Water homeostasis. 3

Preparing the mound

Do termites use mound-building to manage the water balance of the nest?

The circumstantial evidence points to the answer being "yes." Soil transport into the mound is correlated with rainfall episodes, and the time delay correlates to the percolation time of rain showers into the soil.

What is needed to settle the question is an experiment. This page covers the result of an experiment designed to test just that.

We asked the question: if we added percolated water to a nest, on top of what it already receives from rainfall, would the termites respond to this extra water input by moving more water and soil into the mound?

With a grant from the National Geographic Society, that is what we did.

The experiment

Here is the experimental test we performed. If termites move water into the mound to export percolated water from the nest, then adding percolated water to the nest should produce greater rates of soil and water transport into the mound.

Experimental mound

flooding the mound

We identified ten mounds in an open grassy field. Five of these were identified as controls, and five were designated as experimental mounds. For these, we constructed a circular dam 8 m in diameter that surrounded the mound. These were irrigated with 750 liters of water each every two weeks (right). This roughly tripled the percolated water in the experimental mounds.

All mounds were surrounded with a concrete catchment trench to capture soil eroding off the mound. Plaster block moisture sensors were placed in the nest, deep within the mound and at two superficial locations in the mound. We also measured deep (-1m) soil moisture at two locations, one 4 m away (which would be enclosed within the dam on experimental mounds), and the other 8 meters away. These were measured once daily. Finally, we planted colored poystyrene beads at various locations in the nest, deep mound and at two superficial locations, north and south mid-mound. These were placed to estimate soil transport. Termites will pick up these beads and carry them as they would grains of soil.

catchment trenchInserting sensorspolystyrene beads

Left: Mound with surrounding concrete catchment trench. Middle: Placing sensors and polystyrene beads. Right: Blue and green polystyrene beads in new build.

If the termites use transport in wet soil to balance the nest's water budget, three things should be evident in the irrigated mounds. Mounds of the irrigated nests should be moister. Polystyrene beads from the nest and deep mound should appear in higher quantities in the irrigated mounds than in the non-irrigated mounds. Finally, the soil recovered from the irrigated mounds should be richer in clay than non-irrigated mounds. This is because newly built soil tends to be enriched in clay.


Soil moisture

Irrigation began in December and continued until mid May. Deep soil moistures outside the dams (Deep soil - 8 m) did not differ between irrigated and controls, but deep soils near the mound were elevated by the irrigation regime (Deep soil - 4 m) , while deep soil moistures.

deep soilcolony and deep mound

In the nest (right, top), moisture was higher in the irrigated mounds, and this was true for moisture deep withint the mound as well (right, bottom).

top and north

Moisture in the superficial parts of the mound was also higher in the irrigated mounds than for the non-irrigated mounds. It's noteworthy that this increased moisture persists into the dry season and after the irrigation regime has ended.

The first prediction is therefore confirmed. Adding extra percolated water produces moister deep soils adjacent to the colony, a wetter colony, and a wetter mound.


Bead recovery

bead recovery

Beads of four different colors were placed in four different locations in the mound and nest. Red beads were placed in the nest. Yellow beads were placed in the middle chimney. Blue beads were placed superficially (about 10 cm deep) on the north surface, about mid mound. Green beads were placed in a similar superficial location, on the mound's south face.

Eroded soil was recovered from the catchment trench, and then processed to recover the beads. This is done with a combination of drying, sifting and then flotation to recover the beads. For each mound, this involved processing about 250 kg of soil. Recovered beads were therefore beads that eroded from the surface of the mound. This could only happen with the physical transport of beads to the mound surface by termites.

Bead recovery was higher from the superficial sites. Roughly 40% of the beads planted were recovered. about 8% of beads planted in the middle of the mound were recovered, and roughly 6% of beads in the nests were recovered.

Comparing recovery rates between irrigated and non-irrigated mounds shows a higher rate of bead transport in the irrigated mounds compared to the non-irrigated mounds.

The second prediction is also therefore confirmed. Adding extra percolated water to the colony induces greater rates of soil transport to the surface.


Clay content

clay recovery

Termites preferentially transport the clay fraction of soil. Fresh build is richer in clay than the parent soil, and clay fraction of surface soils is enriched the closer to the mound the soil is.

Clay fraction was measured in samples of soil recovered from the catchment trenches. The data presented to the right represent average clay fractions for each batch of recovered soil.

When irrigation begins, clay fraction is enriched for soil recovered from the irrigated mounds.

As the rainy season proceeds, clay fraction for both the irrigated and non-irrigated mounds increases.

Late in the rainy season, clay fraction was persistently high for the irrigated mounds.

The third prediction is also therefore confirmed. Adding extra percolated water to the colony increases soil transport to the surface.


Termite pages

Termite home



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


Team Omatjenne