Polar Bears Have Black Skin Under Clear, Hollow Fur

Few animals are as famous for being white as the polar bear. Yet the skin under its coat is black, and the hairs of that coat have no white color in them at all.

A White Coat Made of Clear Hairs

Polar Bears International, the conservation group that studies the bears, puts it plainly: each hair shaft is pigment-free and transparent, with a hollow core.

That hollow core scatters and reflects visible light in the same way ice and snow do. Neither snow nor a polar bear's coat contains white pigment. Both look white because of how light bounces around inside them.

Lay a single polar bear hair on a table and it looks transparent, as Alaska's Geophysical Institute noted when it reported on a physicist's tests of real bear hairs.

The coat comes in two layers: water-repellent guard hairs on top of a dense undercoat, according to the Alaska Department of Fish and Game, which describes the coat as white or yellowish.

Pink at Birth, Black Within Months

Polar bears are not born with black skin. Newborn cubs have pink skin showing through a short white fuzz, and even their little noses and foot pads are pink.

The change starts early. The Toronto Zoo, which hand-raised a cub born in 2011, saw his pink nose and feet begin turning black at two weeks old. By around three to four months, about the time mother and cubs leave the maternity den, Polar Bears International says the pink skin has turned black.

On a grown bear, the black skin is easiest to see on the nose, lips and foot pads. The black footpads on the bottom of each paw are covered in small, soft bumps called papillae.

Why Black? Scientists Are Still Asking

The color change is obvious, but its purpose is not. Polar bear scientist Thea Bechshoft wrote for Polar Bears International that researchers still do not know exactly why the skin turns black.

There are two leading ideas. Darker colors are better at absorbing heat from the sun, and dark skin likely protects the bear from harmful ultraviolet rays.

Staying warm is mostly the fur's job. Its two layers of fur prevent almost all heat loss, and adult males can quickly overheat when they run.

The Fiber-Optic Myth

For years, a famous claim said the hollow hairs worked like fiber-optic cables, piping sunlight down to the black skin to warm the bear. The idea took hold in the 1970s and spread through the media, books and the internet before anyone tested it.

Then physicist Daniel Koon and a student, Reid Hutchins, got a few hairs from a male polar bear at a zoo in Rochester, New York, and measured how much light actually traveled down them.

Almost none did. Less than a thousandth of 1 percent of red light, and less than a trillionth of violet light, made it along a typical inch-long hair. Even less ultraviolet light got through.

Koon published the results in the journal Applied Optics in 1998. The hairs were not fiber-optic cables after all.

A Second Myth, Busted Over the Arctic

Another popular claim said polar bears give off so little heat that they do not show up on infrared cameras.

Government scientists have since shown otherwise. To count seals and polar bears on the sea ice, American and Russian researchers flew over the Chukchi Sea, between Alaska and Russia, with infrared cameras that detect warm-bodied animals.

In its report on the survey, NOAA Fisheries published a thermal image of a polar bear beside an ordinary photo. The bear shows up as a bright hot spot. NOAA's verdict: "Myth dispelled!"

Black Under the White

In the United States, polar bears live only in Alaska, in two populations: the Southern Beaufort Sea bears along the north coast and the Chukchi Sea bears in the west.

Every one of them, like every polar bear across the Arctic, is black underneath its clear coat.