Nest temperature. Is it regulated?

Hot mound?

In the "air-conditioned termite nest", among the nest properties supposedly regulated is nest temperature.

The supposed flows of air between nest and mound carry heat in spent air to the mound surface, where it can be lost to the environment. The beauty of this mechanism is its capacity for self-regulation: if nest temperature gets too high, this enhances the buoyant forces that drive air circulation, which will transport heat to the surface more rapidly.

The regulation of nest temperature in the "air-conditioned termite nest" has inspired an entire school of biomimetic architecture. Particularly in the architectural literature, one finds many claims for how tight this regulation is. Here are some examples:

“The Eastgate building [a famous example of a "termite-inspired" design] is modeled on the self-cooling mounds … that maintain the temperature inside their nest to within one degree of 31 °C, day and night … “

–“Indeed, termites must live in a constant temperature of exactly 87 degrees (F) to survive.”

In fact, nest temperature of Macrotermes michaelseni is not regulated at all.

The coupling problem

thermal coupling

When a nest is uncoupled from a heat sink, as in an arboreal nest, the nest temperatures fluctuates strongly with daily variation in the drivers.

thermal coupling

When the nest is coupled to a large heat sink, as in a subterranean nest, a steady temperature can result without regulation. This is thermal damping, not regulation.

The major problem for the regulation of nest temperature is simply this: nest temperature should be strongly coupled to soil tempersture.

If nest temperature differs from soil temperature, this will drive a flux of heat between the nest and soil that will equalize the temperatures. If the nest is warmer than the soil, the nest will lose heat to the soil, and vice-versa if the nest is cooler.

The problem comes in with the very large thermal capacity of the soil, which allows it to gain or lose considerable heat without experiencing a change of temperature. To regulate the nest temperature, the termites will have to mobilize sufficient heat to offset these fluxes.

A few calculations show just how daunting the problem is. One can estimate a nest's thermal capacity from the volume of the nest, the specific heat of the parent material and the void space in the mound. By our estimates, this amounts to about 2,500-3,500 kJ K-1, meaning that roughly 2.5-3.5 megajoules of energy are needed to raise the nest temperature by one degree Celsius. The metabolic rate of a typical Macrotermes colony has been estimated at 50-100 W, which amounts to an hourly metabolic heat production of 200-400 kilojoules. If the entire hourly heat production was concentrated into an instantaneous flash, this would be sufficient to raise nest temperature by about a tenth of a degree.

In reality, the temperature elevation would be considerably smaller. It is then sobering to consider that the nest is coupled to the surrounding soil. The thermal capacity of a plug of soil 5 m in radius and 2 m deep (excluding the volume occupied by the nest is roughly 120 MJ K-1, about three orders of magnitude larger than the nest's. A "flash" of the metabolic heat produced in an hour would alter this temperature by less than a hundred microkelvins.

The conclusion is hard to avoid: there is simply not enough metabolic energy flowing through this system to budge temperature much. This means there will not be sufficient energy available to regulate temperature.

This does not mean nest temperatures will not be steady. The effect of embedding the nest in a large thermal sink like the soil will be to damp daily fluctuations of temperature. Nest temperature may still be steady, even if it is not regulated. The only means of settling the question is with an experiment.

TOP

Measuring nest temperature

Temperatures are easily measured and logged with portable devices known as iButtons.

iButton positions

We placed several of these into two mounds and in the surrounding soil, and let them log temperatures at 4 hour intervals for one year. One of the colonies turned out to be dead, so we have legitimate results for only one colony. Here are the complete results for all sensors. The live mound is on the left (OM123) and the dead mound is on the right (OM126).

results

The results show that mound temperature and soil surface temperature vary strongly through the day and through the year. As expected deep soil temperatures are strongly damped through the day. Nest temperatures for both follow soil temperatures quite closely.

TOP

Is nest temperature regulated?

The answer appears clearly to be no. Let us focus on the living colony (OM123), and compare its annual marck of temperature with that of the adjacent deep soil at 1 meter depth. Here is the plot.

annual temperature

Clearly, nest temperature tracks soil temperature closely. The plot below is a periodogram that was generated from a Fourier analysis of the temperatures. There is considerable oscillation of temperatures at long periods, with oscillation strongly weighted to the annual. This is just a way of saying that there is a large annual variation of both nest temperature and deep soil temperature. For both nest and deep soil, the range of temperatures through the year is about 10 degrees for the soil, and 17 degrees for the nest.

The conclusion is clear. Nest temperature is strongly coupled to soil temperature. The large thermal capacity of soil provides considerable damping of daily temperature, but little damping of annual temperature. Nest temperature is not regulated. It is not even particularly steady over the year.

Daily variation of nest temperature is greater than daily variation of adjacent soil temperature however, as shown by the "spile" in the periodograms at a period of one day. What's happening there?

TOP

Mound temperature also drives nest temperature

The nest is thermally coupled to both deep soil temperature and mound temperature. A look at the annual and daily march of mound temperatures (above) shows they vary strongly through the day. The reason is simple: the mound is more susceptible to daily heating from the sun, and loss of heat to the night sky during the night. The most extreme temperatures we recorded were in the mounds.

This is demonstrated by a Fourier analysis similar to the one done above for nest and deep soil temperatures. Here, we compare temperatures of the mound's north face (the one most strongly driven by insolation), in the middle of the mound, and the nest.

mound temperature

Notice the large spikes at a period of 1 day for the mound temperatures. This large amplitude of daily nest temperatures is imposing on the nest temperature.

The mound, far from playing a part in regulation of nest temperature, actually destabilizes nest temperature somewhat. Nest temperature would be steadier through the day if the mound was not there.

TOP

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

Omatjenne

Team Omatjenne

Sponsors