In a Siberian zoo east of Moscow, Russia, a polar bear cub plays with its mother. (Ilnar Salakhiev/AP)

Polar bears are white, right? Not quite.

Though closely related to the brown bears you might find in the most northern regions of the United States, polar bears have a special set of adaptations that allow them to live in the cold Arctic. The most famous of these adaptations is their bright white coloring, which lets them blend in as they roam across ice and snow in search of seals to eat. Can you imagine a dark brown bear trying to hide against bright, white snow?

Although while polar bears usually look white, their fur isn’t white at all. And their skin is black! Polar bear fur is actually see-through, but it takes on a white color because of its structure.

Your hair gets its color from something called pigment. Different types of pigment form in different amounts to create various colors when light hits them, sort of like when you mix shades of paint. But polar bear hair has a structural color, which comes from the way light bounces around the structure of the hair itself — no pigments required.

Unlike human hair, polar bear fur is hollow like a straw. These tubes are too small to see without a microscope, but there’s enough room for light to scatter inside. When the bears stand in the sun and all that light bounces off them, they look white.

In coastal Alaska, an adult dries off after taking a dip in the Chukchi Sea, south of the Arctic Ocean. (Brian Battaile/U.S. Geological Survey/AP)

Some scientists used to think these hollow hairs might do more than just help bears blend in. They thought the structure of the hair, along with the black skin beneath, allowed polar bears to absorb way more heat from sunlight than other animals can. It’s a nice idea, because one wonders how animals manage to keep so toasty warm in Arctic conditions. But experiments showed that very little sunlight actually makes it all the way down the hairy tubes to touch bare bear skin. That makes sense, because the time of year when bears actually have to worry about keeping warm — the Arctic winter — is almost totally dark. It would be silly if their hairs were designed to collect as much warmth as possible during the sunny, warm summer.

If you occasionally see a polar bear who looks a little green, hollow hairs are to blame. Tiny plants called algae sometimes grow inside their hollow hairs. The space where light would usually scatter to create white coloring is filled with green stuff instead, so the bears look like they just took a tumble in some fresh-cut grass. This happens only when it’s quite balmy. Climate change is raising the temperature in the Arctic. But green bears have been spotted more often in zoos, where the weather can be even warmer than an unusually hot Arctic summer. Zookeepers have found a solution, according to Julie Hartell-DeNardo of the St. Louis Zoo: Chilled, salty water with a good filtration system can keep algae growth at bay, so even captive bears can generally maintain their bright white sheen all year round.

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