Water homeostasis. 1

water potential experiment

In contrast to nest temperature, nest humidity appears to be tightly regulated.

Humidity regulation is one of the major claims of the "air conditioned termite nest idea", and humidity control indeed is the major factor in air conditioning in buildings.

As it turns out, the mound has only a tangential relation to regulation of nest humidity. Where the mound is involved with colony water homeostasis, it is in a surprising and unexpected way.


Measuring nest humidity

water potential sensors

Humidity in the nest is determined by the moisture in the soil. The two come into equilibrium, so that nest humidity is essentially equivalent to the local soil wter potential.

Water potential can be measured by using plaster of paris blocks that have sensors embedded in them. When these are placed into mounds, termites build around them, embedding them in local soil. The plaster then absorbs or releases moisture until it comes into hydric equilibrium with the soil. One can then measure the water potential of the plaster, which will be equivalent to the water potential of the soil.

Our initial experiment measured water potentials in the same two mounds (OM123, alive, and OM126, dead) that were used for temperature measurements, and the placement of sensors was the same.

sensor array

In the soil, moisture was measured at two depths, shallow (-10 cm) and deep (-1 m). The soil sensors were laid out in a grid so that the moisture in the surrounding soil could also be monitored.

sensor array2

With the help of the staff at the Omatjenne Research Farm, we followed nest and soil moisture for about two years.


shallow soil mositure

Annual variation in soil water potentials

Soil moisture at Omatjenne is driven by strong seasonal patterns of rainfall, with winter drought and a summer rainy season consisting largely of convective disturbances that produce episodic torrential rainfalls.

Soil moistures were measured once per day. We express water potential as a color mao (blue is wet, red is dry) that plots day of the week on the vertical axis and week of the year on the horizontal axis.

The episodic nature of the rainfalls is evident in the patterns of moisture in superficial soils (10 cm deep, plot to the right). The plots are highly variegated, signifying rapid saturation and equally rapid drying.

Deep soil moisture was measured during a different time frame, but is more extensive. Three things are noteworthy.

deep soil moisture

First, the seasonal variation of deep soil mosture is less variegated than that of superficial soil moisture. This simply means that deep water sources are gradually charged during the rainy season, and gradually discharge during the dry season. It also means that deep soil can serve as an effective water reservoir that termites can exploit during the rainy season.

The second noteworthy observation is how spatially heterogeneous deep soil water is. Near OM123 (ST 1 -1m and ST2 -1 m), for example, there is a substantial area of persistently quite moist soil. Near OM126, it is dryer.

Third, there are anomalous periods duriong the dry season when previously dry soil suddenly becomes moist and then dries again ( ST2D), or become persistently moist (ST1 D, ST4 A, ST6 C and ST6 D). This reflects local termite activity, and this is an important clue to the relationship between nest humidity regulation and the mound.


Moisture in the mound and nest

Within the mound and nest, the annual march of moisture differs somewhat from the undisturbed soil.

OM123 moisture om126 mositure

The first point of difference is in the moisture of the mound (TS, SS, NS and MC). This exhibits the same seasonal variation as was apparent in the soils. Indeed, some of the superficial sites, such as TS, show even stronger seasonal variation. This is to be expected because mound temperatures are more strongly driven by environmental circumstances.

The second point of difference is the temporal variation of deep mound moisture. Where the rise and fall of moisture in deep undisturbed soils was temporally uniform, the annual rise and fall of deep moisture in the mound varies in onset. This is particularly evident in OM123. This cannot be attributed to rainfall. It is caused by termote activity within the mound.

Finally, note that nest moisture of OM123 is very uniform throughout the year, despite being surrounded by soils that vary in moisture considerably through the year. The situation in OM126 is less clear. Nest moisture there is highly variable and patchy. .

sensor placement

OM126 is a special case. At the time sensors were placed, the mound was assumed to be similar in structure to OM123. We discovered only later that this colony had shifted its location laterally so that the LN sensor was in fact grazing the outer margin of an abandoned nest. This is why the soil mositure there is so patchy.


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