Termite mounds harness diurnal temperature oscillations for ventilation

Abstract

Many species of millimetric fungus-harvesting termites collectively build uninhabited, massive mound structures enclosing a network of broad tunnels that protrude from the ground meters above their subterranean nests. It is widely accepted that the purpose of these mounds is to give the colony a controlled microclimate in which to raise fungus and brood by managing heat, humidity, and respiratory gas exchange. Although different hypotheses such as steady and fluctuating external wind and internal metabolic heating have been proposed for ventilating the mound, the absence of direct in situ measurement of internal air flows has precluded a definitive mechanism for this critical physiological function. By measuring diurnal variations in flow through the surface conduits of the mounds of the species Odontotermes obesus, we show that a simple combination of geometry, heterogeneous thermal mass, and porosity allows the mounds to use diurnal ambient temperature oscillations for ventilation. In particular, the thin outer flutelike conduits heat up rapidly during the day relative to the deeper chimneys, pushing air up the flutes and down the chimney in a closed convection cell, with the converse situation at night. These cyclic flows in the mound flush out CO2 from the nest and ventilate the colony, in an unusual example of deriving useful work from thermal oscillations.

Keywords: ecosystem engineering; niche construction; termite mound; thermodynamics; ventilation.

Fig. 1.

Mounds of O. obesus. Viewed from (A) the side, (B) top, and by (C) cross-section. Filling the mound with gypsum, letting it set, and washing away the original material reveals the interior volume (white regions) as a continuous network of conduits, shown in D. Endocast of characteristic vertical conduit in which flow measurements were performed, near ground level, toward the end of flutes, indicated by the arrow (E).

Fig. 2.

Diurnal temperature and flow profiles show diurnal oscillations. (Top) Scatterplot of air velocity in individual flutes of 25 different live mounds ($•$). Error bars represent deviation between upward and downward $\approx$ 1.5-min flow measurements. The dashed red line is the average difference between temperatures measured in four flutes and the center (at a similar height), $\mathrm{\Delta }T$, in a sample live mound (Representative error bar shown at left). (Middle) Corresponding flow and $\mathrm{\Delta }T$, continuously measured in the abandoned mound. (Bottom) CO₂ schedule in the nest ($•$) and the chimney 1.5 m above ($•$), measured over one cycle in a live mound (Movie S1).

Fig. 3.

(Top) Thermal images of the mound in Fig. 1A, during the day and night qualitatively show an inversion of the difference between flute and nook surface temperature. Bases of flutes are marked with ovals to guide the eye. (Middle and Bottom) Mechanism of convective flow illustrated by schematic of the inverting modes of ventilation in a simplified geometry. Vertical conduits in each of the flutes are connected at top and in the subterranean nest to the vertical chimney complex. This connectivity allows for alternating convective flows driven by the inverting thermal gradient between the massive, thermally damped, center and the exposed, slender flutes, which quickly heat during the day, and cool during the night.

Fig. 4.

Temperatures along the center of healthy, $\sim$ 2 m tall, mound at three heights: “nest” ($\sim$ 30 cm below ground), “middle” ($\sim$ 50 cm above ground), and “top” ($\sim$ 130 cm above ground), and in bases of flutes at four cardinal directions. Error in values along the center is $\left(\pm 0.4°\text{\hspace{0.17em}C}\right)$, and $\left(\pm 1°\text{\hspace{0.17em}C}\right)$ in the flutes. The independence of behavior on cardinal direction shows direct solar heating is not of primary importance.

Fig. 5.

(Left) A hollow conical sample from a mound flute. (Right) Flow velocity as a function of back pressure measured by sealing the bottom and pulling air into the sample (Top), and loss of combustible gas by diffusion in same sample (Bottom).