| MadSci Network: Evolution |
That is a very well thought out question Shama. The answer is that it is much easier for cells to make organic molecules that absorb light than it is for them to make organic molecules that reflect light.
Melanin contains many aromatic rings, and has a large number of double bonds, which can easily absorb photons of different wavelengths. Photons in the Ultra Violet (UV), Infra Red (IR) and Visible ranges all get absorbed by melanin in the top layers of the skin, instead of the delicate tissues beneath the skin. Melanin is so good at this absorption that it is used to protect the skin from light damage.
However, there are some other strategies that can be used to protect the skin from light damage. Polar bear hairs reflect and scatter light, exactly as you suggest. However, these hairs contain no pigment; instead they have a hollow core that runs through the center of each hair. When light hits the hairs, it is scattered and reflected by the air in the center of the hair. This is the same reason that grey hair seems so reflective in people; grey hairs have bubbls of air trapped along their lengths, which scatter light.
In addition, the keratin which makes up polar bear hairs is extremely efficient at absorbing UV and IR wavelengths of light. If you try to take a picture of a white polar bear using only UV or IR light, the bear will appear completely black, because all of that light is absorbed by the hair.
A polar bear's skin is black with melanin however, so that if any light does make it past the hairs, it gets absorbed by the melanin.
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