Polar bears (Ursus maritimus) do not change the color of their coat. Their fur is not pigmented white. The appearance of a white coat is an illusion created by the structure of their hair interacting with light in their icy environment. This adaptation allows them to blend into their snowy habitat, which aids both hunting and camouflage.
The Science of Translucent Fur
The coat of the polar bear is not white due to pigment, but because of the physical structure of each individual hair shaft. Polar bear fur is composed of two layers: a dense undercoat for warmth and a layer of long, coarse guard hairs. These guard hairs, which form the bear’s visible outer layer, are entirely transparent or translucent, lacking any melanin or other coloring agent.
The outer layer of hair is primarily made of keratin. Crucially, the long guard hairs feature a large, hollow core, known as the medulla, which is filled with air. This air-filled interior provides a lightweight yet highly effective layer of insulation against the frigid Arctic temperatures. The absence of color and the unique structure of this hair layer govern the bear’s perceived coloration.
Why Polar Bears Always Look White
The hollow, transparent nature of the guard hairs creates an optical effect that makes the fur appear uniformly white. When sunlight hits the coat, the light penetrates the clear hair shaft and bounces off the interior walls of the hollow core. This process, a form of light scattering, causes all wavelengths of visible light to be reflected back randomly and equally.
Since a combination of all visible light wavelengths is perceived as white, the bear appears to be the color of the snow and ice surrounding it. This is the same principle that causes clear ice crystals in snow to look white. The illusion is further contrasted by the polar bear’s skin, which is actually black, an adaptation that efficiently absorbs any light that passes through the fur to help retain heat.
Temporary Shifts in Coat Appearance
While the bear’s biological color remains constant, its coat can temporarily take on different hues due to external factors. The fur may acquire a yellowish or brownish tint from the oxidation of the hair over time or from the oils and blood of their prey, seals. This yellowing is often more noticeable in older bears or toward the end of the year.
In warmer, more humid environments, such as those found in some zoos, the bears may even develop a greenish tint. This unusual coloration is caused by algae growth inside the hollow guard hairs, where the warm, moist conditions provide an ideal breeding ground. Additionally, the ambient light conditions of the Arctic can influence the perceived color, making a bear appear grayish under overcast skies or occasionally reflecting the pinks and reds of a sunset.