Polar Bears: The Crazy Science Behind Their Black Skin and Transparent Fur

Mike Lavery | | BrainsBrains
Polar Bears
A red bear at sunset: Photo: Jon Cornforth/Dailymail

Forget everything you’ve seen and learned – polar bears aren’t actually white. What? What color are they? Well, it’s complicated…

An object’s color in general is determined by what color of light it reflects. A red car is reflecting red light, and absorbing all other colors. A black object is absorbing all colors, and reflecting none. White, on the other hand, is a reflection of all colors. This is why wearing white on a hot summer day generally feels better than black.

Underneath that heavy coat, polar bears have black skin.  This is an adaptation to help them absorb as much heat as possible from the sun’s rays. I know you’re thinking, “doesn’t that white fur just reflect it all away?”

Polar Bears
Black Skin: Photo: WWF

Polar bear fur isn’t actually white! It’s hollow and made of transparent keratin, with little to no pigmentation. When sunlight hits the bear, it undergoes a complex scattering process within the fur (more on that in a bit), with a small amount of light eventually being reflected. Since the sun’s light is white, it’s the color we perceive polar bears to be.

How polar bear fur actually functions is still a subject of research and debate. Initial studies concluded their fur functioned like fiber optic cables. It was thought that light entered a hollow hair fiber and bounced back and forth off the inside until it was transmitted directly into the bear’s skin. Subsequent studies have shown that a single hair’s ability to transmit light is weak and only under perfect circumstances can light travel far enough in one fiber to reach the skin [1].

Polar Bears
Polar bear fur up close. Photo [1]
More recent research has concluded that the hair works collectively as a system to transmit light and heat.  According to a study by M.Q. Khattab and H. Tributsch [1]:

The polar bears fur combines two distinctive optical phenomena: the light collection through optical scattering and the luminescence light collection with the aim of capturing light and concentrating it at the basis of the hairs, where it is converted to heat. The transfer of the light is facilitated by the combination of those two optical phenomena. 

In simpler terms, light only travels down an individual hair for a short distance before being scattered and leaving the hair. Another nearby hair picks up the scattered light, and the process repeats over and over until the light is absorbed by the bear’s skin or dissipates into heat and gets trapped by the thick undercoat. Some of that light is backscattered and is what gives the bears their white appearance. If not for this scattering process, the bear’s fur would be completely transparent and we’d be able to see their black skin [1]. Polar bears rely on their white appearance for camouflage while hunting, so a black polar bear probably wouldn’t last too long.

Polar Bears
Demonstrating how polar bear fur scatters the light of a laser. Photo [1]
Polar bears can end up being a variety of colors in the right kind of light. On cloudy days they’ll be a bit gray (I mean, who isn’t?) and even can turn red or orange at sunset!  Once again, nature is the ultimate engineer.

Reference:

[1] Khattab MQ, Tributsch H.Fibre-Optical Light Scattering Technology in Polar Bear Hair: A Re-Evaluation and New Results. Journal of Advanced Biotechnology and Bioengineering 2016;3,38-51, Synergy Publishers. LINK


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2 thoughts on “Polar Bears: The Crazy Science Behind Their Black Skin and Transparent Fur

  1. I read that a polar bear at a zoo looked purple because she was given medication that temporarily made her skin turn purple. So after reading this article that story does not seem true. Is it?

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