Table of Contents

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Table of Contents

Comparative Analysis of Biomechanical Properties of Mammalian Fur

Table of Contents

Introduction

The presence of hair, or rather keratin structures in nature is abundant. A wide range of animals have keratin structures present in their physiology that serve diverse and varied purposes. Any animal’s fur is optimized for their environment, which leads to the wide range of keratin structures observed in nature.  The mechanical properties of fur are integral in determining their function. Keratin structures can be separated into two main categories – alpha (α) keratins and beta (β) keratins. Alpha-keratins are found in vertebrates, while beta-keratins can only be found in reptiles and birds (Greenwold et al., 2014). The two keratin structures vary in their shape, with alpha-keratins being in an alpha-helical coil, and beta-keratins in a twisted sheet structure (Bragulla and Homberger, 2009). These structural differences lie at the molecular level, but using these basic building blocks, much more advanced and intricate structures are formed. Through variation in these advanced systems comes the wide range of biological purposes that hair/fur serve. 

When analyzing the macroscopic structure of hair, it is important that these properties being analyzed are quantifiable for comparison. It is also necessary to highlight the specialization of the keratin structure. To exemplify these various specializations, animals from diverse environments must be examined. Analyzing the mechanical properties of keratin structures in porcupines, sea otters, and polar bears provides varied specializations that highlight the true scope and range of function of keratin structures. The porcupine uses fur as a defense mechanism against predators. Sea otters, however, have fur that is designed to insulate them most effectively in their marine environment. A polar bear’s fur is both thermally insulating and is optimized for the optics of the surrounding environment. These varied functions are a result of differing mechanical properties of the animal’s hair/fur. 

The analysis of keratin structures in a biomechanical context can be separated into two categories: mechanical properties and optical properties. When considering the optical properties of the hair/fur of an animal, quantifiable values like wavelength reflection and absorption as well as diffraction patterns are considered. Through these quantifiable factors, larger conclusions can be drawn about the exact function of the keratin structure. Similarly, mechanical properties are determined by the measurement of quantifiable factors. Properties like shape, stiffness, aspect ratio, and density are all considered in the analysis. Using this data, the unique function of the hair/fur of the animal can be determined and justified. 

Mechanical Properties

Shape and Stiffness

To start, the shape and structure of fur determine its stiffness. These properties can detail functions like protection, shock-absorption, waterproofing, and insulation (Dawson et al., 2013). Hence, structures determine function. 

Porcupine

Unlike most mammalian species, porcupines have three sets of pelts covering their bodies. The first two layers are composed of short-haired underfur and guard hairs, which cover on average most of the animal’s body, except its snout and the soles of its feet (DeMatteo and Harlow, 1997). The third layer is a type of spiked-shaped fur called quills. These quills grow mostly on the back (rosette area) and the sides of the porcupine’s body, and each has a unique length (Everson, 2015). Structurally, quills have a thick and hard outer tube made out of keratin, and their ends are tapered and pointy. Due to their thick outer layer, porcupine quills are brittle, and break under minimal pressure (Everson, 2015). At a microscopic level, the tips of the quills are extremely sharp, and their surface is fully covered with backward-facing barbs (Fig. 1).  

Fig. 1 Magnified view of a porcupine’s quill end. [Adapted from Everson, 2015]

Specifically, these barbs are responsible for the difficult removal of quills from soft tissues, making them a great defense mechanism against predators in the wild. Contrary to popular belief, porcupines cannot shoot their quills at their predators. However, porcupines have other types of quills that can have unique functions. Some species have specialized quills, called “rattle quills,” that stretch the end of their tail. 

As the name suggests, these quills produce sound when vibrated due to their hollow, capsule-shaped shape, like a rattlesnake. As an additive protective measure, these species of porcupines use this rattling to scare off any approaching predators (Everson, 2015).

Polar Bear

Moving to a colder and dryer environment, we have the polar bear fur. As most mammalian pelts, the polar bear pelt has two layers, which include a dense underfur and guard hairs with varying length (“Polar Bear characteristics” | Polar Bears International, 2020). An individual hair is smooth and transparent, containing a hollow core, that allows for light scattering and reflection (Fig. 2), mimicking its snowy environment. Due to them being hollow from the inside and structurally not being as thick as quills, polar bear fur is flexible and not that stiff. Under pressure, these furs will remain intact, providing increased insulation to the polar bear due to their flexible movement covering the entirety of the animal (Dawson et al., 2013). 

Fig. 2 The cross-section of a polar bear fur, showing hollow interior channel. [Adapted from Khattab and Tributsch, 2016]
Sea Otter

Moving on to aquatic environments, the sea otter also has the two signature layers of mammalian fur. However, both the thick underfur and the guard hairs have adapted to the wet environment, by creating a mechanism that allows the fur and skin to stay dry (“Sea Otter” | National Geographic, 2019; “Southern Sea Otter” | The Marine Mammal Center, 2020). This mechanism will be discussed in a later section. Their individual hair is not hollow. Instead, it is made out of compound keratin, like human hair (Cassidy, 2015). Hence, similarly to human hair, sea otter fur is flexible, allowing it to bend and twist to cover its entire body. 

At a microscopic level, their fur has spikes (Fig. 3). These tiny geometric barbs help the pelage stay compact and tight, allowing the sea otter to keep itself warm in the cold waters, and help it float. They function like Velcro straps, binding together to form a compact – but light – pelage for the sea otter to stay warm (Cassidy, 2015). These bards are not meant for any defensive function – they are mostly to help the sea otter swim and conserve its body temperature.

Fig. 3 Magnified view of a sea otter hair, showing microscopic spikes. [Adapted from Cassidy, 2015]

Size and Aspect Ratio

Aspect ratio is the relationship between the width and height of an object (16: 9 is a common aspect ratio for computer monitors for example). As it is simply a ratio between two separate measurements, aspect ratio does not have any units. Alone, aspect ratio is not inherently useful in providing information on the properties of hair/fur, but used in conjunction with other properties, aspect ratio can provide a full picture of the hair of interest. 

Porcupine

The North American porcupine’s quills are generally 40–50 mm in length and 1.5–2.0 mm in diameter (Vincent and Owers, 2009). Using the graph’s trendline y = 15.15x + 2.93, where y is the length in millimeter and x is the diameter in millimeter, to model the correlation between length and diameter of a porcupine quill, the aspect ratio is 1:18.08.     

Fig. 4 Length vs diameter of porcupine quills. [Adapted from Vincent and Owers, 2009]
Polar Bear

Polar bear hair can be separated into two layers. The layer closest to the skin is used for insulating purposes, and the outer layer reflecting light. The insulating layer can range from 20-30mm in length, with the outer hairs measuring up to 90-170mm in length (Carrlee, “Bear, Polar”). For the insulating layer, the diameter measures approximately 32.5 microns. The fur of the outer layer measures approximately 75-100 microns in diameter (Carrlee, “Bear, Polar”).

Table 1 Polar bear insulatory and outer fur measurements. [Adapted from Carrlee, “Bear, Polar”]

By calculation (refer to Appendix for sample calculation of aspect ratio), the aspect ratio of insulating layer of fur is 1:769, and the outer layer has aspect ratio 1:1486.

Sea Otter

Sea otter fur is like that of the polar bear, as there is an outer layer of fur surrounding the insulating layer. Interestingly enough, since fur is an inflexible insulator, the otter has systems in place for overheating during exercise (Bodkin, 2001). In its rear flippers, there is little to no fur and high vascularization, making it possible for the otter to cool off. Thermal properties are directly correlated to the volume of fur, as the more air that can be trapped means higher levels of insulation. 

Table 2 Sea otter insulatory and outer fur measurements. [Adapted from Carrlee, “Otter, Sea”]

Density 

An important mechanical aspect to consider is the relationship between density of fur and function. Density contributes to the level of heat retention from an animal’s pelage. When analyzing density, one can look at both a macro and micro level. At a macro level, one can consider the distribution of fur follicles on the animal, and on a micro level, one can consider the density of individual hair or quill. 

Porcupine quills, polar bear fur, and sea otter fur, are all made up entirely of alpha-keratin (Murr, 1970). The density of keratin is generally consistent but ranges from 1.283 g/cm3 to 1.335 g/cm3 (Murr, 1970). Polar bear fur, sea otter pelage, and porcupine underfur density is consistent with what could be considered “typical” fur, with a cuticle, cortex, and medulla layers (Cho et al., 2012). However, the porcupine quill is distinctive with harpoon-like cuticle scales, directed down the quill that form a notched edge consisting of thick keratin plates with a softer, pithy interior. 

Porcupine

As seen in Fig. 5 and 6, the thickness of walls of quill medulla cells varies from 0.6 to 1.2 µm. The total thickness of the cuticle and the cortical layer were, on average, 0.19 and 0.05 mm, respectively (Cho et al., 2012). Seen in the transversal view, the thick keratin lining of 0.19mm of the quill provides a strong support that makes the quills a formidable mechanism for defending from potential threats to the porcupine (Ivlev, 2020), but combined with the pithy interior, the quill does not have good buckling support, and thus snaps easily.

Fig. 5 Magnified view of a porcupine’s quill in a transversal view. [Adapted from Cho et al., 2012]
Fig. 6 Magnified view of a porcupine’s quill in a sagittal view. [Adapted from Ivlev and Chernova, 2005]

Taking a more macro analysis of the porcupine’s pelage, the density of quills and underfur are essential for heat retention. From analysis, the back region of the porcupine consists of a high density of quills, but sparse fur (DeMatteo and Harlow, 1997). This region and its insulative properties, also known as the rosette area, will be discussed further in this report. 

In the rosette area, quill presence is at its most and fur is almost absent. Hair on the side areas is relatively dense, but it becomes gradually sparse and fine in texture moving towards the abdomen (DeMatteo and Harlow, 1997). The shoulder area also appears to be well insulated due thick fur, with some number of quills, and some densely packed guard hairs (DeMatteo and Harlow, 1997). Therefore, as seen in Fig. 7, most porcupines in nature have adapted their lifestyle to cover their poorly insulated area to conserve heat.

Fig. 7 Porcupine sitting in a lotus position to minimize exposure of the poorly furred surfaces of the thorax and the abdomen. Photographed by Suzy Whi. [Adapted from Whi, n.d.]
Polar Bear

Not inhabiting the same evolutionary niche, the polar bear’s fur is very different from the unique porcupine quills and has evolved very different properties. The structure of polar bear fur is multifunctional and adapted for the polar bear’s environment, providing both visual camouflage and crucial thermal insulation. The polar bear has a bulky fur, consisting of two well-defined layers: a softer layer of dense white underfur topped by an outer layer of 5-15 cm long guard hairs (Khattab and Tributsch, 2016). 

As discussed earlier, the hair shaft of the polar bear is hollow. This allows for some of the properties that will be discussed in a further section on optical properties. On a macroscopic level, polar bears are furred completely except for the tip of their nose (Ivlev, 2020). This permits them to have almost zero loss of body heat. This is due to the fact that polar bear fur contains several different sizes of hair, with a large density of interfaces. The insulation value of the fur may be affected by both the density of hairs and their structure (Khattab and Tributsch, 2016). 

The guard hairs layer makes up the largest portion of the polar bear’s fur depth but maintains a low probability per unit coat depth that a light ray will be intercepted by it due to its low density (Khattab and Tributsch, 2016). The under-fur layer, however, is composed of highly dense hair that is effective at trapping light rays. As a result, the guard hairs layer allows for forward-scattering of radiation into the lower layers of the fur above the polar bear’s skin, while the emitted thermal radiation of the body is continuously absorbed by the lower layer of the fur due to the under-fur layer`s high probability per unit coat depth that traps these forward-scattered rays (Khattab and Tributsch, 2016). 

The guard layer and the underfur layer work in conjunction with one another, and at the boundary between the fur`s layers, the radiation through the guard hairs towards the lower regions of the fur is likely be intercepted by underfur hairs due to the high density of the pelt in the under-fur layer. This allows for intercepted radiation to be re-scattered, converted to thermal energy, and finally absorbed by the black skin of the polar bear (Ivlev, 2020). These two layers of fur provide the polar bear with thermal insulation that can maintain constant temperature at temperatures of -50 ℃ (Khattab and Tributsch, 2016).  

As seen in Fig. 8, beyond providing insulation, the fur of the polar bear’s paws provides another function by giving the pads of the feet a more secure purchase on the slippery sea ice surface and adding another layer of insulation between the bear’s foot and the ice and snow (Khattab and Tributsch, 2016).

Fig. 8 Polar bear’s densely furred paws, providing insulation, as well as grip on icy surfaces. [Adapted from Khattab and Tributsch, 2016]
Sea Otter

The sea otter is another animal with interesting biomechanical properties concerning the density of furs. The sea otter’s fur is crucial for insulation. Sea otters are unique in that they are unlike most other marine mammals, the sea otter has no blubber and relies on its exceptionally thick fur to keep warm (Allain et al., 1980). With up to 150 000 strands of hair per square centimeter, its fur is the densest of any animal on Earth (Murr, 1970).

The sea otter is completely furred except for their noses. The hair density of the otter is quite constant over the regions of the trunk but is lower at the head (about 60 000 hairs/cm2 on the cheek) (Nickerson, 1989). The hair follicles were arranged in specific groups with different bundles of varying size, normally comprising dominant numbers of wool hair follicles (Allain et al., 1980). The ankle’s and interdigital webbing’s hair densities are approximated to be 107 000 hairs/cm2 and 3 300 hairs/cm2, respectively, compared to a hair density of about 125 000 hairs/cm2 for the back (Ivlev, 2020) (Fig. 9). 

Fig. 9 Volume of the thermally insulating air layer in fur of different sizes, submerged in water. [Adapted from Ivlev, 2020]

These dense hairs have an important impact on elasticity of fur, with the equivalent elasticity modulus of the fur proportional to the elasticity modulus of individual hairs E and their density and must depend on the length l and the thickness d of hairs (Allain et al., 1980). Elasticity of fur helps trap air, which keeps the sea otter insulated and is very efficient, providing the sea otter four times the insulation of the same amount of fat or blubber. 

The elasticity of fur of a sea otter entering the water needs to be sufficiently high so that the fur does not “collapse” due to hydrostatic loads. In a sea otter about one meter long, the hydrostatic load of water exerted on the fur is 1000 Pa. The equivalent elasticity modulus of the fur of the sea otter measured on land is only 15 – 300 Pa (Allain et al., 1980). This is not enough to prevent significant tightening of the fur. As a result of tightening hairs held together by surface tension forces and increased rigidity, the equivalent elasticity modulus of the fur at the first moment of contact with water increases 10 – 100 times. It is assumed that the elasticity of the sea otter fur reaches about 5 × 104 Pa. This value is significantly higher than the maximum excess loads on the boundary of the sea ​​otter fur, thus providing sufficient insulation in cold water temperatures (Allain et al., 1980).

Optical Properties

Polar bear

Since the hair of polar bears is made up of pure keratin, which is transparent for visible and near infrared light, they are translucent; the absence of melanin in the hairs makes them completely transparent from 300 to 2600 nm (Khattab and Tributsch, 2016) (Fig. 10). Through electron microscopy, the hairs of polar bears each have a long, central cylindrical hollow core that occupies about a third of the width of its diameter (Khattab and Tributsch, 2016; Zhan et al., 2019) (Fig. 11).

Fig. 10 Hair Transparency. [Adapted from Khattab and Tributsch, 2016]
Fig. 11 Hollow hair core. [Adapted from Khattab and Tributsch, 2016]

As seen in Fig. 12, keratin produces pronounced absorption peaks only near 3000 nm and 7000 nm (Khattab and Tributsch, 2016). Hence, in the absence of scattering particles, the polar bear pelt would be transparent for the visible light and the black skin of the bear would be visible (Khattab and Tributsch, 2016).   

Fig. 12 Transmission spectrum for an individual hair of the polar bear. [Adapted from Khattab and Tributsch, 2016]

When light is incident on the hair of a polar bear, it is not directly absorbed — it is rather scattered in the hair’s medulla (Khattab and Tributsch, 2016). When it is scattered, light is coupled into the hair fiber, it then travels for a short distance within the fiber before being coupled out again by a subsequent scattering process (Khattab and Tributsch, 2016) (Fig. 13). However, this light is again coupled into neighboring hairs, until the light is either released as heat or absorbed by the bears’ skin (Khattab and Tributsch, 2016). Therefore, the pelt’s transparent hairs appear white and absorb most of the incident radiation (Khattab and Tributsch, 2016). In this way, the fur significantly minimizes the radiation losses from a polar bear’s body in the cold environments by effectively scattering and trapping the long-wave radiations (Khattab and Tributsch, 2016).

Fig. 13 Scattering of light in polar bear hair. [Adapted from Khattab and Tributsch, 2016]

The magnitude of absorption of incident light is dependent on the thickness and density of hair tufts (Khattab and Tributsch, 2016). If light is incident on a thin layer of hair, transmission of light is high and only a small amount of light is absorbed (Khattab and Tributsch, 2016) (Fig. 14).

Fig. 14 Absorption properties of thin polar bear hair tufts. [Adapted from Khattab and Tributsch, 2016]

If light falls on a thicker layer of hair, the rate of transmission is much lower and a higher amount of light is absorbed (Khattab and Tributsch, 2016) (Fig. 15). It can thus be concluded that the rate of absorption increases with the thickness of hair. 

Fig. 15 Absorption properties of thick polar bear hair tufts. [Adapted from Khattab and Tributsch, 2016]
Porcupine and Sea Otter

The porcupine quills and sea otter fur, being both opaque, do not allow light transmission; hence, they do not have optical properties which affect their function. Their colors are black rooted, with light brown extension towards the ends for the porcupine quill, and dark brown with light brown guard hair for the sea otter pelage.

Insulation

By exploring the mechanical and optical properties of fur of different mammalian pelts, we were able to conclude that fur has a multitude of various functions depending on the species and the environment they live within. Although hair and fur are multifunctional from one species to another, one hair function that is consistent for all mammals is insulation. Insulation, also known as heat conservation or thermoregulation, is a direct consequence of the presence of fur, which acts as an insulator between the body of the animal and the environment (Fig. 16). This insulation shields the animal from low temperatures, while maintaining the heat naturally created by its body. Hence, for all mammalian species, hair and fur were evolutionarily developed to retain and regulate body heat (“Thermoregulation and mammalian fur” | OpenLearn | The Open University, 2012).

Fig. 16 The insulative property of mammalian fur between two environments. [Adapted from Willmer et al., 2000. Courtesy of “Thermoregulation and mammalian fur” | OpenLearn | The Open University, 2012 ]

Although fur plays the main role in conserving body heat, the real star of the show when it comes to insulation is air. Under normal conditions, air can rapidly transfer heat through convection, making it a bad insulator. However, when it is confined in small spaces, like the ones created by fur layers (Fig. 17), it can no longer support broad convection, hence turning air into a good insulator (“Convection” | OpenStax, 2020).

Fig. 17 Air pocket trapped inside fur, creating a lightweight insulator. [Adapted from “Convection” | OpenStax, 2020]

This air trapped by the fur of the animal is what allows mammals to conserve their body heat and regulate their temperature from one environment to another — from the warmest to the coldest, and from the driest to the wettest. 

Porcupine

In the case of the porcupine, there are three layers of fur that help insulate its body, as mentioned in an earlier section. These are the fur pelage and guard hair that entirely covers its epidermis and the quills that grow on their back. According to research, it was found that the presence of quills alongside the pelage increased the thermal conductance, making quills a bad insulator for the porcupine (Ivlev and Chernova, 2005; Clarke and Brander, 1973). This poor insulation can be explained by the relative positioning of the quills compared to the remainder of the pelage. Due to quills growing upright – unlike the undercoat and the guard hairs, which grow densely near the epidermis — they are incapable of trapping air in tight spaces, leading to thermal dissipation (DeMatteo and Harlow, 1997). Hence, body areas that are heavily dense with quills, like the rosette area on its back, loses more radiant heat compared to the side and shoulder areas, which are densely packed with fur instead of quills (Clarke and Brander, 1973). It has been shown that during winter, the back region does indeed lose more heat than the lateral and shoulder areas, which would seem to indicate that quills are poor insulators aside from minimizing wind penetration (Clarke and Brander, 1973).  However, it has also been concluded that the total insulation of the porcupine is 10 % greater than other mammals of its size (Ivlev and Chernova, 2005; DeMatteo and Harlow, 1997). This weird phenomenon can be explained by the porcupine’s low metabolic rate and the presence of depressed quills, which are better suited to trap air (DeMatteo and Harlow, 1997). Although generally being bad insulators, quills can also be positioned in a way to maximize the porcupine’s insulation, which comes to show that the presence of quills is not only a defensive mechanism but can also be an insulation measure.    

Polar Bear

The polar bear on the other hand has its two layers of dense underfur and guard hair to insulate against the cold and windy climates of the Arctic. Its underfur has adapted to prevent any heat loss, which makes it a great thermal insulator in harsh climates. By being hollow and flexible, polar bear fur has the capacity of holding heat inside the pelt by absorbing radiation from the sun and transferring it near the surface of the epidermis (Stegmaier et al., 2009; Zhan et al., 2019). Combined with the air that is trapped inside the pelt, the heat that is conserved in the fur allows the polar bear to stay warm in the Arctic climate. However, polar bears are not only land mammals, but they are also semi-aquatic (Liwanag et al., 2012; Ivlev, 2020). In water, polar bears are incapable of producing enough oils that completely waterproof their pelt. In consequence, wet fur cannot conserve heat because trapped air is capable of escaping, making it a bad insulator when wet. In these situations, the polar bear relies more on its layer of fat to keep it warm, instead of its fur (“Polar Bear Characteristics” | Polar Bears International, 2020). Although having dense fur that keeps them warm in the Arctic weather due to its high insulation and heat-holding properties, the polar bear’s pelage becomes almost useless when it swims. This can become dangerous for any bear that does not have enough body fat for the swim.

Sea Otter

Similarly, to the polar bear, the sea otter is also a semi-aquatic mammal. As discussed earlier, the sea otter has one of the densest pelts in the mammalian class (Liwanag et al., 2012; “Southern Sea Otter” | The Marine Mammal Center, 2020). This high density is capable of trapping air bubbles into the pelage and combined with the microscopic spikes present on each individual hair, it becomes almost impossible for the air to escape (Cassidy, 2015). Adding to its density, sea otter hair does not have arrector pili muscles, which are responsible for hair erection, also known as goosebumps (“Sea Otter” | National Geographic, 2019). By having no control over the relative positioning of their hair, the pelage of the sea otter remains flat over the surface of the epidermis, continually trapping air bubbles. With a semi-permanent layer of air, the sea otter fur becomes a great insulator for any land environment. However, sea otters are also aquatic animals, making them need good insulation while swimming. Unlike the polar bear, otters do not have excess body fat to keep themselves warm while swimming. Instead, they rely on natural oils secreted over their pelt, which allows their fur to become waterproof (Cassidy, 2015; “Sea Otter” | National Geographic, 2019). By adapting to have hydrophobic fur, the sea otter can keep both its fur and skin dry while in the water. This property allows the fur to still trap air while underwater, which provides thermoregulation while wet (Ivlev, 2020) and provides buoyancy for the otter to float restlessly on the surface of the water (Cassidy, 2015).

Analysis

As explored throughout the report, porcupine quills, polar bear fur and sea otter pelage have inherent characteristics that are optimized for their varied environments. Through the analysis of some key characteristics, namely size, aspect ratio, density, and optical properties, their optimal function can be explained and compared to one another for further understanding of these different, but quite similar fur types. 

Hair PropertyPorcupine QuillPolar Bear FurSea Otter Pelage
Average Size (Diameter by length)Following: y = 15.15x + 2.93 y: Length (mm) x: Diameter (mm)Insulator Fur: 0.0325 x 25 mm Outer Layer: 0.0875 x 130 mmInsulator Fur: 0.0125 x 10.2 mm Outer Layer: 0.105 x 17.55 mm
Aspect Ratio1:18.08Insulator Fur: 1:769 Outer Layer: 1:1486Insulator Fur: 1:816 Outer Layer: 1:167
DensityQuill density is highest on back, underfur density is highest on ventral areaDensely furred over entire body excluding nose, especially on pawsMost densely furred animal in the world, averaging 150 000 hairs per square centimeter
ColorBlack root, with light brown extension towards the endsWhite, completely transparentDark brown with light brown guard hairs
Table 3 Comparative measures for porcupine quills, polar bear fur and sea otter pelage. [Information taken from Carlee, “Bear, Polar”. Table and calculations are original.]

Following the collected data in Table 3, we notice that, when comparing relative sizes, the porcupine quills are the largest compared to the polar bear and sea otter. Their relative sizes can be further understood through their shapes. The porcupine quills are thick, spiked-shaped, which provides them with protection and some insulation. The polar bear and sea otter on the other hand have hollow and vibrissae (human-like) fur, respectively. To provide good enough insulation, the polar bear and sea otter hair needs to be short and flat. 

Relating to density, the sea otter has the highest density of fur for any animal on Earth, followed by the polar bear, and finally the porcupine. This explains why the sea otter has the highest insulation out of the three animals, but also shows what type of hair/fur is better for insulation from one environment to another.

Finally, comparing the insulative properties of the three animals, we notice that none of them have a thermoregulatory advantage over the other. However, we can notice that sea otter fur is the only one that allows the animal to have complete insulation in any environment. Although porcupine quills are found to generally reduce insulation, on certain parts of its epidermis, the quills were able to improve insulation due to their depressed positioning. For the polar bear, its hollow fur allowed it to trap sun radiance and increase its heat conservation. However, in an aquatic environment, the polar bear fur was not as useful, leading the bear to rely on its fat instead of its fur for insulation. The sea otter, compared to the polar bear, can combat this issue by secreting oils to waterproof its pelage, allowing both on land and in water insulation.

Evidently, we were not able to compare the optical properties of porcupine quills, polar bear fur, and sea otter pelage because two out of the three animal’s fur does not exhibit any form of reaction towards electromagnetic radiance. It is, however, safe to say that the polar bear fur’s optical properties enable it to additionally practice crypsis, also known as camouflage, to blend-in with the surrounding snow.

Biomaterial Implications of Mammalian Fur

Thanks to its various properties, mammalian fur has inspired the development of many synthetic materials, and even inspired medicinal treatments. In this final section of our report, we will explore three inventions and applications inspired by mammalian fur.

Starting off with our spiky buddy, the porcupine quill strangely looks like a needle, especially their tips. As Cho and colleagues observed, quills have an ease penetrating into soft tissues, and they also tend to remain in position, thanks to its barbs’ strong adhesion (Cho et al., 2012). As the natural design of a porcupine quill is to insert itself with minimal force into soft tissues, these skin appendages have inspired one of the medical field’s most important inventions: hypodermic needles (Collier, 2013). However, contrary to the porcupine quill, needles were designed to generate the least amount of tissue damage when inserted and removed. Cho and colleagues further synthesized synthetic polyurethane quills and quill-mimetic hypodermic needles to test the durability and force requirement needed to puncture tissues (Cho et al., 2012; Collier, 2013). As a conclusion, these new designs allowed the production of a new line of needles-like devices, which can penetrate skin tissue with minimal force requirement and remain inserted with strong adhesion (Collier, 2013). These needle-like devices, known as transdermal devices or patches, are the current most efficient way to administer drugs and medication to the body, without the need of big needles.

Moving forward, polar bear hairs are distinctive of other hairs due to their hollow cores; they determine polar bears’ white color and are optimized to prevent heat loss. Material scientists from the University of Science and Technology of China (USTC) have used polar bear hair as a model to design a synthetic heat insulator (Zhan et al., 2019). They successfully reproduced the structure of individual polar bear hairs by making a new, super-elastic lightweight carbon tube aerogel (CTA) consisting of hollowed-out carbon tubes wound into an aerogel block (Zhan et al., 2019) (Fig. 18). This aerogel does not degrade noticeably over its lifetime and proves to have excellent thermal insulation and elastic characteristics, which can help in its engineering applicability (Zhan et al., 2019). However, the production of CTAs still needs to be scaled up from a centimeter scale to a meter scale for future applications in the aerospace industry (Zhan et al., 2019).

Fig. 18 Electron microscopy of the hollow bioinspired carbon tube aerogel. [Adapted from Zhan et al., 2019]

Finally, the fur of a sea otter, like a polar bear, has very effective natural thermal insulation. In otters and other semi-aquatic mammals, the hydrophobic insulation lies in the thin layer of hair closest to the animal’s body. This layer entraps a layer of air that protects the animal from their harsh environment. Researchers at MIT explored the air-entrapping properties for use in materials applications, particularly a sea-otter inspired wetsuit. Using a simplified model of synthetic hair being plunged into liquid, they then compared it to a smooth surface (Nasto et al., 2016). While this research is still in very early stages, they found that the hairy surface entrapped more air than the smooth surface, and was therefore more insulating (Nasto et al., 2016). By controlling the length, density, and arrangement of “hairs” on the wetsuit, they can be optimized for the wetsuit-wearer’s particular environment, for example a wetsuit designed for surfing vs. a wetsuit designed for deep diving (Nasto et al., 2016). The researchers have yet to develop a model for a furred object remaining static in the water (Nasto et al., 2016). This research, while in its early stages, provides a way to quantify the insulating properties of fur and use that knowledge to advance our own technologies. 

Conclusion

Through the analysis of porcupine quills, polar bear hair, and sea otter fur, it was determined that their different keratin hair structures make them specialized for their respective functions. Aspect ratio, density, stiffness, shape, optical, and insulation properties are the biomechanical properties of mammal hair structures that were examined. Mammalian fur has been shown to have multiple functions such as color appearance, defense against predator attacks, and most importantly, insulating properties. The distinct hair structures of these animals allow them to perform their functions by different pathways; for instance, while polar bear fur can enhance insulation optically, sea otters are insulated by trapping air between their fur and seawater. The thickness and density of fur also have a major role in the efficiency of the functions of fur; the thicker the fur layer, the more it provides insulation and optical properties to determine the color appearance. To get a more in-depth idea about the structure of hair/fur, further research should be done to compare their properties and functions on the biomolecular and cellular levels.

Appendix

Sample Calculation of Aspect Ratio: using the polar bear’s insulatory fur as an example.

            Diameter = 32.5 microns (Carrlee, 2009)

            Average Length = (20 + 30) /2 = 25 mm (Carrlee, 2009)

Conversion: 1 micron = 0.001mm → 32.5 microns = 0.0325 mm 

Aspect Ratio is:

Diameter : Length = 0.0325 : 25

Multiply by 1/0.0325 and round to nearest integer

= 1: 769

1:769 is the aspect ratio of the insulatory fur of a polar bear.

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