Before we get to the bears, let's look at snow. Why is it white? After all, snow is simply crystalized water droplets, and everyone knows that water is transparent.
Light is made up of many different frequencies, and each of these particles (photons) exist at a different energy level. As light hits an object some of these particles are absorbed, some pass through the object and others are reflected off. Those that don't pass through or are not absorbed are seen by our eyes as that object’s color.
Snow is made up of frozen water crystals all stuck together with lots of little air pockets mixed in. The ice molecules are translucent (not transparent as you might have assumed), which means that energy particles that pass through this material are deflected slightly so that they exit at a slightly different angle than they entered. As light passes through the ice crystals this deflection continues, aided by the scattering effect of the air pockets, with each layer it encounters, bouncing around until it finally exits the mass of snow again. Remember, each returned light frequency contributes to what we perceive as an object's color. Because this process occcurs with each wavelength, all frequencies of the light spectrum are returned, so our eyes see the snow as all of these "colors", which is essentially what the color white is.
A polar bear reacts with light just like a big pile of snow. There are actually two layers of fur; an under layer made up of soft, wooly hairs which help to insulate the skin, and a top layer with longer, stiffer hairs (called "guard hairs") which are covered in oil to keep the bottom layer from getting wet. Even after an ocean swim, the bear simply shakes the water off this top layer and the skin is completely dry. Both of these layers have hairs that are translucent shafts. Together the hairs act just like the snow piles described above. When sunlight hits the body, the photons of each wavelength ricochetes through the hair and is scattered by the air pockets until they shoot back out again, making the animal appear to be covered in white fur.
Above is a closeup shot of a polar bear’s fur. Even here the coat looks white, but if you focus on a single follicle you’ll see that it is nearly transparent. Note that like with snow, translucency is more important for the hair than transparency. If it was transparent you would see right through the fur like glass (all photons would pass through the material) and the bear would appear to be completely black. This is because beneath the white fur, that is exactly the color a polar bear’s skin is.
There was a theory a while back that these hollow, translucent hairs were an adaptation to the cold. The idea was that the hairs acted like fiber optics, trapping sunlight and funneling the energy through the hairs and heating up the skin. Because the color black (actually the absence of color) absorbs heat, the thinking was this would be a perfect way for a polar creature to warm its body. As it turns out a bear’s under layer of fur isn’t even hollow (although the outer guard hairs are), so this theory has been tossed. Besides, even if this was true it wouldn’t be much use to an animal that may spend four or five months at a time in darkness, since polar regions have long spells of daylight followed by long spells of nighttime, each stretch longer and longer the closer you get to the pole. Here’s a description of the study that debunked this theory.
Some polar bears appear yellow, especially during the summer and in older individuals. This is caused by oxidation within the hairs by sunlight. Bears kept in captivity and housed in freshwater pools rather than saltwater ones, sometimes appear green. This is from algae in the water that has attached to the fur and discolored it. Since the algae doesn’t survive in salt water the bears are treated with either a salt solution or with hydrogen peroxide to return them to their normal white color.
Now meet Pelusa. A few years ago this 23 year-old resident of the Mendoza Zoo in Argentina came down with a nasty case of dermatitis. The vet prescribed a treatment using a medication called Gentian Violet, which turned the animal bright purple and made her a minor celebrity both in South America and on the net. Articles I’ve read say the medication was sprayed on and she will return to normal once the treatment is stopped (she did). It looks too uniform for me to believe it was just from being sprayed on so my guess is that the meds were absorbed by the sebaceous glands and the color was distributed throughout the skin and into the hair follicles. If you use Frontline on your pet, that is how this anti-flea medication is spread across your dog or cat's skin.
Polar bears (Ursus maritimus) are the apex predator of the arctic. They feed almost exclusively on seals, which are grabbed by the bear when they surface to breathe through holes and gaps in the sea ice. For a big predator to survive in polar regions it obviously must have some way of keeping itself warm. It turns out that polar bears have several ways of doing this in addition to the two-layered fur.
You might guess it’s the bear’s blubber that keeps it from freezing, but unlike seals, the blubber of a bear is concentrated in the legs and belly and used as an emergency source of energy whenever food gets scarce. You’ll notice that although a bear has enormous feet, these are heavily padded to reduce heat loss on the ice. Also, the ears and tail are very short and these would normally be two places where body heat would easily escape from.
But the most important things keeping the bear from freezing are those air pockets between the hair we mentioned earlier. These work sort of like a wet suit, and if you’ve ever surfed or snorkeled in cold water you know how horrible those first few minutes can be. The thin layer of cold water that seeps between the suit and your skin is warmed up by your body and then serves to insulate you. These air pockets, warmed by the animal’s body and trapped by the fur is its best heat-conserving mechanism. In fact this is so effective that a polar bear is one of the few animals that can’t be captured with infrared photography. You’ve all seen how this works where different colors of an animal or body of water represents different temperatures (here are some very cool infrared photos of animals). In an infrared photograph of a polar bear all you see is a small red spot in front of the bear’s mouth, heat lost as it breathes.
[Update] I wanted to include another analogy to try to better describe how this double layer of fur works. When we go out on a cold and rainy day we can keep warm by wearing a sweater and a jacket. But look at a sweater sometime and you'll wonder how this loose-knitted, full-of-holes thing could keep you warm. The holes are air pockets that your body heats up to insulate you. But this won't work if the air pockets fill with water or wind exchanges the air in them faster than you can heat it. The jacket's role is to keep the wind and rain off the sweater so the sweater can do its job. The inner, wooly hair is the bear's sweater and the oil-covered guard hairs make up its jacket.
For a bit of a mea culpa, the title of this diary was a little misleading. Even though the individual hairs are clear, if we see an animal is white then it’s white. And hopefully we’ll get to see these incredible animals for many years, but things are not looking good for Ursus maritimus. Climate change has already cost the bears some of its sea ice habitat, which is vital since seals cannot be caught in open water regardless of how well this species of bear can swim. And they can swim extaordinarily well, but they can’t swim forever. For the first time ever researchers have been finding dead bears floating far from any ice packs, signs that some of these animals are drowning trying to travel from one ice sheet to another as the space between ice sheets expand. This poor guy above may very well be royally screwed.
Visit Polar Bear International, one of the best groups around fighting for this species.
Other diaries in this series can be found here.