To: All Msg #92, Jun1893 08:10PM Subject: Polar Bear Hair I finally found the reference I

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From: Mark Isaak To: All Msg #92, Jun-18-93 08:10PM Subject: Polar Bear Hair Organization: The Aurora Group, Palo Alto, CA From: (Mark Isaak) Message-ID: <> Reply-To: (Mark Isaak) Newsgroups: I finally found the reference I remembered to the myths about polar bear hair. It's in "Letters", _Scientific American_, Sept. 1988, pg. 8, by D. M. Lavigne, Dept. of Zoology, Univ. of Guelph, Ontario. (In the process, I also found a letter (from Aug. 1991) debunking the idea that humans are the only mammals which can't drink and breathe simultaneously. But I digress.) I quote the polar bear letter herewith, in its entirety: To the Editors: "Solar Polar bears" ["Science and the Citizen," March] is only the latest version of a story that has been in circulation now for almost 10 years. The fact that the story is largely fictional seems to go unnoticed. The solar-polar-bear tale is a classic example of scientific folklore. Each time it is told, new elements are added, but the essential ingredients remain the same. Obviously the story has wide appeal, and most readers, including many scientists, it seems, are not aware of its problems. "It must be true," they say; "I saw it in the newspaper"--or in _Time_ (December, 1978), _Natural History_ (October, 1981) or _BBC Wildlife_ magazine (February, 1988). Now that it has reached _Scientific American_, can textbooks be far away? Space does not allow me to outline the entire history of this legend, but I should like to put your own story into perspective. It has been known since the late 1960's that the pelts of certain white animals are capable of trapping sunlight and converting some of it into warmth on sunny days in cold environments. The white hairs reflect and transmit much of the visible and near-infrared rnadiation (the major components of sunlight) down through the pelt, where it is absorbed by a dark skin. Skin temperatures well above the deep body temperature (37 degrees C) have been recorded in such animals. The heat that is emitted from this warm skin then becomes trapped by the white hairs, warming the layer of insulating air in the pelt. Such a "greenhouse effect" is not unique to seals or polar bears; it was reported in certain arctic and subarctic plants and "woolly bear" caterpillars as early as 1955. The seminal work that spawned the legend of "solar polar bears" was a more recent observation (but not as recent as suggested in your story). Working at the University of Guelph in the early 1970's, Nils Oritsland and I found that the pelts of several white animals, including whitecoated harp seal pups and polar bears, _absorb_ much of the ultraviolet component in sunlight. (Short-wave UV rays make up less than 10 percent of sunlight; they contribute to the synthesis of vitamin D, cause sunburning and tanning and in highly reflective environments may cause damage to the human cornea, a condition known as snow blindness.) Since ice and snow reflect UV, we found that UV photographs could reveal "black" seal pups and polar bears and also military equipment painted "white" for camouflage in the Arctic--but with UV-absorbent paint! (Other details of our work as they were outlined in the story are incorrect, but since these details are not essential elements of the legend, I shall not go into them here.) With this brief overview we can now evaluate your version of the solar-polar-bear legend. Your piece states that polar-bear fur is a "natural collector that convert part of the solar-radiation spectrum into heat with an efficiency exceeding 95 percent." It does not say specifically which part of the solar spectrum might be involved, but I am not familiar with any such data. The information with which I am familiar suggests that polar-bear hair is not nearly as efficient as contended in utilizing visible and near-infrared components of sunlight, and not nearly as efficient as certain seal pelts. Even cow pelts appear to be more efficient in this regard than polar-bear fur. Your piece also states that the polar bear is the "most efficient absorber" of the UV component in sunlight, but this is also not correct. Every white bird and mammal we have examined absorbs UV with wavelengths of between about 290 nanometers (the shortest wavelength found in sunlight) and 320 nm., and dark-colored birds and mammals are much more effective than any white animals, including polar bears, in absorbing the entire range of UV wavelengths in sunlight (between 290 and 400 nm.). The observation that individual polar-bear hairs are "actually colorless, not white," attributed to Richard E. Grojean sometime within the past seven years, was actually published by Oritsland and Keith Ronald in 1978. Their observations pertained _only_ to visible and near-infrared radiation, not to UV. And to compare a polar-bear hair to a "quartz fiber" is nonsense. Quartz transmits near-UV (that is why quartz lenses are used for UV photography), whereas, as the piece correctly notes, polar-bear hair absorbs UV. It is for this reason that mechanisms proposed to explain how polar-bear hairs trap UV radiation to gain heat will not work. In early versions of the legend it was proposed that the UV somehow entered the hollow core of the polar-bear hair, which then functioned as a "light pipe" to direct the radiation to the skin. Of course, since the UV is absorbed by the protein in the outer layers of the hair, little if any of it would actually reach the hollow core to be transmitted magically down the light pipe. Presumably in an attempt to overcome this problem, the legend has been modified, as is shown in the diagram for your piece. Now it is proposed that the UV is captured by the outer solid part of the hair (labeled "Transparent shaft") and aimed "toward the skin." Once again there is a serious problem: the "shaft" of the polar-bear hair is _not_ transparent to UV; it absorbs it. Yet if a UV-absorbent material could significantly improve the efficiency of solar collectors, then, as I have indicated above, researchers would choose a highly UV-absorbent dark pelt rather than a white polar-bear pelt for the purpose. Indeed, if one wants to fill solar collectors with hairlike fibers in order to _absorb_ more of the solar spectrum, why not fill the collectors with black hairs? This brings me to what is perhaps the most perplexing aspect of this bit of folklore: the tenuous connection between UV radiation and heat from the sun. No one to my knowledge has demonstrated that UV contributes anything to thermoregulation in polar bears and other mammals. And since UV is only a small part of incident solar radiation, it seems peculiar to concentrate on this part of the spectrum in order to improve solar collectors or, as in earlier versions of the legend, cold-weather clothing for the military. D. M. Lavigne Department of Zoology University of Guelph Guelph, Ontario -- Mark Isaak "There lives more faith in honest doubt, Believe me, than in half the creeds." - Tennyson


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