White, Yellow, Green?

Polar bears, Ursus maritimus, live in the Arctic and are heavily dependent on the ice floes where they hunt, breed, and spend a majority of their lives, making them the only bear to be classified as a marine mammal.  They are strong swimmers and are able to maintain a steady 6 mph swim speed for several hours, crossing long distances between ice floes.  Polar bears are a relatively new species, having branched off from the brown bear family about 150,000 years ago.  Their close family relationship enables them to breed with brown bears, and grizzly / polar bear cubs have been observed, although not often.

Many other marine animals inhabit the world of ice including the polar bear’s favorite prey: seals coming out of the water over the edge of an ice floe.  Bears often wait at these locations for a chance to grab a seal with heavy, sharp claws.  But seals are wary and slippery, and bears do not often capture them, losing 9 out of every 10 they hunt.  Their white fur should help to conceal them against a background of snow and ice.  But as you look at the following pictures, are they really white or yellow or tan or even green? 

A polar bear’s fur has a top layer of guard hairs and a bottom layer called the undercoat.  Guard hairs are much longer and completely cover the undercoat.  Both hair types are transparent with no pigment of any color.  The guard hairs are hollow tubes filled with air that scatter visible light into all the different wavelengths of the spectrum.  When we see a structure scattering light, it appears to be white.  However, polar bears rarely look completely white, except in the spring after their annual molt when they shed their old fur and grow a new coat.

Most of the time, polar bear coats are shades of yellow or tan, possibly with some darker colors.  Oils rubbed off from their prey, including fat from seal and whale bodies, tinge the fur in shades of light yellow or tan.  When they are on land, constant contact with dirt and other environmental elements may add darker colors to their fur.  Polar bears work hard to keep their fur clean and in good condition.  When combined with a thick layer of fat on the inside of their skin, it is the best insulation against the outside environment where they live.  Clean fur also provides good camouflage when stalking prey.  They may roll in snow or wash off in the ocean to clean their fur.

Bears that live in captivity, outside the Arctic, may display shades of green in their fur.  Algae growing in warmer waters of captive environments gets inside the guard hair tubes, which make an ideal environment for it to grow.  Neither saltwater nor extremely cold water, both found in Arctic environments, support the growth of algae.  A long soak in saltwater is a popular restorative cure for bears in zoos.

Come to see our polar bear at the Jurica-Suchy Nature Museum to learn more fascinating facts about this iconic species.  For information or to make a reservation, please click here:   Visit JSNM