Water homeostasis. 2

termite drinkers

Regulation of nest humidity can be analyzed through a simple water balance method. Put simply, a constant nest humidity imposes the requirement that water loss rate must always equal water import rate.

Macrotermes live in environments that are strongly seasonal, with torrential rainfalls and high humidities in the warm summers, and very low humidity and cool temperatures during the winter dry season. This means that the balancing the water budget imposes different physiological challenges through the year.

The water balance of the nest and mound

dry season water balance

water balance wet season

Although the nest humidity is damped considerably by the fungus combs, this cannot keep nest humidity constant over seasonal time frames. The strongly seasonal climate of this region means that there will be different challenges in balancing the water budget through the year. In all instances, termites act as active carriers of water, either by drinking from wet soil and storing it in their crops, or carrying it in wet soil. You can see a video of this here.

During the dry winter, the mound and soils dry considerably. The nest will therefore lose water to these surrounding dry soils. To keep nest humidity constant, termites must actively transport water into the nest. This probably comes from termites vertically down through the soil to water tables. Anecdotal reports from the mining literature indicate that termites will go hundreds of feet down in the soil to find water.

The termites face the opposite problem in summer. Here, the termites face substantial inflows of water during the episodic periods of torrential rains. Now the problem is a surfeit of water, which the termites must balance by transporting water out of the nest. Because the mound is generally the dryest soil nearby, the termites move water up into the mound.

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Engineered soil water stores

calcrete layer

A layer of calcrete, exposed in a trench adjacent to a mound

calcrete nodules

Close up of nodular calcrete cobbles. Note the many tiny places where water could collect.

During the dry winter, termites must seek water from stores in soil. Much has been made of anecdotal reports from the mining literature of termites burrrowing hundreds of feet down in search of liquid water. More likely, termites exploit shallower reservoirs of water.

One of the common features of the soil in this region is a layer of nodular calcite (calcrete) that forms about two meters below the surface of the soil. To some extent, this is probably biogenic: methane (produced in termites' guts, water, carbon dioxide and soluble salts of calcium can combine into insoluble calcium carbonate. This commonly precipitates as nodules that can grow and form a cobblestoned impermeable layer that can trap water. This forms a so-called perched water table that can collect water percolating down during torrential rains. This provides a ready store of liquid water that termites can tap.

One of the interesting features of this calcrete layer is that it dips below a termite mound. This deformation is probably due to the ongoing focal perturbation of soil structure and chemistry near the colony. It may be that this depression of the calcrete layer forms a "saucer" beneath the colony, which allows perched water to flow toward it from a wide area around, collecting perched water for the termites to drink.

schematic of perched water tables

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The link between water transport and soil transport

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Termites can transport water simply by imbibing it from soil into their crops. Note the termite in the time lapse video to the right, and how its abdomen swells as it imbibes water from the soil. This water can then be shared with other termites.

More commonly, water movement takes place in conjunction with soil transport. When termites build, the soil slurry they use as mortar is about 30% water. You can see this also in the video to the right. Note how wet the freshly built soil is/ Termites also tend to translocate soil from wet areas to dry areas.

The link between water transport and soil transport can be seen in the temporal patterns of deposition of soil to the mound surface. During the dry season, there is no building on the surface that ever takes place, except in the context of mound repair. During the rainy season, soil deposition is episodic, and is correlated with episodes of torrential rainfall. On the graph below, we followed episodes of soil deposition for four mounds over the course of a rainy season. Specifically, note how episodes of soil deposition (green horizontal bars) follow a few days after a rainfall episode (blue vertical bars) has finished.

episodic soil deposition

This happens to be the same time frame for rainfalls to percolate down to deep soils. This raises the intriguing possibility that mound building - or more specifically, the soil transport that produces the mound - is actually a means of exporting excess water from the colony.

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Termite pages

Termite home

Structure

Endocasting

Social homeostasis

Nest temperature

Water homeostasis 1

Water homeostasis 2

Water homeostasis 3

Fungal symbiosis

Fungal symbiosis and water 1

Fungi and water homeostasis 2

Gas exchange 1

DC vs AC Gas Exchange

Gas exchange 2

Gas exchange 3

Gas exchange 4

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