyces). These termites cultivate the fungus on a sort of honeycomb, fungus comb, built with the cellulose-rich droppings of the foragers. Fungus-cultivating termites evolved in Africa in the relatively recent geological past and have since spread to southern Asia.

In savannahs, termites are faced with a continuous plant cover (grassland instead of isolated trees), and to exploit this they construct their nests in the soil. Some feed partly or wholly below-ground, eating leaf litter, humus, or roots. Many other termites emerge onto the soil surface at night to collect living plant matter or dead remains (forager termites) or the dung of herbivorous animals. To minimize dangerous exposure to predators and air, surface- foraging termites build networks of tunnels 2-6 in (5-15 cm) below the soil surface, though others tunnel much deeper. Some also build surface galleries covered with earth or even mud sheaths around a fallen branch or pile of dung (see photo above). All this tends to concentrate nutrients near the nest, and this is reflected in the distribution of the vegetation. In extreme cases, shrubs and small trees may grow precisely on the termite mounds scattered throughout a nutrient-poor savannah, giving rise to the landscape known as termite savannah (see photo 19).

Most savannah termites build dispersed nests spread out over a relatively large area. Others build centralized nests, perhaps to improve their perimeter defenses against ants, their main predators. These concentrated nests are, however, more vulnerable to attacks by vertebrate predators such as anteaters, and so they often protect their nest with thick, hard walls. Normally, savannah termites build underground nests, but a minority build termite mounds that rise above the ground level, with the nest within or below the mound. Due to its conical shape, the rain can run off the sides leaving the interior dry. Tall termite mounds are common in sites where the soil is regularly flooded, as in the depressions in central Africa known as dambo.

Centralized nests with a large termite biomass produce a lot of metabolic heat. The soil temperature at a depth of 4 in (10 cm) is almost constant, day and night, because soil is a good insulator. For the same reason, however, it is difficult for an underground nest to dissipate metabolic heat, and this may lead to overheating. The fungus-growing African termites of the genus Macrotermes seem the species most at risk of overheating. The nests contain several million individuals with a total biomass of several kilos, while the fungus combs may weigh twice as much as the termites. These also metabolize, producing heat and waste gases. In hot areas, some species of Macrotermes build complicated passive ventilation systems that make use of the wind blowing over the mound. In eastern Africa, M. jeanneli builds chimneys up to 23 ft (7 m) tall that use the Venturi effect to suck air into the nest, thus cooling it by the evaporation of water. As this implies the consumption of large amounts of soil water, these chimneys are a sign of relatively abundant water supplies.

Defense of the termite's mound is also very important, and inside it there are often mazelike structures to confuse intruders, mainly ants. The so-called "cathedral mounds" of the eastern African Macrotermes bellicosus neatly combine a ventilation system with elaborate defense systems against attacks by army ants. The central nest is completely surrounded by a large empty space crossed by a few narrow bridges. It is built on top of a circular plate supported in the center by a narrow pillar, a structure that might seem to have been designed by an expert in impregnable medieval fortresses. Furthermore, the underside of the plate has a thin, smooth spiral vane hanging down vertically. It evaporates water to cool the nest but also functions as a deterrent maze. Any ant that tries to reach the nest from below by climbing the pillar must either try to climb across the slippery vane or follow the spiral round and round.

Termite mounds are complex structures built by organisms with complex social lives.
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Publication:Encyclopedia of the Biosphere
Date:Mar 1, 2000
Words:877
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