The humble polar bear has one interesting quirk to its fur, which has netted it two different problems. One is algae, and one is a piece of pseudoscientific nonsense.
In the YouTube video above, you're going to see a couple of green bears. This is actually pretty common among zoo bears, especially in hot and muggy climates. It's not just that algae grows on their hair. It's that algae grows inside their hair. While most animals produce hair that's a long, thin cable, polar bears produce hair that's a hollow tube - like the world's most dangerous drinking straw. The tube is colorless, but it's made with translucent material. Any light that comes in is scattered, and so it appears white. (This is one of the reasons polar bears look brightest in direct sunlight.) In cold or harsh climates, the tube of hair stays empty. In hotter ones, even if the bear is being regularly washed, algae starts growing inside the tube of hair, turning large sections of the bear green.
In the 1970s, people noticed the similarities between bear hair and fiber optic cables. They also noticed that a shaved polar bear has black skin. Suddenly people thought that they had discovered how the bears stayed warm. Their hair was channeling heat right to their skin, which was absorbing it as only black material can. It was a great notion, nature going high tech, but sadly it was shown to be false. Finally, Daniel Koon, a researcher at St. Lawrence University tested out the notion.
It was thoroughly debunked. The the hair channeled between a thousandth and a trillionth of a percent of visible light for one inch, and it absorbed UV light entirely. Biologists pointed out that being the perfect fiber optic cable might not work well for bears. They're already built to withstand the cold, but being on an exposed piece of ice in all-day sun with no shade, night, or relief would cause them to overheat. You can still find the myth in plenty of places, though.
Via Applied Optics, Alaska Science Forum, Bear.org, and The Library of Congress.