Termite nests have been widely studied as effective examples for ventilation and thermoregulation. However, the mechanisms by which these properties are controlled by the microstructure of the outer walls remain unclear. Here, we combine multiscale X-ray imaging with three-dimensional flow field simulations to investigate the impact of the architectural design of nest walls on CO 2 exchange, heat transport and water drainage. We show that termites build outer walls that contain both small and percolating large pores at the microscale. The network of larger microscale pores enhances permeability by one to two orders of magnitude compared to the smaller pores alone, and it increases CO 2 diffusivity up to eight times. In addition, the pore network offers enhanced thermal insulation and allows quick drainage of rainwater, thereby restoring the ventilation and providing structural stability to the wet nest.

Here, we use multiscale X-ray imaging combined with three-dimensional flow field simulations ( 27 ), borrowing techniques from research in subsurface flows in rocks ( 28 ), to provide new insights into the micro- to macroscale architectural design of the outer wall of two non–fungus-growing termite nests from different locations in Africa. Although the materials used for nest construction in the two selected nests are considerably different, we show similarities in the microscale structural features of the outer wall and their role in providing effective CO 2 exchange to the atmosphere and thermal insulation to the inner parts of the nests. We also demonstrate that these microscale structural features help in rapid water drainage after rain and during nest construction from the moist soil, providing both structural stability and ventilation.

CO 2 exchange is more critical in non–fungus-growing closed nests such as Trinervitermes geminatus termite nests that have no large millimeter-scale openings in the outer wall and lack specific aeration tunnels ( 9 , 22 , 23 ). The outer wall contains microscale pores that have been assumed to be disconnected in previous two-dimensional visualization studies ( 16 ) and is therefore considered to have no external opening to the atmosphere ( 10 ). It is unclear how these non–fungus-growing termites exchange CO 2 to the atmosphere for effective ventilation of their nest. In particular, the role of the three-dimensional microscale morphological features of the outer wall in controlling gas exchange and heat transfer is unknown. If the outer wall is porous, then are the pores connected and permeable? And if so, how do they contribute to air circulation or ventilation? In addition, does the porous structure of walls control the thermal insulation and the structural stability of the nest? Resolving these questions will bring us one step closer to understanding mechanisms that will be useful in designing energy efficient buildings ( 24 – 26 ).

There are two types of termite nest: fungus growing and non–fungus growing. In the former case, termites cultivate fungus for food in the interior fungus-comb chambers of the nest. However, the fungus results in enhanced levels of CO 2 in the nest ( 14 ). For the survival of the millions of termites living in the base chambers of the nest, the CO 2 must be dissipated to the atmosphere. This is mainly achieved by gas exchange through millimeter-scale external openings in the outer wall of the nest. Termites open and close these openings frequently in response to the amount of CO 2 accumulated in the nest and local breeze outside the nest ( 15 ). In the outer wall, in addition to the millimeter-scale openings, there are smaller pores that have also been hypothesized as a source of gas exchange for ventilation ( 11 , 12 , 16 – 18 ). The mechanisms by which termite nests achieve effective ventilation and control the nest temperature have been studied extensively in the past. These mechanisms include temperature-driven convection current ventilation ( 11 , 12 , 17 ), passive diffusion ( 11 ), external wind flow ( 15 , 19 ), metabolic heat ( 20 ), evaporative cooling ( 21 ), or a combination of any of these effects.

Many animals and plants have developed advanced physical structures and operational skills, which have inspired a number of design innovations ( 1 – 8 ). Among them, termite nests have long been investigated for ventilation and thermoregulation. Their structure maintains a stable temperature throughout the year and permits self-sustainable CO 2 exchange with the atmosphere for ventilation ( 9 – 13 ). These self-sustaining temperature and ventilation properties have been a key motivation for designing eco-friendly buildings.

RESULTS

We investigated two African non–fungus-growing T. geminatus termite nests (Fig. 1A), one from Nguekokh, Senegal (Fig. 1B) and the other from Kankan, Guinea (Fig. 1C), which are about 1100 km apart. Both nests were constructed by the same termite species. The termites in both nests have a body size of about 3- to 5-mm height and 4- to 14-mm length. We first imaged the excavated nests (Fig. 1, D and E) with a medical x-ray computed tomography (CT) scanner at a voxel resolution of 0.3 to 0.6 mm, which allowed us to investigate the complete structure of the nests in three dimensions nondestructively.

Fig. 1 Termite nest locations, excavation, and x-ray tomographic imaging. (A) Two sampling locations, Nguekokh (Senegal) and Kankan (Guinea)—marked by yellow circles, were selected for this study. The Senegal nest was taken from a region that is sandy on the surface (brownish in the satellite map), whereas the Guinea sample was taken from a more vegetated region. The map in this image is taken from Google Maps, with the data provider listed at the base of the image (Imagery 2017 Landsat/Copernicus, Data Scripps Institution of Oceanography, National Oceanic and Atmospheric Administration, U.S. Navy, National Geospatial-Intelligence Agency, General Bathymetric Chart of the Oceans, Map data 2017 Google). (B and C) Nests in the field in Senegal (B) and Guinea (C). (D and E) These nests were excavated from the field and brought to the laboratory for x-ray imaging. The yellow dashed lines in (D) and (E) show the boundary between the upperground and the underground portions of the nests. [Photo credit for parts (B) to (E): Christian Jost]. (F and G) The excavated nests were imaged in three dimensions nondestructively with a medical x-ray tomography scanner at a pixel resolution of 0.3 to 0.6 mm. The imaging plane was clipped vertically to show the interiors of both nests. Red and yellow represent solid material and the inner channels of the nest, respectively. (H) Histograms of the thickness of the outer walls of the nests, which was measured from the tomographic images (F) and (G) in the upper parts of the nests that were exposed to the atmosphere.

From the medical x-ray CT scanning, both nests appear to have a similar architecture (Fig. 1, F and G). In addition, the thickness of the inner solid walls and channel width is identical (Fig. 2). Here, the channels are defined as millimeter-scale openings in the nest, which are used for passages by termites. The outer walls of the nests (which separate the atmospheric air from the air in inner channels; Fig. 1, F and G) have a thickness of 11.1 ± 4.9 mm (mean ± SD) and 14.9 ± 5.6 mm (Fig. 1H) for the Senegal and Guinea nests, respectively. These values are approximately 1.4 to 2.1 times larger than the values obtained for the inner walls of the nests, which are 7.7 ± 3.3 mm and 7.2 ± 2.6 mm for the Senegal and Guinea nests, respectively (Fig. 2).

Fig. 2 Thickness profiles of the inner solid walls and channels of the nest. (A and B) Thickness maps of the Senegal nest, showing the width of inner channels (A) and inner solid walls (B). The regions close to the outer wall were not considered in this analysis. (C and D) Thickness maps of the Guinea nest, showing the width of inner channels (C) and inner solid walls (D). The semitransparent box indicates the parts of the nest that were underground. (E and F) Histogram of thickness maps of the inner channels (E) and the inner walls (F).

From the satellite map (Fig. 1A), it can be seen that the color of the surface soil at the two studied locations differs noticeably (Fig. 1A). The site in Senegal appears sandy (brownish color), whereas the site in Guinea is green with a vegetation cover, indicating a higher clay content and a larger moisture and nutrient retention capacity. This observation is consistent with our x-ray diffraction (XRD) analysis (fig. S1 and table S1), which shows the presence of a majority of quartz minerals with small clay fractions in the Senegal nest and a larger fraction of clay (kaolinite and illite) in the Guinea nest. We hypothesize that the higher clay fraction in the Guinea nest affects the thermal conductivity and other morphological properties of the nest.

Three-dimensional microstructure of the outer walls of the nests To investigate the microstructural features of the walls of the nest, we drilled multiple samples from the inner and outer walls of the nests (see Materials and Methods) and scanned them nondestructively using a high-resolution x-ray microtomography scanner at voxel sizes of 2 and 5 μm. The notable feature of both nests is the presence of large microscale pores in the walls (Fig. 3, A, B, D, and E). In the Senegal nest (Fig. 3, A and D), a network of large and small pores is observed. The small pores could be interparticle (intrapellet) pore spaces inside individual soil pellets collected by termites. The large pores could be interpellet pore spaces formed during construction when soil pellets were fused together. Here, a soil pellet refers to the soil material collected and shaped by a termite. The volume of pellets has been reported to be of the order of 0.59 ± 0.36 mm3 (equivalent to a radius of 0.52 ± 0.44 mm) for Odontotermes obesus termites that are similar to T. geminatus termites (29). If we consider a tetrahedral packing of pellets (considering pellets as spheres), then the radius of inscribed sphere in the pore space between pellets would be approximately 117 μm [=0.225R, where R is the radius of a pellet (30)], which is in the range of pore radii reported in Fig. 3G. Fig. 3 X-ray microtomography analysis of termite nests. (A and B) Two-dimensional grayscale cross sections of the x-ray microtomographic images of the outer wall of the Senegal (sample 1) (A) and Guinea (sample 1) (B) nests at a voxel size of 5 μm. Gray and white spots represent solid matrix and metallic elements [consistent with the XRD analysis], respectively, while black represents empty (void) space filled with air. In the Senegal nest (A), the smaller pores (intrapellet pore space) and the larger pores (interpellet pore space) are distinguishable, whereas the Guinea nest (B) shows only interpellet pore space, because of a larger fraction of clay indicated by shrinkage cracks in the solid matrix. (C) The solid part of the Senegal nest was dissolved in water to form a slurry in a glass vial, which was then air-dried without compaction, hereafter called Senegal random pack, and imaged with x-ray microtomography. (D to F) Three-dimensional images of the Senegal, Guinea, and Senegal random pack, with their color coding for pores, solids, and metallic elements. (G) Probability density function (PDF) and cumulative frequencies of the pore size of the samples shown in (D) to (F). We also show data (red) for a subset (SSc) taken from the Senegal nest sample in which the smaller pores were isolated for pore size comparison (refer to text and fig. S2 for further details). The vertical dashed gray line indicates the upper bound of the pore radii of smaller pores, obtained from subsets SSa-SSd. (H) Porosity-permeability relationship for the Senegal, Guinea, Senegal random pack, and four different subsets of isolated smaller pores of the Senegal nest. The error bars of the porosity are extracted from the porosity values of each slice along the z axis of the image. In general, the larger pores (as seen in the walls of the termite nests) are not expected to form if the nest material (e.g., soil particles) was packed randomly as in the case of a random sand pack (31, 32). To test this hypothesis, we prepared a slurry (semiliquid mixture of solid material and water) of the Senegal nest material by dissolving it in water in a glass vial, which was then stirred gently by hand using a thin plastic rod. The slurry was then air-dried without compaction and imaged with x-ray microtomography (Fig. 3, C and F). This air-dried sample will be referred to as the Senegal random pack. The images in Fig. 3 (C and F) show that only interparticle (smaller) pores are present in the Senegal random pack, with pore sizes similar to the smaller pores in the Senegal nest walls (compare the red and yellow lines in Fig. 3G). For this analysis, we isolated the smaller pores in the Senegal nest from the larger pores by taking a cubical subset where the pore space was entirely composed of smaller pores (fig. S2). The average size of these smaller pores in the Senegal nest is 19.3 ± 10.6 μm (mean ± SD), which is of the same order of magnitude as in the Senegal random pack (19.6 ± 6 μm). On the other hand, the average pore size in the complete Senegal nest sample (containing both smaller and larger pores) is 68.5 ± 63.4 μm, which is larger with a wider spread in the pore sizes (Fig. 3G). Like the Senegal nest sample, the outer wall of the Guinea nest also contains large pores (Fig. 3, B and E). Only a few small pores are observed in the outer walls of the Guinea nest, possibly due to the presence of a larger fraction of clay in the wall material (fig. S1 and table S1). However, we observe various microscale shrinkage cracks in the clayey material (Fig. 3B and fig. S3). The porosity (defined as ratio of the volume of void space in a sample to the total sample volume) of the Guinea nest wall, which is dominated by the contribution of large pores, is 15 ± 2.8% (mean ± SD). Here, we calculated the SD from the porosity values of each two-dimensional slice of an x-ray tomographic image along the axis of the sample. This porosity value is significantly lower than that of the Senegal nest wall 27.9 ± 2.9%. Despite this difference in porosity, the sizes of the pores in the Guinea nest wall (72.9 ± 44.5 μm) are of the same order of magnitude as observed in the Senegal nest sample (Fig. 3G). This similarity in pore sizes occurs due to the dominance of larger pores in both nest walls. In addition, both nest walls show a high percentage of connectivity of the pore space (97 to 98% and 92 to 93% of the pore space in the Senegal and Guinea samples, respectively), which is likely to play a crucial role in air flow and ventilation. For the connectivity analysis, we considered the connected voxels belonging to the pore space, which spanned across the full length of the sample (along an axis).

Flow and thermal properties—The role of larger microscale interconnected pores The interconnected larger (microscale) pores in the nest walls are formed either according to simple construction rules similar to the way ant colonies build their nests (19, 33) or as a consequence of physical constraints resulting from the natural way of putting pellets together. These larger pores have many advantages for the nest structure. First of all, the presence of interconnected larger pores results in enhanced permeability. Permeability is the property of a porous medium, which measures the ability of a fluid to flow through it. It is part of the coefficient of proportionality in Darcy’s law that relates pressure drop to fluid velocity (see Eq. 6 in Materials and Methods) and is expressed in the units of square meters (28, 34). The permeability of different samples of the Senegal nests, computed by solving the Navier-Stokes equation (see Materials and Methods), is in the range of 0.6 × 10−12 to 3.5 × 10−12 m2 (Fig. 3H). The computed flow fields are focused in large interconnected pores (Fig. 4, A and B), indicating higher velocities in large pores. To investigate the effect of small and large pores on the permeability and flow fields, we isolated many regions in various tomographic images of the Senegal nest wall by taking cubical subsets at locations where the pore space consisted entirely of smaller pores (fig. S2). The computed permeability of the smaller pores in the isolated subsets is in the range of 2.7 × 10−14 to 6 × 10−14 m2, which is one to two orders of magnitude lower than that in the complete nest sample (Fig. 3H). Qualitatively, the flow fields are uniformly distributed in the smaller pores, with fewer locations where the flow fields are focused with higher flow velocities (Fig. 4, D and E). Fig. 4 Flow fields and CO 2 diffusive fluxes. (A and B) Flow field simulation using Navier-Stokes equation on the Senegal nest (sample 1), showing normalized pressure (A) and velocity (B) fields. The black arrow in (B) shows the inlet face and the direction used in all simulations (A to K). The boundary condition used in the flow simulations corresponds to a pressure gradient of 1 Pa/mm. A higher pressure is imposed on the inlet face of the sample. (C) CO 2 flux visualization in the porous matrix of the Senegal sample, which was estimated by solving Fick’s second law. The boundary condition used in this simulation corresponds to a 5% change in CO 2 concentration (relative to an atmospheric CO 2 concentration of ~0.0164 mol/m3) across each 1 cm thickness of the nest wall. (D to I) Similarly, these simulations were conducted on the subsets of the Senegal nest containing smaller pores and the Guinea nest. Pressure (D), velocity (E), and CO 2 flux (F) in the subset (SSc) containing only smaller pores in the Senegal nest. Pressure (G), velocity (H), and CO 2 flux (I) in the Guinea nest (sample 1). Pressure (J) and velocity (K) fields in the Senegal random pack. (L) PDF of the logarithm of the pore-scale velocities in different samples. We also observe that the flow fields are focused in the larger pores of the outer wall of the Guinea nest sample (Fig. 4, G and H). Although the porosity of the outer wall of the Guinea nest is significantly lower than that of the Senegal nest, the permeability values of 1.1 × 10−12 to 2.3 × 10−12 m2 are in the range of the Senegal samples (Fig. 3H). The permeability of the outer walls of the Guinea nest would be close to zero without larger pores, because the clayey soil with some microscale cracks (fig. S3) is approximately impermeable, providing no air ventilation. Both the Senegal and Guinea nest samples show a wide distribution of local velocities, with stagnant flow zones at lower velocities predominantly in the Senegal sample (Fig. 4L). The subsets containing smaller pores in the Senegal nest also show stagnant zones (fig. S2), which suggests that the presence of dead-end pores and the clay-blocked throats between neighboring pores could cause this behavior. On the other hand, the Senegal random pack (Fig. 3, C and F), which has comparatively smaller pores (Fig. 3G), has a more uniform flow field (Fig. 4, J and K), without having significant stagnant zones (Fig. 4L). This behavior indicates the presence of a well-connected pore space in the Senegal random pack. To estimate the apparent CO 2 diffusivity and CO 2 flux in the porous matrix of the outer walls of both nests, we solved for transport using Fick’s second law in the pore spaces of these samples (see Materials and Methods and Fig. 4, C, F, and I). The apparent diffusivity of the outer walls of the Senegal nest is in the range of 3.9 × 10−7 to 7.2 × 10−7 m2/s, which is a factor of three to eight larger than that of the smaller pores in the outer wall (8.8 × 10−8 to 1.3 × 10−7 m2/s), indicating that the interconnected larger pores in the Senegal nest enable passive diffusion of the CO 2 . The apparent diffusivity of the outer wall of the Guinea nest (1.9 × 10−7 to 3.1 × 10−7 m2/s) is similar to that of the Senegal nest, indicating that the larger pores have a dominant role in controlling CO 2 exchange. The CO 2 flux streamlines also show similarities in the Senegal and Guinea nest samples (Fig. 4, C and I). The above approximations consider only diffusive CO 2 exchange. To account for advection due to wind flow outside the nest, we estimate the Péclet number for mass transfer (Pe m ), which is the ratio of advective-to-diffusive transport flux over the thickness of the outer walls (see Materials and Methods). Considering a wind velocity of 0 to 5 m/s outside the nest and using the values of apparent diffusivity and permeability from numerical simulations, we obtain a Péclet number in the range 0 to 7.35 and 0 to 6.06 for the Senegal and Guinea nest samples, respectively. For higher wind velocities outside the nest, when Pe m > 1, advective transport dominates over diffusion, whereas for Pe m < 1, diffusive exchange of CO 2 is dominant. The value of Pe m = 1, at which advection and diffusion contribute equally, corresponds to wind velocities of 1.8 to 5 m/s and 2 to 2.3 m/s for samples of the Senegal and Guinea nests, respectively. Overall, these calculations show that both diffusive and advective transports of CO 2 are important depending on the wind velocity outside the nest. We also compute thermal conductivities of each sample by performing steady-state simulations on the heterogeneous solid matrix (see Materials and Methods). The apparent thermal conductivity (here defined as the overall thermal conductivity of a porous medium in the presence of negligible conducting void space) of the outer wall of the Senegal nest, ~4.18 W/m/K, is 16.6% smaller than that computed on a subset containing smaller pores, ~5.01 W/m/K. This difference indicates that the larger pores and higher porosity provide air insulation to the nest. The simulated heat flux streamlines also show small differences (Fig. 5, A and B). On the other hand, because of a higher clay fraction in the walls of the Guinea nest, the apparent thermal conductivity is considerably lower, 1.58 W/m/K, therefore further enhancing thermal insulation by reducing thermal exchange between inner and atmospheric air. The thermal streamlines also show significantly lower heat transfer for the same temperature drop across the Guinea sample (Fig. 5C). The presence of a larger fraction of metallic elements in the Guinea nest (compared to Senegal nest samples) could affect this analysis; however, their poor connectivity and the presence of a larger amount of connected clay minerals could explain the observed behavior. Fig. 5 Heat flux simulations. Heat flux streamlines colored by the magnitude of the heat flux obtained for an applied temperature gradient of 1 K/cm in the Senegal nest (A), smaller pores in the Senegal nest (SSc) (B), and the Guinea nest (C). The black arrow in (A) shows the direction of temperature boundary condition used in all simulations (A to C). The light and dark gray in the solid matrix show sand grains and clay, respectively, whereas white represents metallic elements. To compare the advective heat transport through pores with heat conduction through the solid matrix, we calculated the Péclet number for heat transfer (Pe h ) over the thickness of the outer walls (see Materials and Methods). Using the values of apparent thermal conductivity from numerical simulations, we obtain a Péclet number for heat transfer in the range of 0 to 8.43 × 10− 4 and 0 to 1.45 × 10−3 for the Senegal and Guinea nest samples, respectively. These low values of the Péclet number indicate that heat transfer is controlled by conduction through the solid material (for wind velocities in the range of 0 to 5 m/s). From the above analysis, it is clear that the large interconnected pores in both nests are critical for (i) thermal insulation of the inner parts of the nest and (ii) ventilation and CO 2 exchange. For a higher Péclet number, when the outer wind velocity is high, advective transport dominates over diffusion. In this case, again, the larger pores in the nest wall help the flow of air due to their higher permeability values. We also note that the porosity of the walls of a fungus-growing nest reported in a previous study (12) is considerably larger (37 to 47%) compared to the values observed here. In the future, a similar analysis can be conducted on such high porosity nest walls to compare the pore sizes and flow and thermal properties. Further, for an accurate prediction of effective temperature control of termite nests, it is also important to consider the thermal capacity of the system, particularly the difference between the thermal capacity of the solid matrix and air. However, in this study, our focus is on the effect of small-scale features and how they affect the ventilation, thermal, and drainage properties of the nest walls.

What happens after rainy periods? The pores in the nest walls also play a crucial role in the stability of the wet nest during its construction and in ventilation during rainy days. After a short episodic period of rain, the rain water can spread out into the porous matrix. The water from larger pores near the nest surface can be pulled into the smaller pores of the inner parts of the water-wet (hydrophilic) nest material until capillary equilibrium is reached. Under this scenario, the larger interconnected pores become available for gas exchange rapidly, helping in ventilation and further drying of smaller pores that retain water due to capillary forces. In the case of prolonged periods of rain, the pores of the nest wall can be filled with water and block the passage of air. To investigate this situation, we conducted a water imbibition experiment on a small sample (sample 2) of the Senegal nest wall. We first imaged the sample under dry conditions, which shows the interconnected larger pores (Fig. 6A) similar to those observed in sample 1 in Fig. 3A. We then injected water from the top of the sample. It was allowed to drain for approximately 2.5 hours and was imaged again. The evaporation during scanning was restricted by wrapping a thin sheet of cling film around the sample. The drained sample is shown in Fig. 6B in which air and water are black and white, respectively. We observe that air is present mostly in larger pores, and the smaller pores remain water-filled. This observation is supported by the pore occupancy analysis shown in Fig. 6G in which the water occupies smaller pore radii, whereas larger pore radii are occupied by connected and disconnected air. Here, the connected air (blue in Fig. 6, C and D) was separated from disconnected air clusters (different colors in Fig. 6C) by performing label analysis and isolating the connected air cluster that spanned across the sample. The disconnected air occupies a range of pore radii with a majority of intermediate sizes (Fig. 6G). The air in these pores can become disconnected because of its entrapment in dead-end pores during water injection (35, 36). It should be noted that some large air-filled pores at the boundary of the image may appear disconnected because of image processing artifacts of boundary voxels (e.g., shown in green in Fig. 6C). Fig. 6 Drainage of the Senegal nest. (A) A two-dimensional grayscale cross section of the dry Senegal sample (sample 2). Air is represented by black, and the solid phase is represented by gray. (B and C) The sample was saturated with 0.16 ml of potassium iodide (KI)–doped water (for improving x-ray absorption contrast, see Materials and Methods) from the top, which was then allowed to drain. The x-ray tomographic image represents the sample at approximately 2.5 hours (B). An axis connectivity analysis was performed on the air phase (C). For the connectivity analysis, the connected voxels belonging to the pore space that spanned across the full length of the sample (along an axis) were considered. Here, light blue represents the connected percolating air cluster spanning across the length of the sample. Various other colors show disconnected air clusters. Light green at the base could be connected; however, because of its presence at the border of the image, it is assigned as a disconnected phase during image processing. (D) Three-dimensional visualization of the percolating air cluster in the drained sample. (E and F) Flow field simulations on the air phase of the drained sample showing pressure (E) and velocity (F) fields. (G) Pore occupancy analysis of the drained sample. The plot shows the fraction of the pore space occupied by water [white in (B)], connected air cluster [light blue in (C) and (D)], and disconnected air [various colors in (C)]. (H) PDFs of the velocity fields for the dry and the drained Senegal sample. The drained sample shows a considerable amount of stagnant regions with flow focused through fewer open paths. There are two possible reasons for the presence of air in larger pores. First, it is possible that the some air clusters become isolated and trapped during water imbibition, which typically occurs in larger pores for water-wet (hydrophilic) porous materials (35). The second possibility is that if the pore space of the sample was completely filled with water, then the water-filled pores could be invaded by percolating air. In this case, air as a nonwetting phase would fill larger pores first due to their lower resistance to air invasion. The higher permeability of the larger pores helps the water to drain at a higher rate. The water in the smaller pores (white in Fig. 6B), which is held because of strong capillary forces, would take many hours to dry (37). The air saturation analysis shows that the 52.9% of the pore space is occupied by air, of which approximately half (51.6% of the air phase) is connected and available for air circulation and ventilation. The permeability through this available space is 2.6 × 10−14 m2. The flow field simulations show that the majority of flow occurs through a single flow channel (Fig. 6, E and F) and the rest of the available space is stagnant to flow (Fig. 6H). Although the permeability of the drained sample is approximately one order of magnitude smaller than that of the dry sample (8.5 × 10−13 m2), the percolating air path (connected across the sample) is sufficient to start slow ventilation and drying of the rest of the wet nest wall. This analysis shows that the presence of larger pores in the outer walls of the nest helps to reestablish ventilation and drying of wet walls after rainy periods. We also performed percolation simulation analysis to quantify the invasion and percolation of air in a completely water-filled sample, which is explained in fig. S7.