Are Polar Bears Really White? Fur, Skin and Warmth

Why polar bear fur looks white

Color comes either from pigment or from the way a surface handles light. Polar bear hairs contain no pigment. Instead, each guard hair is clear, with a rough inner core full of tiny air spaces, and these scatter all wavelengths of visible light roughly equally. When all colors are scattered back to our eyes together, we see white.

Snow, clouds and table salt are white for the same reason. A single snowflake or grain of salt is clear, but millions of surfaces bouncing light in every direction look white. In the same way, the polar bear’s millions of clear hairs give it a pale coat that blends in with snow and sea ice, which helps it creep up on seals without being noticed.

Polar bears have black skin

Under the fur, polar bear skin is black. You can see it on the bare parts of the body: the nose, the lips, the edges of the eyes and the footpads. It also shows through when a bear climbs out of the water and its wet fur clumps together.

Dark skin absorbs heat well, so it was long thought that black skin helps polar bears soak up warmth from the sun. In practice, the fur is so thick that very little sunlight reaches the skin, so its color may matter less than once believed. Dark skin is also typical of bears in general, and the polar bear evolved from brown bears within the last few hundred thousand years.

How the fur and fat keep polar bears warm

A polar bear’s coat has two layers. The dense underfur, roughly 5 cm (2 in) long, traps a layer of warm air next to the skin. Above it are the long guard hairs, about 5–15 cm (2–6 in) long, which shed water and snow and protect the underfur. The guard hairs are partly hollow, which adds insulation and some buoyancy.

Below the skin lies a layer of fat that can be up to about 10 cm (4 in) thick. Fat is the main insulation in water, because wet fur loses much of its ability to hold warm air. Together, fur and fat keep polar bears so well protected that they lose very little heat. Their bigger problem is often overheating: a bear running fast on a mild spring day can get too hot within minutes, which is one reason polar bears usually walk slowly.

Why polar bears look yellow, cream or gray

Polar bears often look cream, yellow or even grayish rather than snow-white. Several things cause this:

• Oil and food stains. Fat from seal blubber soaks into the fur while the bear eats, and dirt builds up over the year.
• Age of the coat. Bears molt once a year, from about spring to late summer. A fresh coat looks whiter, while an old one turns yellower.
• Light. Low-angled sunlight in the Arctic, especially in the evening, can make the coat glow yellow or golden, while in shade it looks blue-gray.

So the color you see in a photograph depends a lot on season, light and how recently the bear has fed.

Myth: polar bear hairs work like fiber-optic cables

A popular idea from the 1970s and 1980s held that hollow polar bear hairs act like optical fibers, channeling ultraviolet sunlight down to the black skin to warm the bear. The story was repeated in many books and articles.

Measurements published in the late 1990s showed that it is not true. Polar bear hairs absorb most ultraviolet light instead of guiding it along their length, and only a tiny fraction could ever reach the skin. Scientists still study how the hairs scatter light, and their structure has inspired research into insulating materials, but the “fiber-optic bear” is a myth.

Green polar bears in zoos

In 1979, three polar bears at the San Diego Zoo in California turned noticeably green. The cause was tiny algae growing inside the hollow guard hairs, which thrived in the warm, humid climate and the bears’ pools. The algae did not harm the bears, and treating the fur restored its usual color.

Similar green tinges have been reported in other zoos in warm climates. It never happens to wild polar bears, because the Arctic is too cold and dry for algae to grow in their hair. A related case is the arctic fox, which molts from a brown or gray summer coat into a white winter coat, while the polar bear stays pale all year.

Polar bears and thermal cameras

Polar bears are so well insulated that they barely show up on infrared (thermal) cameras. In thermal images, the body of a bear on the ice looks almost as cold as the snow around it. Usually only the breath, the eyes and the muzzle show as warm spots.

This makes thermal cameras of limited use for counting polar bears from aircraft, so scientists usually rely on visual aerial surveys, satellite images and capture-recapture studies instead. Learn more about how bears are counted in how many polar bears are left?