Polar Bear Hair Myth
David B. Schaffer, M.D., Lourene Nevels, PhD.,
Kimberly Lengel, Senior Curator of Mammals
Philadelphia Zoo, Philadelphia, PA
Introduction: Living in severe northern climates, the polar bear’s adaptations to its habitat seem to fascinate many; it is a subject of much discussion and has permeated various popular scientific and lay media. (1-6)
There is considerable agreement on several characteristics: its tongue, nose, and skin are black; it has a 7-11cm thick subcutaneous layer of blubber; (7,8) and it retains heat so effectively that it is virtually invisible to infrared photography, with only the bear’s expired breath showing up on the film. (8) Although this retention of heat is amazing, it is the alleged ability of the polar bear to gather heat in the severe polar climate that has commanded the most interest.
The polar bear’s fur appears white and is composed of two layers: an outer layer of 5-15cm long, tubular, air-filled, pigment-free transparent guard hair and a softer layer of white underfur. (9) It is the long transparent, hollow guard hair that was said to be the mechanism by which the bear gathers heat. (10) The theory was that the transparency allows the ultraviolet rays of the sunlight to be captured by the air in the hollow hair shaft and then to be transported to the bear’s skin in the same way warm water flows through a pipe; a process likened to fiber optics. In their 1980 study, Grojean et al (10) hypothesized that the transported light is converted to heat by the black skin, and the animal is warmed. This single paper gave rise to the “fiber-optic” theory, which has remained prominent in lay media, although many investigators have pointed out obvious fallacies.
For instance, during the Arctic fall, winter and spring (roughly eight months, September through April), subzero temperatures (>/= -40o C) and partial to total darkness prevail. (11,12) Chill factors are made even worse by incessant winds and periodic blinding snowstorms. Moreover, male polar bears and non-pregnant females do not hibernate during this time. They can switch over to a “walking hibernation” (slightly lower body temperature, decreased heart and breathing rates) at any time (summer or winter) when food is scarce, a phenomenon unique to this species. (8) Pregnant female polar bears do not truly hibernate either, but they will den up for 2-4 months at the end of the polar winter to give birth and nurse their young.
White coloration results from the total reflection of all visible wavelengths of light from an object. (13) We see polar bears as white because their fur reflects or scatters all visible wavelengths of light from both the transparent guard hair and the soft underfur. This fact, as well as the polar climate, raises some questions : if all the visible light is being reflected, and only UV light absorption is taking place, how does the bear keep warm during the long, sunless, polar winter? How can ultraviolet light be a viable source of warmth when it is nearly absent for eight months each year? Finally, since the bear does not hibernate, how can this theory explain the bear’s ability to survive the partial or total darkness during the Arctic late fall, winter, and early spring?
These are the questions that led us to examine the hair of polar bears to determine just how unique it is compared to that of other mammals, and to re-examine the “fiber optic” theory still prevalent in lay media.
Methods: Samples of fur were collected in a non-invasive manner from polar, brown, spectacled and sloth bears; cheetah, snow leopard, Amur tiger, Pallas and domestic cat; the Arctic fox and five different domestic dogs, and two humans. Almost all of the specimens of fur from the exotic mammals (ursids and felids) were collected by keepers at the Philadelphia Zoo by sweeping the unoccupied holding areas. Fur samples from the brown bear were obtained through a donation from the Albert Rix Bear Park; the Arctic fox fur from the Riverbanks Zoological Park & Botanical Garden, South Carolina, and from Alaska (obtained by Dr. Mary Murphy from a trapper’s pelt of an adult fox in Denali National Park). The domestic dogs and domestic housecat samples came from private owners, and the human hair from two of the authors.
Because the heavy melanotic pigmentation of some fur prevented viewing its structure adequately, samples were subjected to a 3% solution of hydrogen peroxide (H2O2) for 3, 5 and 7 days. Adequate bleaching for a more detailed structural evaluation was obtained after seven days. As a control, to be sure no changes resulted from the bleach, the white polar bear fur was also identically bleached, and no microscopic structural changes were noted when compared to the untreated samples.
All samples were examined at the Philadelphia Zoo’s Pathology Laboratory by binocular light microscopy at 40x, 100x, 400x, and 600x magnifications, and microphotographs were obtained. Paraffin imbedded longitudinal and cross-sections of both unstained and hematoxylin & eosin (H&E) stained fur samples were made at the University of Pennsylvania’s School of Veterinary Medicine.
Results: Unbleached and bleached typical polar bear guard hairs are transparent, although varying in size, and show a relatively large hollow, central lumen that occupies about one third of the total width of the shaft, and shows a varying amount of segmentation and debris that stained acidic (red) with hematoxylin and eosin. The underfur of the polar bear is smaller, solid and also transparent.
The brown bear’s unbleached guard was darker because of its brown pigmentation, but both it and the bleached hair are essentially the same and showed the identical characteristics of the polar bear’s guard hairs. H & E stains revealed the same acidophilic staining of the cellular debris in the central lumen. Whole mounts of brown bear underfur were also similar to polar bear underfur. At all magnifications they appear indistinguishable from each other.
The remaining animals also showed varying degrees of similarity. The marked similarities throughout were that guard hairs all showed a central, segmented lumen; many of the guard hairs had lumens with widths comparable to the polar bear and all were filled with debris. The underfur in all of the mammals studied were solid, rod-like structures.
Discussion: Having seen that the polar bear hairs (both guard and underfur) were not unique, our investigation into the theory of the solar heating properties of the polar bear’s hair led us to examine the conclusions of earlier studies. Grojean et al (1980) believed that the polar bear hair looked like quartz glass. Using a quartz glass capillary tube to simulate the hair, they showed that there was excellent transmission of UV light, and thus they became the proponents of the “fiber optic-like” theory. While this may be true of quartz, it led them to false conclusions with regard to the polar bear, whose hair has different properties with respect to UV light than quartz. (14,15)
Hair (fur) is keratin, and all keratin, regardless of color, absorbs UV light much more strongly than visible light. However, unlike quartz, keratin does not conduct the absorbed UV light. (14,16) Indeed, when a “white” polar bear is photographed with ultraviolet light-sensitive film, it appears black because of this absorption. The question remained: if visible light is not absorbed, do either the infrared or ultraviolet spectrums of sunlight get transmitted down the keratin (not quartz) tubular hair to the black skin for absorption and conversion into heat?
This question was put to an actual test by Koon, (16) who exposed polar bear guard hair to both infrared and UV light to see if and how far the transmission of either occurred. The study revealed that less than 0.001% of infrared light traveled 2.54cm down the strand of a polar bear hair, and less than one trillionth of UV light got transmitted that same distance. By extrapolation, no meaningful amounts of either would reach the bear’s skin, 5-15cm away. So, visible light is reflected or scattered, some infrared and UV light is absorbed, but none is transmitted to the bear’s skin for heat conversion.
Now, consider the polar bear’s habitat. Sunlight (visible, UV and infrared) in the Arctic occurs only for six months during which time the sun is never directly overhead. According to Dr. Bernard Stonehouse from the Polar Ecology and Management Research Group at the Scott Polar Research Institute, UV light levels are indeed generally low in the Arctic and UV contributes only a very small proportion of total energy in comparison to visible light, (17) and temperatures drop below zero (>/= -40o C) during the dark winter. (12) Precisely the time of the year when the bear most needs warmth corresponds to the months when little or no daylight is available.
The behavior of the polar bear also contradicts the solar energy theory. During the summer months, the only time when the sun is high enough in the Arctic to produce warming of animals, the polar bear’s greatest thermoregulatory challenge is staying cool, as they do not tolerate overheating. Swimming is one effective way for the bear to cool down, as its fur flows freely away from the skin in the water, (18) allowing the body temperature to drop.
Moreover, if the guard hairs of the polar bear had a heat conducting function, the bear would be in serious trouble during these sunlit months. The properties of the hair shaft, a dead tissue, are fixed and not under any nervous or hormonal control. If the hair transmitted UV and/or visible light to the body for warmth, overheating and dangerous hyperthermia would occur during this time. The fact that polar bears survive well outside in zoos such as San Diego, Dallas, and the warm summer months of Philadelphia speaks for itself.
During the months of little to no visible, infrared or UV light (late September through late March) polar bears are most active, hunting on land and in the sea almost constantly during this time. And in the freezing waters of winter, the fur still floats freely, allowing the icy water in contact with the skin. The ineffective insulation provided by the fur is further illustrated by the fact that cubs are kept from the water until after they are six months old and have developed a sufficiently effective fat pad. (8)
Conclusions: Based on the almost identical appearance of polar and brown bear hair, as well as the similarities observed among all the hair samples from the other mammals examined, polar bear hair is not unique.
The polar bear is an endothermic mammal, not an exothermic reptile. Like other mammals, the bear’s own internal metabolism is the primary heat producing mechanism, not the hair.
As for the function of the white fur, it primarily functions as camouflage and protection; while its role in thermoregulation is most likely to provide the same protection as the sweater or coat we wear, helping to keep the bear from losing heat rather than gathering heat.
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