Animal Architecture: The Mechanics Behind Some of Nature’s Most Ingenious Structures

Adi Orlov, Berine Wehbeh, Yuheng Liu, Sara Fraser

Abstract

This essay compiles a selection of different animal homes and the mechanics behind their assembly, stability, and functioning. The more you contemplate the living conditions of animals, the more questions arise. For everything to function the way it does, there must exist a particular explanation for the mechanical and material properties of these habitats. In fact, most animal homes are so intricate that every single step in the building process has a complex mechanical reasoning behind it, developed over the constant evolution of species. Accordingly, this paper aims to highlight said mechanical characteristics. The sections each tackle the architectural and mechanical properties of a specific type of animal habitat, namely the beaver dam, the bird nest, the beehive, and the termite mound.

Introduction

It is no secret that different animals have vastly different living environments. Even within the same species, there is a noticeable disparity in the shelters built by animals that are found in varying regions. Specific animals have individual needs which are reflected in the homes they construct. These homes serve as shelter from the obstacles faced by the species, such as predators and harsh environmental conditions. Without this protection, the animals, regardless of their position on the food chain, would be left at the mercy of larger threats. Consequently, animals tend to design their homes in the most mechanically sound and efficient way possible using the raw materials available in their living area, as these dwellings play quite a considerable role in the survival of the faunae (Fagundes et al., 2021).

Proper use of mechanics in the construction of homes provides a form of evolutionary advantage for the builders. While the mechanisms of animal homes cannot be generalized, this essay attempts to explain the physics behind four distinct types: beaver dams, bird nests, beehives, and termite mounds. In the corresponding sections, various aspects of each of the aforementioned shelters are analyzed.

Beaver Dams

Beavers are found all around the world and impact the environment in many ways. They are known as ecosystem engineers; they can modify the environment surrounding them by transforming raw materials into useful ones via the use of mechanical work. These fascinating engineers build dams, a structure built with surrounding material to create a proper inundated shelter away from predators, a place to gather food and keep during winter or transportation corridors that are inaccessible to predators. By doing so, beavers modify the topography of their surroundings, specifically water-related characteristics such as flow, height, and temperature (Johnston & Naiman, 1990; Majerova et al., 2015; Nyssen et al., 2011). The following sections will explore the relationship between dam structure and water properties and explain how they are interlinked.

How Beaver Ecology and Topography Affect Structure

Beavers do not always build dams. The necessity of building a dam is to have appropriate water level height to cover the entrance of the lodge and/or to be able to swim and store food within a pond (Fig. 1). This is done specifically in fluctuating, shallow or narrow currents (Macdonald et al., 2008). Beavers build dams by collecting sticks and placing them on the bottom of the stream until a sufficient height is reached and sealing the dam with mud or other sediments. Other structures that beavers build which will not be further explored are lodges, which contain the living quarters above the water level and are only accessible from below the water.

The structure of the beaver dam is dependent on multiple variables. Beaver ecology affects dam height and predetermines how tall their structure should be. The two most common species of beavers that are found around the world are Castor canadensis and Castor fiber, and they differ in the way they build their structures as the latter has more restricted building activity (Gurnell, 1998). Furthermore, different types of dams exist, and each has a different size. Primary dams are on average 0.46 m taller than that of secondary dams, possibly due to their difference in roles since primary dams are used to create a pond large enough and deep enough to cover the entrance of the lodge and submerge the food cache (Fig. 1) whilst secondary dams are used to create a water level large enough for protection from predators and transport of materials (Hafen et al., 2020). The topography of the region also plays a role in determining the structure of the dam. Research done by Hafen et al. (2020), whose goal was to characterize the relationship between dam structure and geomorphologic parameters, demonstrated that certain parameters, such as estimated discharge for a flood, the width of valley bottom at beaver dam location, and channel slope, affected the height of the dams in a statistically significant way. Their research showed that the larger the channel slope of where the dam will be built the taller the dam will be while the height decreases with stream power and the width of the valley bottom at the beaver dam location (Hafen et al., 2020).

Beaver dam length is affected by the environment that surrounds it. This is due to the materials that can be found in close proximity to the dam-building site; if more appropriate material can be found without the threat of predators, beavers will have more resources to build longer dams. A study done by Ronnquist & Westbrook (2021) that focused on the physical properties of dams showed that dams in wetlands that have peat soil were significantly longer than dams in mineral sites and that sites that had aspen growing within 100 m were additionally longer than their non-aspen comparisons. Figure 2 represents the difference between dam length in the presence of peat and minerals and how the presence of aspen directly correlates to longer dams. This demonstrates how these ecosystem engineers can use information from their surrounding and alter their construction of the dam based on the information they have collected.  

Beaver Dams and Influence on Water Properties

A beaver dam can exert its influence on the hydrological properties of the current of water that it modifies by building such structures. A beaver dam modifies the water storage upstream of the dam, as it seeks to increase the water level and produce a habitable pond to survive. This phenomenon is all directly related to dam water tightness, dam size, stream depth, flow volume (Westbrook et al., 2013). Much research has been done to find the relationship between beaver dam structure and pond characteristics, such as size, flow rate, etc. Woo & Waddington’s (1990) research provided a template for categorizing six different classes of beaver dams and their ability to retain water. They found that the overflow class created the largest storage-water surface potential, allowing more water to be collected (as cited in Westbrook et al., 2013). Furthermore, the research of Johnston & Naiman (1990) aimed to find how pond characteristics changed with the introduction of beaver dams. They concluded, after 40-year research, that the number of pond sites increased dramatically during their research and that all of them were still distinguishable after 40 years, signifying that beaver dams can retain water to their appropriate needs with little failure. Figure 3 shows to which significance pond areas increased with the presence of beavers. In 1940 with the introduction of beavers in Kabetogama Peninsula of Voyageurs National Park in Northern Minnesota, the cumulative pond area was less than 500 ha but this number increased by more than six times by 1986, proving that beavers dams can increase storage levels of water (Johnston & Naiman, 1990). This can be particularly valuable as a source of water for an ecosystem in the vicinity of the beaver dams especially during droughts, which are increasingly frequent with the current rate of climate change (Ronnquist & Westbrook, 2021).

Similar to how beaver dams allow ponds to store water and be an important source of water during droughts, their ability to keep more water also permits them to mitigate the effects of climate change via its impact on floods. The water storage of dams, as previously explained, is dependent on multiple variables such as dam height relative to the height of the riverbank, the width of the dam, and other properties of the dam concerning the geomorphic and hydrological properties of the river (Westbrook et al., 2013). However, this increase in water storage also affects the downstream transmission of water, which decreases flow velocity, essentially slowing down the effects of the flood (Ronnquist & Westbrook, 2021). This phenomenon is further shown in the research of Nyssen et al. (2011) whose objective was to determine beaver dams’ influence on water discharge by measuring how much water is released over a year time before and after the introduction of beavers to parts of Belgium. They concluded that there was a lowering of discharge of water peaks as a result of this introduction of beavers and that these structures set a limit to how fast the water would be able to flow through it, delaying the water movement by one day. In addition, these beaver dams do not fail during times of extreme flooding, showing consistency in how it is able to buffer the flow of water through it (Ronnquist & Westbrook, 2021).

This increase in water retention that is observed due to the dam-building process also affects the water temperature of the pond. Beaver dams increase the temperature of the water by a statistically significant amount. This is proven by the study done by Majerova et al. (2015) in which one of the objectives was to measure how the temperature varied within a mountain stream after the introduction of beavers and the subsequent beaver dam construction. Figure 4 shows how the temperature varied between different regions of the stream at different dams in Curtis Creek in northern Utah, where the measurements were being taken, with the 24h moving average (Fig. 4b) showing that the dams all presented a positive variation in temperature. Majerova et al. (2015) concluded that the temperature of the water increased by 0.38 ̊C after the introduction of beavers, which shows that beaver dams do have an influence on water temperature. This effect can be partly due to the total water surface area change observed after the beaver introduction, which had more than doubled, as the increase in water surface area would allow more water to be exposed to the sun (Majerova et al., 2015).

Beavers demonstrate great engineering capabilities in modifying the environment around them through the construction of dams, and this itself is influenced by the environment as well. The topographic properties of the region affect the structure of the dam being build which in turn affects the resulting hydrologic properties of the current. These modified properties should be further examined to determine if it would be possible to use at a larger scale to counter the effects of climate change, as early research shows many signs of promise in this ongoing problem. Similarly, as to how beavers can take resources from their environment and transform it into useful material, birds are able to do similarly with their construction of nests.

Bird Nests

While bird nests may seem to many to be a haphazard assembly of parts, the feathered creatures are in fact some of the smartest architects on the planet. Not all birds make their own nests, but those that do often craft them with tremendous complexity. The exact kind of nest varies greatly from one species to another, with the structure of seemingly randomly packed twigs most commonly associated with the term bird nest. Though these homes might play vastly different roles in the lives of different types of birds – from a place adequate to comfortably lay eggs, to one suitable for the raising of offspring, and finally as an essential component of some mating practices – they all have one thing in common. All such nests are structurally sound because of the mechanics behind their fabrication. Caliology, the study of birds’ homes, still has a long way to go before it can accurately explain all the properties of bird nest mechanics, however, all the existing research agrees on the fact that these relatively simple dwellings are actually utilitarian constructions.

Different birds settle in vastly different homes. Although the simple cup nests, the intricate structures of weaver birds, the eagles’ eyries, the unconventional mud nests, and the trunk dwellings of woodpeckers are all as unique as can be, they all share a certain degree of complexity that can only exist because of their material and mechanical properties. In order to achieve a satisfactory understanding of the physics behind the structure of various bird nests, it is essential to properly consider the different mechanical aspects.

Jamming and Nest Stability

When trying to construct a cup nest, a bird might encounter issues with its rigidity. The grains typically employed for this are far from tough enough to ensure proper stability. The transition from solid to fluid states is typically associated with thermal properties. In the case of bird nests, this transition relies heavily on mechanics and is purely athermal. The procedure by which this happens is commonly referred to as “jamming,” the seemingly random assembly process of fluid-like grains into a solid nest. Generally, jamming can be defined as a peculiar kind of phase transition that acts in systems regardless of their thermal states. It typically occurs when the mobility of the components is restricted but the stress applied is insufficient to cause any sort of yielding. Jamming thus requires considerable geometric constraints so as to prevent the particles in the system from having enough free surrounding space to flow (Heinrich, 2015). This can clearly be observed in bird nests, particularly in the traditional cup nest (Fig. 5) which is an example of chaotic granular packing. The jamming of grains plays the most crucial part in nest stability, as it makes the birds’ home rigid enough to withstand the required weight and other challenges it might face (Weiner, 2020).

Ability to Manage External Stress

Stability alone is not enough. A nest should be able to handle significant amounts of external pressure without falling apart. In addition to supporting the weight of the bird itself and its eggs, the nest must be capable of resisting substantial stress exerted by outside factors, such as extreme weather conditions. As an example, swallows build their mud homes to endure up to 40N of outside pressure. When a storm hits, the nest is acted upon by a force of around 8N, in addition to its weight of approximately 4N. This remains well under the 40N limit, thus ensuring the safety of the nest and its residents (Yeonsu et al., 2021). Stress-resistance is also evident in the home of the white-nest swiftlet, more commonly referred to as the edible-nest swiftlet. When we analyze the general stress distribution on the nest, we notice that the pressure is mostly concentrated at the location where the bird usually stands (Fig. 6). However, the nest seems to be overdesigned so as to ensure the safety of the entire structure. Even if the excess weight is enough to damage the outer rim, the nest as a whole will remain intact. Furthermore, the design guarantees that the part holding the eggs is more or less isolated from the stress, with the nest components, primarily made from the swiftlet’s saliva, redistributing the pressure away from that area. It can thus be safely assumed that bird nests are mechanically designed to allow any applied external force – up until a specific threshold value – to be spread out in a way that ensures that it does not surpass the breaking strength of the material. This can be attributed to the positioning of the fibers throughout the structure in a stress-diffusing pattern (Hadass et al., 2019).

Internal Stress and Friction

To counter the effects of stress mentioned in 2.2, a bird nest has a certain level of internal strength associated with its strength. Let us consider the cup nest once more. While the grain components might originally act fluid-like, as they pile up on top of each other, the individual interactions trigger a force chain motif. More grains lead to more contact, which in turn yields increased levels of static friction as each piece acts on the adjacent ones (Weiner, 2020). This phenomenon can also be observed in slightly larger nests such as those of the Dead-Sea sparrow. The structure itself consists of interlocked dry branches. With each branch the sparrow adds while building, more friction is progressively added to the system. This consequently augments the internal pressure, which is crucial to the overall stability of the nest. Moreover, the branches actually remain in place because of the frictional force exerted on each branch by the others (Jessel et al., 2019).Nevertheless, friction is not the only force crucial to the success of the nest.

Role of Adhesive and Cohesive Forces

Adhesion is defined as the attraction between different elements, whereas cohesion is that between similar ones. When it comes to mud nesters’ homes in particular, adhesive forces act between mud granules and foreign substances, drawing them together, while cohesive forces dictate the attractive tendencies among the mud substrates. These bonds can only withstand a limited amount of tension before falling apart, defined here as the tensile stress. It is interesting to note that the cohesive and adhesive properties of mud alone are far from adequate to form any sort of nest, much less attach said nest to a support. This is why swallows, phoebes, and other mud nesters use their own saliva to facilitate the nest building process. Mucin, a binding agent in that saliva, leads to a noticeable increase in the tensile strength of the mud granules, thus allowing them to stay bonded together long after the saliva evaporates (Fig. 7).Without the added cohesive forces, the birds would never be able to construct their mud dwellings. This is also true for the adhesive forces binding the nest to its resting place (Jung et al., 2021).

Bird nests are thus a striking architectural phenomenon. However, the ingenuity that birds employ while constructing their homes is not limited to these animals only. Other species, such as bees, showcase a similar ability to incorporate mechanical elements to better the structures they build. 

Beehives

Honeybees are among the world’s most efficient workers and architects. These social animals live in the beehive — an enclosed structure that is built entirely with bee wax. Their home is a remarkable execution of high precision engineering. It features vertical, six-sided honeycomb cells with each wax partition being less than 0.1 mm thick (Fig. 8). Each of the six walls is the same width, and the walls meet at an angle of 120 degrees, producing a nearly perfect hexagonal shape (Hillock, 2020).

Beehives can be found in rock cavities, hollow trees, or even hanging down in an exposed environment in warmer climates. With only a single entrance to the beehive, honeybees use a variety of mechanical techniques to achieve thermoregulation. Furthermore, the reason behind the elegant hexagonal structure inside beehives can be explained by mechanical properties as well.

The Mechanics Behind a Hexagonal Shape

A beehive (Fig. 9) is not the only one in nature with a hexagonal honeycomb structure. Other examples of this structure include the eyes of a dragonfly (Fig. 10) and volcanic columnar jointing (Fig. 11.). The application of the hexagonal honeycomb is due to its efficient mechanical properties; it is the most efficient way to maximize area with the least total perimeter (Hales, 2001). This is referred to as the honey conjecture (Hales, 2001). Of equal importance, honeycombs stack together in an offset arrangement with six walls surrounding each chamber, providing an exceptionally high mechanical strength and stability (Zhang et al., 2015). Furthermore, the hexagonal structure provides excellent heat dissipation, particularly compared to triangular and square structures, which is important for the waxy structure during warm weather (Fig. 12., Fig. 13.).

Moreover, in a scientific article in 2013, honeybee’s perfectly staggered hexagonal honeycombs were found to be explained with simple physical forces. In fact, the ability to measure angles and lengths is not needed for bees to build the hexagonal structure. In reality, the honeycomb chamber is built with a circular shape by the bees, and it quickly transforms into the well-known hexagon within a couple days (Fig. 14.). To do so, it requires the bees to soften the wax with body heat and knead them into hexagonal chambers with the help of surface tension at the junction points (Karihaloo et al., 2013). Nonetheless, bees are skilled builders that help shape the circular honeycombs into hexagonal chambers.

Thermoregulation in Beehives

In a typical beehive, there are usually around 10,000 to 60,000 bees living together in narrow and confined spaces. This large number of bees could result in their home getting overheated during the hot summer. In winter, bees do not have a tolerance to cold temperatures either. Furthermore, the central brood area of the beehive must be kept at an average temperature of 35°C for the colony to survive (Chandrasekaran, 2019). How does the beehive maintain its homeostatic temperature throughout the cold winters and hot summers?

Honeybees use a variety of techniques from physics to sustain a desired temperature. To survive the winter, honeybees use propolis, a sticky glue-like resin that is strongly adhesive, to seal off any unwanted gaps and strengthen the beehive’s insulation (Jarimi et al., 2020). Bees will also clump together to reduce the surface area exposed to cold air. Then, by the principle of heat transfer, this would reduce heat loss via convection. During the summer, the beehive is ventilated, similar to using a fan to cool the inside of a house. This is done by fanner bees fanning their wings and forcing air circulation throughout the beehive, and bringing cool humid air inside. Additionally, honeybees use evaporative cooling, so some worker bees will carry water to the beehive through their bodies (Jarimi et al., 2020).

Applications of Beehive’s Structure

The beehive’s structure, with numerous mechanical properties, is a suitable application for innovative architectural design. The hexagonal honeycomb not only provides internal structural support, but also optimally divides the beehive’s internal space into individual cells that are equally sized. Moreover, the honeycomb can distribute and disperse the external forces on the beehive evenly and effectively. Due to the simple but unique hexagonal shape, the honeycomb has great expandability — bees can simply create more hexagon segments that fit perfectly to the perimeter of the honeycomb (Nejad, 2016). As a result of these advantages, the hexagonal beehive structure can be a suitable design for buildings. Constructing a building with the HexaGrid design, a building design modelled after beehives, is more cost-efficient as it reduces the amount of steel used by roughly 10% to 15%. The hexagonal design can also minimize skyscraper structural failure, since it has a better ability to redistribute stress than the typical frames used in most tall buildings (Nejab, 2016). Portrayed in Figure 15, the horizontal stress of a HexaGrid design building is distributed relatively evenly, with no outliers of high-level stress distribution.

In conclusion, the beehive is a structure with numerous mechanical properties, including good thermoregulation, optimization of materials and spacing, and even stress distribution. It is the efficient design of thermoregulation that allows bees to survive the four seasons (Jarimi et al., 2020). The spatial optimization ensures that bees have the maximum internal space with least total perimeter (Zhang et al., 2015). The even stress distribution of the hexagonal shape provides strong stability and excellent strength (Nejad, 2016). All in all, these characteristics make the beehive structure a good application to architectural design. However, bees are not the only insect engineers; termites must also build homes to protect their colonies.

Termite Mounds

Termite mounds are structures that take years to build (Fagundes et al., 2021), and can stay intact for centuries (Zachariah et al., 2020). These monoliths, reaching 8-9 meters in height (Fagundes et al., 2021), protect the colony from predation and external weather while sheltering fungus culture sites for feeding (Chen et al., 2019). This section will focus on land mounds only, as opposed to those found in trees.

Thermoregulation

Termites are found in many different regions of the world with different climates. This is an issue for termites, as they must maintain a homeostatic nest temperature which is ideal for nutritional fungus growth. This is why mounds in colder climates have thicker insulated external walls, while mounds in warmer regions have thinner breathable external walls (Fagundes et al., 2021). External temperature variation may also result from the amount of shade the mound is in. To counteract this, mounds located in the shadow of trees are dome-shaped and point straight upward, while mounds exposed to the sun display a taller conical shape, with the tip pointing at the average zenith position of the sun, which is influenced by geographical latitude. The latter configuration aims to minimize solar irradiation on the external surface and maintain a cooler nest temperature (Fig. 17.). The solar heat absorbed by surface conduits in the walls is dissipated by convection of the air outside. As for the metabolic heat generated by the nest, it is dissipated into the cooler deep soil.

Wind speed can also affect structural stability. Figure 18 shows how termites build their mounds in environments with different average wind speeds. They aim to optimize aerodynamics to ensure structural stability, leading to more dome-shaped mounds at higher wind speeds.

In nature, termites must consider the combination of these factors (zenith angle, wind speed and solar irradiance) present in their environment when building their mounds. Under these constraints, they choose a shape that is stable, minimizes nest temperature and enables efficient ventilation. Figure 19 shows the best mound shapes for different optimization objectives in ambient conditions of India and Brazil. The middle mound shapes represent the compromise between the two objectives and are most likely to be found in nature (Fagundes et al., 2021).

Water Content Regulation

In southern Asia, termites must adapt to a new challenge: the dichotomy of the rainy and dry seasons. Water content in the mound is important to maintaining a homeostatic microclimate for fungus foraging. The outer walls provide an initial barrier to prevent water from entering the mound during the rainy season. If this layer is compromised and water enters the mound, the inner tunnels act as a drainage system. In addition, termites can remodel these tunnels to drain any excess water. Both factors, tunnel structure and termite intervention, are essential to effective drainage, and allow the mounds to remove water efficiently (Fig. 20).

During the dry season, the mound walls trap moisture which termites can carry deeper into the mound through tunnels to preserve the microclimate conditions (Chen et al., 2019). These tunnels must be reinforced to allow termite travel and to stabilize the overall structure.

Inner Structure and Soil Cementation

Transportation of materials in the mound is essential for the colony to function. Inside the mound, there is a network of tunnels through the porous soil that allows termites to move around the structure. Figure 21 shows a micro computerized tomography (CT) scan of a mound chunk cross-section (Oberst et al., 2021).

The particle density is higher in the inner walls than in the outer walls of the tunnels. This creates a bone-like scaffold that support the mound’s weight. Microindentation tests near the nest reveal a much higher Young’s modulus, hardness and indentation modulus than control compressed soils. The difference reaches across orders of magnitude and the upper limit of Young’s modulus for termite composite exceeds that of man-made concrete (Oberst et al., 2021). Additionally, organic additives confer greater weathering resistance to the termite soil composite. Termites prefer to build with soil composed of different particle sizes that is close to its liquid limit, but once dried, this soil alone does not withstand water erosion well. Termites form saliva-coated boluses of soil in their mouths, acting like bricks for construction. Although the biocementing interactions between the saliva and soil are not well understood, soil manipulated by termites in this way has better water resistance when subjected to repeated cycles of wetting and drying, like rainfall in nature (Fig. 22) (Zachariah et al., 2020).

These properties make termite structures strong and water resistant, which are important properties for any building material subjected to environmental conditions.

Applications to Construction

Due to its mechanical properties, termite soil composite is a suitable construction material for human buildings. Bricks made from termite soil meet the established criteria for operational bricks in construction. The soil’s plasticity due to its clay content allows the bricks to keep their shape when moulded (Legese et al., 2021). Termite soil composite also makes viable masonry units. This raw material is readily available globally and is considered a waste material. Further, it is environmentally friendly and sustainable (Mahamat et al., 2021).

Conclusion

This essay has provided insight into the mechanical properties that govern animal architecture. Each of the homes discussed is a good example of a structure built by animals that can be considered phenomenal from a mechanics point of view. While they all share that level of intricacy and complexity, every one of them is also remarkably unique. It is interesting to note the similarities in the construction techniques employed by various species and the mechanical factors that drive these similarities. However, it is just as fascinating to see how these different animals have their own distinct ways to adapt to the conditions surrounding them. Animals do not receive enough credit for their architectural and engineering prowess. The dexterity with which they employ various physics principles to their benefit is often incomparable. These include, but are not limited to, fields like acoustics and thermodynamics, in addition to concepts such as water retention, and internal or external stress. Humans may consider themselves to be the only engineers on the planet, but they fail to notice that the wildlife around them are prone to using engineering concepts that sometimes surpass even our knowledge and understanding.

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