You can find amazing and wonderful things when you browse old science journals. And when I say “old,” I don’t mean that you even have to go back very far. While I was tracking down an article in the journal Applied Optics from 1980, I came across a paper by R. E. Grojean, J. A. Sousa, and M. C. Henry1, “Utilization of solar radiation by polar animals: an optical model for pelts.”
Yes, it’s a paper that looks at whether the fur of polar bears works as a solar heat converter!
You would think that this would be a relatively simple question to answer. We’re not talking quantum physics, we’re talking polar bear fur, but the exact optical purpose of polar bear fur, if any, has been surprisingly controversial — and as far as I can tell has not been solved! Let’s take a look at the history, the science and the mystery.
The paper by Grojean, Sousa and Henry (GSH) is very much a product of its time. It begins with a very practical motivation,
The existing energy crisis and the impending shortages of fossil fuels have given added impetus to the investigation of direct conversion of solar energy to more immediately useful forms.
The paper came out in 1980, on the heels of the 1979 oil crisis, a result of a drop in oil production on the heels of the Iranian Revolution. I grew up singing this catchy 1978 tune, which was intended to teach Americans to conserve energy, which was in scarce supply:
The polar bear research was a collaboration between Grojean at Northeastern University and Sousa and Henry at U.S. Army Natick Research & Development Command, Clothing, Equipment, & Materials Engineering Laboratory.
The purpose of the research was two-fold. On an immediate level, the researchers wanted to find ways to improve arctic military uniforms and equipment. More generally, the researchers hoped that their results would provide insights into improving solar energy technology.
Polar bear pelts are a good example of the tradeoffs that evolution must make to optimize the survival of species. Living in the arctic, where the temperatures are extremely cold, it would superficially seem that black fur would be best for a bear, so that it soaks up as much sunlight as possible to convert into heat. But living in the snow, as a predator, a polar bear needs to be white to match its background. So how does a polar bear balance its need for warmth and its need for camouflage?
In the paper, GSH open with a discussion of existing human-made solar converters, and then quickly move to discussing optical observations of polar bears. Polar bears are very hard to see with visible light — which is the point — and also are extremely thermally insulated, so they are almost impossible to see with infrared radiation.
But polar bears strongly absorb ultraviolet light, and that offered a clue to the researchers: do polar bear pelts draw in UV light to bolster their heat?
The researchers put polar bear hair under a microscope to determine its structure. They found that the hairs are essentially transparent to visible light and contain a partially hollow core with a rough surface. Their illustration of the structure is shown below.
This structure suggested a fascinating possibility to the researchers. Fiber optic cables guide light through the use of total internal reflection, in which a light beam bounces endlessly as it travels along the cylinder. I’ve blogged about total internal reflection before; you can even guide light in a stream of water, and if the stream is thick enough, you can even see the light bounces!
GSH proposed more or less the following: light enters a hair from outside, and hits the rough inner surface of the hollow region and gets scattered in all directions. Most of that light will leave the hair, giving the pelt a white color. Some of that light, however, scattering at an angle close to parallel to the hair length, will be trapped by total internal reflection and be funneled towards the black skin of the bear, where it will be absorbed as heat. Roughly speaking, light is more likely to be totally internally reflected the higher the refractive index of the hair, and this refractive index depends on wavelength. The index is much higher for UV light than visible light, which means that more UV light will be trapped than visible. Thus, we end up with more visible light reflected, giving the bear its white color, and more UV light funneled to the bear’s skin, providing warmth.
It seems like a great explanation, and a magnificent example of natural selection at work, but is it correct? GSH were immediately challenged by Craig Bohren and Joseph Sardie, in a comment on the GSH paper2. Bohren, in particular, was someone who could not be ignored — he is one of the world’s leading expert on light scattering by small particles, and co-wrote one of the classic books on the subject.
Regarding the darkness of a polar bear belt under UV light, Bohren and Sardie offered a simpler explanation: hair is made primarily of keratin, and keratin strongly absorbs in the UV range of the spectrum. Fiber optic style light guiding is unnecessary, they said, and the answer is simply mundane absorption. They also argued that the roughness of the inner core of the hair is unnecessary to produce the whiteness, because a large number of nearly transparent fibers will naturally appear white! (This is definitely true: this is exactly why paper appears white: it is a collection of transparent fibers all tangled up.)
Even more, Bohren and Sardie note that there simply isn’t that much UV radiation at ground level, even in the arctic, to make UV conversion worthwhile. They note that only 7.3% of all solar radiation is in the UV range, and a large amount of that is scattered by the atmosphere, so there really isn’t that much UV energy available for heat conversion in polar bear fur anyway!
GSH had to respond to that, and they wrote a “reply to comment” to Bohren and Sardie’s comment3. It is difficult to provide much detail in a reply to comment, so their responses are very qualitative and general. For one thing, they acknowledge that their model is still very preliminary and incomplete. To answer the question of keratin, they note that there are arctic animals with animal fur that doesn’t absorb in the UV, such as the arctic hare. The presence of keratin, they argue, therefore doesn’t prove that polar bear hair absorbs strongly in the UV. Finally, they note that the amount of UV radiation that a polar bear is exposed to is likely higher than the estimate Bohren and Sardie make, and they provide anecdotal evidence to support this, such as the prevalence of UV-induced snowblindness in the arctic.
The argument between GSH and Bohren and Sardie wasn’t settled, though both sides at least agreed that more measurements needed to be taken. But things got even stranger about a decade later, in 1990, when researchers from the Hahn-Meitner Institute in Berlin released the paper4, “Light collection and solar sensing through the polar bear pelt.” As the title suggests, the German researchers not only continued the study of polar bear pelts as light collectors, but also hypothesized that it works as a light sensor, as well!
The Germans noted that there does seem to be significant absorption of UV light in polar bear hair, which would seem to disprove GSH’s hypothesis. However, they also noted that the hairs appear to show unusual luminescence characteristics: that is, the hairs absorb UV radiation and reemit it at a much longer wavelength, much closer to visible light. This suggested to them a modified model of polar bear hair light guiding: imagine that UV light hits the polar bear hair and causes fluorescence, and that fluorescence propagates with much less absorption to the skin of the bear!
Here is where things get interesting. Referring to experimental measurements of polar bear skin temperature and estimates of how much solar energy is collected by the bear’s pelt, they conclude that the amount of heat provided by the pelt is pretty much insignificant compared to the bear’s own internal heat regulation system. If a bear’s fur has evolved as a light collector, then, what purpose does it serve? They suggest that it helps the bear navigate using the sun!
Though the solar collection doesn’t really affect the overall warmth of the bear, it locally influences the skin temperature of the bear — the part of the bear facing the sun gets warmer than the side away from the sun. A bear wandering the blinding snow on a sunny day (remember snowblindness?) would not be easily able to see where it is going, but it could sense direction based on the heat on its skin! Their illustration of the process is shown below.
The latissimus sheets are muscle groups which have a rapid adjustable blood flow to keep the skin temperature stable and could be used as a temperature reference, to sense which part of the bear’s skin is warmer and thus which direction the sun is shining!
It is a remarkable hypothesis, and if true it is amazing! Is it true? The authors note that more field studies would be needed to confirm or refute it. They also suggest that this sensory system could be at a very early stage of evolution and very crude; but little advantages can still make a big difference.
There is at least one more wrinkle to this story: in 1998, Daniel Koon of St. Lawrence University published a paper5 “Is polar bear hair fiber optic?” Koon reanalyzed the data of GSH and the German researchers, supplemented with his own measurements of the light transmission properties of polar bear hair, and concluded that the light gets absorbed by the hair over too short a distance in order for it to be effectively guided to the bear’s skin. In short, he estimated that light energy would be reduced over ten orders of magnitude while propagating through the length of a typical polar bear hair. This would be a negligible amount of energy reaching the skin, and would suggest that the hair serves no heat conduction role at all. However, this is one study, by a single researcher, and as far as I know it has not been independently confirmed, so it seems that the various explanations for the optical purpose of polar bear hair remain up in the air.
So in the end, it turns out that it is very difficult to determine whether or not there is an evolutionary optical purpose for the structure of polar bear hair! And we cannot rule out the possibility that it is all a red herring, and that the strange structure of the hair serves a very different purpose unrelated to optics. For example, a 2011 paper by Chinese researchers focused on the hollow core of the hair, and the irregular “chambers” within it. They note that the little chambers act as obstructions to the flow of heat, preventing the hollow core of the hair from easily transferring heat back away from the bear and to the atmosphere. So the hollowness may have more to do with obstructing heat flow than collecting light energy!
This is one of the fun things about science — even seemingly simple questions can turn out to be surprisingly difficult to answer!
- R. E. Grojean, J. A. Sousa, and M. C. Henry, “Utilization of solar radiation by polar animals: an optical model for pelts,” Appl. Opt. 19, 339-346 (1980).
- Craig F. Bohren and Joseph M. Sardie, “Utilization of solar radiation by polar animals: an optical model for pelts; an alternative explanation,” Appl. Opt. 20, 1894_1-1896 (1981).
- R. E. Grojean, J. A. Sousa, and M. C. Henry, “Utilization of solar radiation by polar animals: an optical model for pelts; authors’ reply to an alternative explanation,” Appl. Opt. 20, 1896-1897 (1981).
- H. Tributsch, H. Goslowsky, U. Küppers, H. Wetzel, “Light collection and solar sensing through the polar bear pelt,” Solar Energy Materials 21 (1990), 219-236.
- Daniel W. Koon, “Is polar bear hair fiber optic?,” Appl. Opt. 37, 3198-3200 (1998).
- J-H. He, Q-L. Wang, J. Sun, “Can Polar Bear Hairs Absorb Environmental Energy?” Thermal Science 15 (2011), 911-913.