Exploring the Role of Skin Pigmentation in the Thermal Regulation of Polar Bears and Its Implications in the Development of Biomimetic Outdoor Apparel

Abstract

A popular belief for why polar bears have black skin is to increase solar heat gain from solar radiation that penetrates through a translucent fur layer made of unpigmented hollow hair. To examine the relative importance of skin color on solar heat gain, we measured thermal gradients, heat flux, and solar transmittance through a polar bear pelt under solar irradiation while thermally anchored to a temperature-controlled plate set to 33 °C. We found that for 60–70% of the dorsal region of the pelt where the fur layer is thickest, solar energy cannot reach the skin through the fur (solar transmittance ≤ 3.5 ± 0.2%) and therefore skin color does not meaningfully contribute to solar heat gain. In contrast, skin pigmentation was important in the remaining areas of the pelt that were covered with thinner fur. This information was used to select commercially available materials according to their solar optical properties to build biomimetic outdoor apparel with enhanced solar heat gain by a factor of 3 compared to standard outerwear constructions.

1. Introduction

Polar bears are iconic creatures whose fur has long served as inspiration for biomimetic insulating materials [1,2,3,4,5,6,7,8]. This has been enabled by decades of research on polar bears and polar bear pelts. This research history, and some assumptions extended from it, has led to a general understanding of how polar bear fur and skin help keep the animal warm in the cold Arctic environment. One popular belief is that their unpigmented hollow hair directs or allows solar radiation through the fur layers to pigmented black skin to enhance the amount of solar heat gain [9], defined as heat load on the skin. The details and relative importance of fur solar transmission and skin pigmentation have long been debated. For example, early hypotheses have been refuted that the hollow fur acts as optical fibers for solar radiation, or at least the UV component, channeling to the skin surface for solar heat gain [10,11]. It is curious that such a hypothesis ever gained traction in the first place, given that the UV portion of the solar spectrum accounts for only about 3% of the total energy available for solar heating, with the vast majority contained in the visible and near-infrared regions [12]. Recognizing that “little additional energy can be gained by harvesting solar radiation”, some researchers have proposed that the polar bear pelt serves as a sensory system in which temperature patterns created by directional solar heat gain can be used to navigate the Arctic environment [13].

Biomimicry of polar bear fur has generally followed two approaches: (1) replicating the porous hair structure or (2) replicating the solar-energy-transmitting fur on top of solar-energy-absorbing skin. As an example of the first approach, freeze-spinning of a silk fibroin–chitosan aqueous solution followed by freeze-drying yielded fibers with aligned pore structures [1]. A woven fabric prepared from these biomimetic porous fibers was found to be a better insulator than a commercial polyester fabric of similar thickness. Such porous fibers can be coated with a thermoplastic polyurethane to yield core–shell fibers with cross sections that appear like cross sections of polar bear hair [2]. These biomimetic fibers were knitted, and the thermal insulation properties of the resulting fabric were better than comparable commercial fabrics using nonporous commodity fibers. Despite these positive results, independent observers noted the “slow and energy-intensive” production process means that “synthetic polar bear fur clothing” based on these fibers “is a long way from appearing in a mainstream clothing outlet” [3].

The second biomimetic approach focuses on replicating the solar-energy-transmitting fur on top of solar-energy-absorbing skin. For example, solar thermal collectors for warm water production or heating have been fashioned from multilayer textile composites consisting of translucent ethylene tetrafluoroethylene (ETFE) membranes, open polyester monofilament spacer textiles, and black silicone coatings on the backside [4,5]. Heat can be captured by flowing fluid inside and through the plane of the spacer textiles [6]. In another example, a solar-transmitting polypropylene spun-bonded nonwoven was layered on top of a nylon fabric vapor-coated with poly(3,4-ethylenedioxythiophene) (PEDOT) that is solar-absorbing [7]. The PEDOT-nylon underlayer also reportedly exhibits low infrared emissivity and thus high infrared reflectivity, which contributes to heat retention.

Polar bears have also inspired the creation of new functional materials with infrared-reflecting underlayers, even in instances where the connection between the newly created materials and the polar bear pelt is less apparent. For example, cellulose fiber membranes were coated with carbon nanotubes to create a solar-absorbing surface, while the backside of the membrane was sputtered with silver to create an infrared-reflecting surface. The researchers called this laminate a “bio-inspired radiative outdoor warming membrane” [8].

Despite this body of successful biomimetic work based on the popular belief that pigmented black skin is relevant for solar heat gain in polar bears, little experimental work has been conducted to test this hypothesis. In fact, studies of mammalian physiology and thermal properties of polar bear fur make no mention of the pigmented black skin [14]. Instead, solar heat gain is reported to depend on a variety of biophysical attributes such as fur color, optical properties, density, and depth, and on environmental factors such as temperature, solar intensity, and wind speed [15,16,17,18]. Skin color can influence solar heat gain if solar radiation can penetrate the fur and reach the skin [19]. Whether it reaches the skin or not, solar radiation that penetrates deeper into the fur deposits heat closer to the body, which is then better insulated from heat loss to cold environments by the overlying fur [20]. In contrast, absorption of solar radiation closer to the surface leaves heat insulated from the body by the underlying fur and more easily lost to a cold environment.

Our interests lie in developing outdoor apparel and footwear to improve performance in all environmental conditions. Solar heat gain is known to reduce metabolic heat production requirements [21] and can therefore be used to enhance performance in cold and sunny conditions. Such conditions can be common for activities such as high-altitude mountaineering. Thus, to better understand the relative importance of the material attributes responsible for solar heat gain, we studied the thermal properties of a polar bear pelt with and without incident solar irradiance. Our studies revealed that solar heat gain does not rely on darkly pigmented skin over a large portion of the polar bear, a surprising finding considering the popular belief that dark skin is an important aspect of polar bear thermal regulation. However, skin pigmentation was found to be an important contributor to solar heat gain in thinner areas of the polar bear pelt. The collective findings were then used to develop biomimetic outdoor apparel with enhanced solar heat gain.

2. Materials and Methods

A polar bear pelt was obtained on loan from the Burke Museum of Natural History and Culture in Seattle, Washington. The origin and other details of this polar bear pelt are unknown. As is typical, the pelt was characterized by dark pigmented skin protected by a dense underfur and a less dense layer of more coarse guard hairs rising above the underlayer fur. Length of both underfur and guard hair varied across the pelt, with shorter lengths near the head and progressively longer hair toward the tail. Likewise, the hair became progressively longer moving laterally from the dorsal median line. To investigate the effect of hair length, three locations were chosen for simplicity and convenience. They are designated as short, medium, and long hair regions and are located approximately 100, 150, and 220 cm from the nose and 10, 10, and 20 cm, respectively, left of the dorsal median line (see Figure 1). Upon request, we were given permission by the museum to make minor modifications to a single small region of the pelt (the medium-hair-length region was chosen) to quantify the relevance of the skin’s solar absorptance on the solar heat gain and therefore thermal comfort of the animal. These modifications are described later in this manuscript.

We devised an experimental setup (Figure 2) to evaluate the hypothesis that pigmented skin plays a role in thermal comfort by evaluating the effect of sunlight on the temperature distribution and net heat flux through the pelt under cold temperatures. The setup consisted of, from bottom to top, a temperature-controlled cool/heat plate [ThermoElectric Cooling America Corporation (TECA, Chicago, IL, USA) AHP-1200CPV High Capacity/Cool/Heat Plate ECN003] equipped with an RTD probe (TECA, 100 ohm 6″ × 0.125″ diameter), a guarded copper block with a one-inch-square heat flux sensor [FluxTeq (Blacksburg, VA, USA) PHFS-01] and thermocouple secured to its top side, double-layer thermally conductive adhesive tape, the polar bear pelt, a 20-thermocouple (type T thermocouples) tower to measure temperature gradients within the fur, and a solar simulator [Sciencetech (London, ON, Canada) Sci-Sun-150]. The solar simulator uses a xenon arc lamp to provide a Class A spectral match across the wavelength range of the solar spectrum and is designed to meet the following standards: ASTM E927-19 [22] and IEC-60904-9-Ed.3 [23]. The entire apparatus was installed inside a 0 °C temperature-controlled environmental chamber at the Bowerman Sports Science Center on the campus of the University of Oregon in Eugene, Oregon. A data logger [Graphtec Instruments (Irvine, CA, USA) GL240], located outside the environmental chamber during testing, was used to record values from the thermocouples and the heat flux sensor.

Testing was performed by setting the temperature-controlled plate to 33 °C to mimic a representative Arctic mammal skin temperature [24,25]. The heat flux sensor and guarded copper block were used to measure the one-dimensional heat flux flowing into (i.e., towards the temperature-controlled plate) or out (i.e., towards the solar simulator) of the pelt. A thermocouple embedded inside the heat flux sensor also allowed measurement of the subcutaneous skin temperature during testing. Double-layer thermal adhesive tape ensured good thermal contact between the pelt skin and the heat flux sensor. The thermocouple tower was constructed to detect temperatures from 2 mm above the skin up to 49.5 mm above the skin in 2.5 mm increments. A comb was used to move the fur back into its natural position after inserting the thermocouple tower. In some measurements, the solar simulator was turned on to simulate 1000 W/m² of solar irradiance on the polar bear pelt. The solar irradiance of the solar simulator was calibrated before each test with a pyranometer [Hukseflux (Center Moriches, NY, USA) SR05-A1]. Data were recorded for at least 2.5 min after reaching thermal equilibrium.

Transmittance across the wavelength range of the solar spectrum (τ_solar) was measured for textile fabrics and sheet insulation using the solar simulator and pyranometer described above.

3. Results

Using the experimental setup shown in Figure 2, a series of tests were performed to evaluate the role of the polar bear pelt skin color on solar heat gain. The first set of experiments was focused on the medium-hair-length region of the pelt. In this location, the underfur length was 20–25 mm, the guard hair length was 60–65 mm, and the natural fur thickness was 40–50 mm. The natural fur thickness was measured as the vertical distance from the skin to the top of the fur when the hairs are lying in their natural oblique resting position.

3.1. Influence of Solar Irradiance on Pelt Temperature and Skin Heat Flux

A first test was conducted around the mid-back section of the pelt (the medium-hair-length location; see Figure 1) to evaluate and compare the skin heat flux and temperature gradients through the fur without (0 W/m²) and with (1000 W/m²) solar irradiance. Results are shown in Figure 3. The hair-side surface of the skin (distance from the skin = 0 mm in Figure 3) was calculated using a simple thermal model from an estimated skin thermal resistance, the measured heat flux, and the subcutaneous temperature (see Section S1 in Supplementary Materials). To differentiate the measured data points from the calculated hair-side skin surface temperatures, these latter temperatures are depicted by symbols with black borders in Figure 3.

The results presented in Figure 3 show, as expected, a significant difference in the thermal response of the pelt to the two solar environments. In the presence of solar irradiance, the pelt experienced a net positive (heating) skin heat flux of 95 W/m². In contrast, it showed a net negative (cooling) skin heat flux of −73 W/m² in the absence of solar irradiance. The temperature through the fur under no solar irradiance progressively decreases from the subcutaneous temperature of 33 °C to the ambient temperature of 0 °C, which is approximately 30 mm above the skin surface. At this location, the natural fur thickness is 40 to 50 mm. When subjected to 1000 W/m² solar irradiance, the temperatures are higher across the entire fur thickness. A discontinuity in the rate of temperature change across the fur thickness occurs between 20 and 25 mm, which is the height of the underfur. While experimental uncertainty cannot be discounted, this discontinuity may be due to the different physical and thermal properties of the fur above and below this diffuse interface. Interestingly, the temperature profile, when exposed to 1000 W/m² solar irradiance, exhibits a maximum of around 4.5 mm above the skin, suggesting that peak solar absorption occurs away from the skin surface. These data demonstrate that significant solar absorption occurs within the dense fur, away from the skin surface, of a magnitude sufficient to heat a portion of the underfur above the subcutaneous temperature.

3.2. Influence of Skin Color on Pelt Temperature and Heat Flux

The results presented in Figure 3 suggest that solar irradiance is absorbed by the pelt, but they did not allow a comprehensive evaluation of where that absorption occurred (e.g., hair and skin or hair only). In a second test, the skin in the same nominal two-inch by two-inch medium-hair-length region initially evaluated in Figure 3 was carefully coated with white paint [Faber-Castell (Cleveland, OH, USA) PITT Brush White] to reflect a portion of the solar energy that passes through the pelt hair (as opposed to absorbing a portion of the solar energy by the natural dark skin of the polar bear). Care was taken to ensure that paint was only applied to the skin surface and not the adjacent underfur. We then measured the thermal behavior of the white painted skin section of the pelt under 1000 W/m² of solar irradiance to compare it to the thermal behavior of the natural dark skin under the same solar irradiance.

The results presented in Figure 4 show nearly identical hair and skin temperatures for the natural dark skin and the painted white skin. Similarly, the skin heat fluxes were measured at 95 W/m² and 94 W/m², respectively. These results suggest, for the medium-hair-length regions of the pelt, that solar radiation is mostly absorbed within the fur above the skin. These results demonstrate that skin color does not play a significant role in solar heat gain in regions of the pelt with this medium length of hair.

3.3. Solar Transmittance of Polar Bear Fur

The surprising results shown in Figure 4 beg the question as to whether the fur transmits any notable amount of sunlight down to the skin. A darker solar-absorbing skin would only contribute to the overall solar heat gain if a meaningful amount of light were transmitted through the fur to be absorbed in the skin directly.

To evaluate this question, we carefully removed a 1.5-inch disk of skin in the same nominal location assessed in Figure 3 and Figure 4 to create a hole while keeping the overlying hair structure intact. A scalpel was used, and care was taken to minimize any disturbance to the hair as it was cut from the skin. The transmittance of incident solar radiation through the fur alone was then measured using the solar simulator and a pyranometer (see Figure 5). The hole was cut in the same location as where the white paint was applied to the skin (i.e., the medium-hair-length location shown in Figure 1). A 1.5-in-diameter light collimator, constructed with a reflective Mylar liner, was used to capture all the transmitted light at the back of the pelt and direct it to a 1.5-in-diameter pyranometer sensor.

The measured average solar transmittance through the fur was found to be 3.5 ± 0.2%. This demonstrates that a relatively low fraction of the incident solar irradiance reaches the skin, consistent with the results shown in Figure 4. In the mid-back section of the polar bear pelt, solar absorption at the skin therefore plays little role in solar heat gain due to minimal transmission of sunlight through the entirety of the fur layer.

3.4. Varying Hair Lengths and Fur Thickness Across the Polar Bear Pelt

While it was found that solar absorptance at the skin does not significantly contribute to solar heat gain in the mid-back medium-hair-length section of the pelt, this finding is expected to vary with fur thickness and density. In addition to the measurements made for the three regions selected for thermal evaluation (see Figure 1 and

Table 1. Description and location of the three regions selected for thermal evaluation (see Figure 1) on the dorsal portion of the polar bear pelt.

Regions	Location	Distance from Nose (cm)	Hair Description	Dimension (mm)
Short hair	Upper back	100	Underfur length	15–20
			Guard hair length	30–40
			Natural fur thickness	15–20
Medium hair	Mid back	150	Underfur length	20–25
			Guard hair length	60–65
			Natural fur thickness	40–50
Long hair	Lower back	220	Underfur length	35–45
			Guard hair length	80–100
			Natural fur thickness	40–50

), eight hair-length and fur-thickness measurements were made at 25 cm intervals both along and 18 cm to the left of the dorsal median line (see Table S2 in Supplementary Materials). Using all measurements, hair length, and fur thickness ranges were estimated. The underfur length gradually increased along the dorsal plane from 15 to 25 mm near the head to 70–80 mm near the tail, and the guard hairs increased from 30 to 50 mm near the head to 105–130 mm near the tail. In contrast, the natural fur thickness (i.e., the distance from the skin to the outer-most guard hairs as they rest naturally) did not increase along the entire dorsal plane but was found to increase from 15 to 30 mm near the head to 40–55 mm at a region 130–150 cm from the nose, remaining at this thickness range across the remainder of the dorsal plane to the tail. Thus, moving from head to tail, the underfur and guard hairs increase in length, but the natural fur thickness remained 40–55 mm over a large portion of the pelt. If the areal density of hairs emerging from the skin remains similar, the bulk density of the fur must increase as hair length increases and natural fur thickness remains constant. Because increased bulk density reduces solar light transmittance, it is expected that solar irradiance will not reach the skin through any region with hair lengths greater than the medium-hair-length region described above.

Defining the dorsal plane as a rectangle with dimensions 70 × 180 cm extending from just behind the head to the tail (see Figure S2 in Supplementary Materials), the surface area of the dorsal plane with hair ≥ medium lengths is 60–70%. Thus, a large portion of the polar bear pelt exposed to the sun allows negligible solar radiation through the fur to the skin, and therefore skin solar absorptance cannot play a significant role in solar heat gain. To avoid further modifications to the pelt, we did not repeat the experiments depicted in Figure 4 or Figure 5 on the short-hair sections of the pelt. However, based on the collective findings described above, much can be ascertained from the temperature-gradient measurements alone.

Using the experimental setup previously shown in Figure 2, we compared the thermal performance without (0 W/m²) and with (1000 W/m²) solar irradiance of the three representative hair lengths in the pelt. The temperature gradients through the fur are presented in Figure 6a,b for 0 W/m² and 1000 W/m² solar irradiance, and the respective skin heat fluxes are shown in Figure 6c. Cross sections of the three hair lengths are shown again in Figure 6d for reference.

Without solar irradiance (Figure 6a), the temperature gradients appear similar in shape, with the long-hair region exhibiting the highest temperatures across the entire fur thickness, including at the skin (patterned data points with black border). Similarly, temperatures for the medium-hair region are greater than for the short-hair region. When the pelt is exposed to 1000 W/m² (Figure 6b), the temperature gradient for the short-hair region exhibits the same general shape but with higher temperatures that monotonically increase to the skin. For the short-hair region, the temperature of the skin is higher than for the longer-hair regions. In contrast, the temperature gradients for the medium- and long-hair regions exhibit maxima about 5 mm above the skin. These data indicate that solar radiation is reaching the skin in the short-hair region of the pelt, leading to heating at the skin surface and increased temperature.

For the longer hair regions, based on the data reported in Section 3, very little light reaches the skin and therefore must be absorbed by the fur, leading to volumetric heat generation. Temperatures are greater for the long-hair region ≥ 14.5 mm above the skin, consistent with more solar energy being absorbed in the fur as the hair length increases. Thus, as the fur becomes thicker, solar heating occurs progressively closer to the fur surface, more insulated from the body by the underlying fur, and more easily lost to a cold environment.

Skin heat flux values are shown in Figure 6c, where negative values represent net heat transfer away from the skin (i.e., cooling), and positive values represent net heat transfer to the skin (i.e., heating). Without solar irradiance, the longer hair region outperforms the other two regions by exhibiting the lowest outward heat loss (−69 vs. −73 and −109 W/m² for the long-, medium-, and short-hair regions, respectively). In contrast, when exposed to 1000 W/m² of sunlight, the short-hair region outperforms the other two regions, exhibiting the highest net inward heat flux (126 versus 95 and 71 W/m², for the short-, medium-, and long-hair regions, respectively). The natural fur thickness is the same (40–50 mm, cf. Table 1) for the medium-hair-length and long-hair-length regions, indicating the observed differences in measured heat fluxes cannot be due to fur thickness alone. The long hair provides a dense insulating layer that minimizes heat loss in the absence of sunlight, but also leads to less solar absorption at the skin. Short hair provides less insulation in the absence of sunlight but is beneficial for solar heat gain by allowing incident solar energy to absorb at, or closer to, the skin.

The results presented in Figure 6 suggest that solar absorption at the skin can play a role in solar heat gain for certain areas of a polar bear. Absorption of solar energy, and thus the pigmentation of the skin, will only matter in locations where the hair is sufficiently short to transmit an appreciable amount of sunlight to the skin. As described in Section 3.4, this is only 30–40% of the back and sides of the polar bear pelt. Importantly, the fur does not need to transmit solar radiation completely to the skin to enhance total solar heat gain. If the hair is sufficiently translucent that solar energy is absorbed deep into the fur, the resulting heat can be protected from loss to a cold environment by the overlying insulating fur. This interplay between the insulating effect of thicker hair and the higher solar transmittance of shorter hair highlights levers that can be employed to create materials for enhancing human thermal comfort.

3.5. Polar-Bear-Inspired Layered Material Constructions for Warmer Cold Weather Garments

Guided by the testing results with the polar bear pelt, we designed layered material constructions that allowed for the transmission of a meaningful amount of sunlight through the outer layers for absorption at the innermost layer, closest to the skin, to generate a local solar heating effect. Each material construction consisted of an outermost shell fabric, a middle insulation layer, and an innermost (i.e., skin side) lining fabric. In total, four different material constructions were assembled by stacking commercially available synthetic woven fabrics using the same polyester sheet insulation (60 g/m², 11.5 mm thick). Three were made using a translucent gray fabric (36 g/m², 0.04 mm thick) as the shell and one of three linings: a white fabric (70 g/m², 0.08 mm thick), a black fabric (70 g/m², 0.08 mm thick), and a light gray fabric (40 g/m², 0.07 mm thick) with a patterned print of heat-retention elements at 50% surface coverage. The heat-retention elements are multilayered (thin metallic layer overcoated with a black pigmented lacquer) and engineered to exhibit both high solar absorptivity and low thermal emissivity [26]. The resulting engineered lining enhances solar absorption while minimizing re-emission heat losses. A fourth material construction was made to represent standard outerwear using a more opaque black shell fabric (70 g/m², 0.1 mm thick) and the same black lining fabric and sheet insulation described above.

Using a similar experimental setup as was used for the polar bear pelt testing shown in Figure 2, we examined the thermal performance of the material constructions under 1000 W/m² of solar irradiance. The experimental setup and the individual fabric layers are presented in Figure 7.

The measured skin heat fluxes for the four different layered material constructions exposed to 1000 W/m² of solar irradiance at 0 °C are presented in Figure 8. Although all samples possess similar thermal resistance to conduction and convection due primarily to the sheet insulation layer, the more opaque black shell sample exhibited the lowest skin heat flux (71 W/m²). In other words, this material construction provides the lowest solar heat gain due to the lower solar transmittance of its shell fabric (τ_solar = 20%). For the material constructions with the translucent shell (τ_solar = 43%), the white (low solar absorption) lining performed the worst (118 W/m² net heat flux toward the skin), followed by the black lining (144 W/m²) and the patterned lining (220 W/m²).

The material construction with the more opaque shell fabric exhibited the lowest solar heat gain because more solar energy was blocked at the outermost surface. Heat absorbed at this location can readily escape into the cold environment. The greater solar heat gain for the fabric stack with the patterned lining was due to the combination of a solar-transmitting shell and insulation layer with a lining fabric engineered to exhibit greater solar absorptance. Solar energy was thus absorbed and trapped closer to the skin, and an almost 3× increase in net skin heat flux was observed. These results might be considered counterintuitive since black is typically viewed as the warmest color in sunlight, so outerwear with black shell fabrics might be expected to be warmer in the sun. As with solar thermal collectors and animal coats, the important concept here is the depth at which the sunlight is absorbed [4,5,18]. By mimicking polar bear fur with material constructions that enhance sunlight transmission through outer insulating layers, solar energy can be absorbed closer to the body, thereby enhancing solar heat gain.

4. Discussion

We developed an experimental setup and methods to evaluate the thermal properties of a polar bear pelt, including the role of skin solar absorption and fur solar transmission. Using a solar simulator and a temperature-controlled plate inside a 0 °C environmental chamber, we measured the net skin heat flux and temperature gradients through the fur thickness with and without solar irradiance, for black and white skin, and for different hair lengths. Our testing showed that in the mid-back section of the polar bear (i.e., the medium-hair-length region shown in Figure 1), solar irradiance increased the skin heat flux and the average temperatures across the thickness of the pelt. This thermal behavior was not changed after painting the skin white. We also found that only 3.5% of solar irradiance was transmitted by the fur to the skin. Overall, our findings show that for the mid-back section of the pelt, most solar absorption occurs before reaching the skin, and therefore skin pigmentation (and resulting solar absorptance) does not play a role in the thermal regulation and comfort of the animal.

We performed additional non-destructive testing at shorter and longer hair sections of the pelt to evaluate whether this conclusion was applicable throughout the whole dorsal portion of the pelt. While longer hair provided less heat loss and higher skin temperatures in the absence of solar irradiance, the shorter hair portion of the pelt (which transmits more sunlight) demonstrated the highest skin temperature and net solar heat gain when exposed to 1000 W/m² of solar irradiation. Further, the short-hair section of the pelt exhibited its highest temperature at its hair-side skin surface, as opposed to inside the dense hair layer above the skin for the longer hair regions. This result suggests that solar absorption at the skin occurs in the short-hair regions of the pelt and that skin solar absorption in these regions could locally play a role in polar bear thermal regulation and comfort. Alternatively, as the short-hair regions occur on the head and upper back near the head, it is interesting to consider whether solar heat gain at the skin contributes to animal orientation and navigation, as suggested by some researchers [13].

Our conclusions are based on investigations of one polar bear pelt. Testing with live animals of different ages, at different times of the year, at different solar angular positions with respect to the oblique natural hair alignment, and with varying levels of hair wetness could all give rise to different conclusions. That said, given that 60–70% of the dorsal portion of the pelt evaluated in this study consisted of sufficiently long hair to prevent a notable amount of solar transmission to the skin, this study does challenge the popular belief that dark skin plays a significant role in polar bear thermal regulation. Another hypothesis is that the skin is pigmented to provide protection from the damaging effects of UV exposure [27,28], which can be amplified on snow due to the high reflectance of UV radiation by snow [29].

Our findings on the pelt motivated us to design material constructions for outerwear that mimic polar bear fur to enhance solar heat gain. Outerwear that takes better advantage of solar heat gain can lead to lighter-weight garments, which is great for energy conservation. We focused our efforts on using commercially available, affordable, and therefore scalable, solar-transmitting shell fabrics and insulation layers atop solar-absorbing lining fabrics. We fabricated four different 3-layer material constructions using the same sheet insulation with varying types of fabrics. By using a translucent shell fabric and insulation layer with a high-solar-absorptance low-emissivity lining, a three-fold increase in net skin heat flux was possible compared to a conventional black shell and lining fabric under 1000 W/m² solar irradiation. Rather than only relying on the thermal properties of different layers, we have demonstrated the importance of tuning solar optical properties to enhance thermal comfort in outerwear and that this was possible using commercially available materials and fabrication processes. This approach allows consideration of other materials and colors other than black for the shell fabrics in outerwear constructions designed for warmth. Additional work on optimizing solar optical properties of different layers has the potential to further enhance solar heat gain and thermal comfort.

5. Conclusions

This study has shed light on the role of skin solar absorptance in polar bears. Contrary to popular belief, a significant portion of the polar bear’s dorsal region does not benefit from pigmented skin with greater solar absorptance because very little solar energy reaches the skin: it is absorbed in the overlying fur. Skin solar absorptance and therefore skin color are expected to matter for smaller portions of the dorsal region (upper back and head). Regardless of skin pigmentation, fur translucence is clearly important for enhancing solar heat gain. The physical principles of transmitting sunlight through insulating layers to allow absorption of solar energy at, or close to, the skin have been employed to develop biomimetic outdoor apparel with enhanced solar heat gain from commercially available materials.