If you were asked, “what color is polar bear fur?” the obvious answer is “white.” It certainly looks white and yes, for all intents and purposes, polar bear fur is white. But perhaps you’ve noticed that sometimes a polar bear looks yellow, gray, or even green. Why is that? Do some polar bears have different color coats? Does the coat color of a polar bear change with the environment? Do polar bears just need better shampoo?
Before we get to the bottom of why polar bears appear to have such a range of colors, it’s important to know a key fact about polar bear fur. Polar bear fur isn’t technically white…it’s translucent. That’s right, a polar bear’s outer coat consists of guard hairs that are actually devoid of pigment and are essentially “clear” hollow tubes.
Now I’m sure you want to know how it is that a polar appears white (and yellow, green, etc.) with all that translucent fur, so let’s talk about the structure of that fur.
The Structure of Polar Bear Fur
Mammalian hair is composed of keratin which is typically organized into three layers. The cuticle is the outermost layer and is comprised of a very thin, translucent layer of overlapping “scales.” Below the cuticle is the cortex which consisted of long, spindle-shaped cells and may contain pigment granules (in the case of colored hair) along with corticle fusi (which are essentially air spaces). Lastly, in certain hairs of some animals, an additional layer known as the medulla is present. The medulla is essentially a structured, air-filled core that is quite well-expressed in many arctic animals such as polar bears.
In the case of polar bears, the guard hairs consist of a thin cuticle encasing a well-developed cortex and a wide medulla. The cortex in these hairs is devoid of pigment and the medulla is essentially a wide airspace. Light hitting this fur is subsequently scattered in all directions giving the appearance of a white bear.
So, when you see a polar bear and perceive it as a big, cuddly mass of white fluff, you’re eyes are actually interpreting the scattering of all wavelengths of visible light rather than a color produced by pigments.
Gray, Yellow, and Green Polar Bear Fur
So what about the sometimes gray, yellow, or green appearance of polar bears?
In the case of gray bears, the color is most likely the result of the thinner coat produced by polar bears in the summer. Since polar bears have black skin, a little bit of the black undertone can show through the thinner summer coat and produce a gray appearance. This is particularly evident around the face of a polar bear near the nose.
In the case of yellow bears, it would seem that prolonged exposure to sunlight oxidizes the hair strands, creating a yellow appearance. Thus, a polar bear appears whitest after a fresh molt when the hair is freshest.
Last, but not least, is the curious case of the greenish polar bears. Greenish polar bears are often seen in zoos located in warm climates and in 1979, researchers Ralph Lewin and Phillip Robinson determined that the greenish appearance is the result of algae.
Under warm conditions, the “hollow” medulla of polar bear guard hairs creates a humid microclimate that is well-suited for fostering masses of green algae derived from the enclosure ponds of these captive bears. Since the rest of the hair is translucent, the green color from the algae in the medulla creates a greenish overall appearance. However, this phenomenon appears to be restricted only to the guard hairs since the medulla of undercoat fibers appear to be too thin (in the area of 20 micrometers) to facilitate algal infiltration and growth.
So there you have it: polar bear fur isn’t actually white at all. Now go take this newfound knowledge and blow some minds.
References and Further Reading
- Derocher, A. E. and W. Lynch 2012. Polar Bears: A Complete Guide to Their Biology and Behavior. Johns Hopkins University Press, Baltimore, Maryland
- Lewin, R. A., & Robinson, P. T. (1979). The greening of polar bears in zoos. Nature, 278(5703), 445-447.
- Microscopy of Hair Part II: A Practical Guide and Manual for Animal Hairs. 2004. Federal Bureau of Investigation, Quantico, Virginia.
- IUCN/SSC Polar Bear Specialist Group.
- Alaska State Museum Bulletin. Issue 45.