Life Science

Polar Bear Fur Isn’t (Technically) White – It’s Translucent!

What color is polar bear fur? The obvious answer is white. It certainly looks white, and for all intents and purposes, polar bear fur is white. But perhaps you’ve noticed that sometimes a polar bear looks yellow, gray, or green. Why is that? Do some polar bears have different color coats? Does the coat color of a polar bear change with the environment? Do polar bears just need better shampoo?

Before we get to the bottom of why polar bears appear to have such a range of colors, it’s important that we first address one key fact about polar bear fur.  Polar bear fur isn’t technically white…it’s translucent. A polar bear’s outer coat consists of guard hairs that are essentially clear hollow tubes devoid of pigment.

Curious polar bears at Bernard Spit in Alaska swimming and approaching a camera.
Curious polar bears at Bernard Spit, AK. Image: Steven Kazlowski/ Barcroft Media

So why does a polar bear usually appear white (…and yellow, green, etc.) with all that translucent fur? Let’s talk about the structure of that fur.

The Structure of Polar Bear Fur

Mammalian hair is composed of keratin which is typically organized into three layers.

  • The cuticle is the outermost layer and consists of a very thin, translucent layer of overlapping scales. These scales can vary in form depending on the animal and sometimes give hairs a rough, barbed texture.
  • Below that cuticle is the cortex, which consists of long, spindle-shaped cells and may contain pigment granules (in the case of colored hair) and corticle fusi (essentially air spaces). 
  • Lastly, an additional structural layer called the medulla is sometimes present. This part of the hair is essentially an air-filled void and you’ll find it well-expressed in arctic mammals because it can help trap warm air close to the body.

If you examine polar bear guard hairs up close, you’ll find a thin cuticle encasing a well-developed cortex and a wide medulla. The cortex in these hairs is devoid of pigment, and the medulla is a large, open air space. Therefore, when sunlight hits this fur, very little gets absorbed, and it’s subsequently scattered in all directions giving the appearance of a white bear.  

Electron microscope imagery of polar bear guard hair showing the cortex devoid of pigment.
Polar bear guard hair under scanning electron microscopy. Source: Nano Nature.
Polar bear guard hair under magnification showing a translucent cortex and hollow medulla.
Polar bear guard hair (400x). The medulla appears black under transmitted visible light.
Source: Alaska State Museum.

Gray, Yellow, and Green Polar Bear Fur

OK, now what’s up with the sometimes gray, yellow, or green appearance of polar bears? In the case of gray bears, the color is most likely the result of a thinner coat. Polar bears have black skin underneath all that fur. In the summertime, when their coats thin out, some of that dark undertone can peak through the fur above it. This can give the bears an overall grayish appearance. You’ll often notice this the most around the face of a polar bear.

In the case of yellow bears, the culprit is probably oxidation. Prolonged exposure to sunlight will gradually oxidize polar bear guard hairs and create a yellowish appearance. This is why a polar bear appears whitest after a fresh molt and then gradually yellower.

Last but not least is the curious case of the greenish polar bears. Greenish polar bears were first observed in zoos situated in warm climates. In 1979, researchers Ralph Lewin and Phillip Robinson determined that the greenish appearance of those captive bears resulted from algae!

Under warm conditions, the hollow medulla of polar bear guard hairs creates a cozy humid microclimate well-suited for growing the types of algae found in zoo enclosure ponds. Since the rest of the hair is translucent, the green color from the algae growing in the medulla creates a greenish overall appearance. Only the guard hairs are affected, however, because the width of the medulla of undercoat fibers is too thin (~20 micrometers) for the algae to infiltrate them and grow.

Polar bears with green fur at a zoo in Japan caused by algae growth.
Polar bear fur with algae growth. Source: Higashiyama Zoo, Japan, 2008.
Polar bears with green fur at a zoo in Japan caused by algae growth.
Polar bear fur with algae growth. Source: Higashiyama Zoo, Japan, 2008.
Polar bears with green fur at a zoo in Japan caused by algae growth.
Polar bear fur with algae growth. Source: Higashiyama Zoo, Japan, 2008.

So there you have it: polar bear fur isn’t technically white at all.  Now take this newfound knowledge and blow some minds.

References and Further Reading