Polar bear fur is made up of a dense, insulating underfur that is topped by guard hairs that vary in length. The fur prevents almost all heat loss—in fact, adult males can quickly overheat when they run.

And surprisingly, the fur isn’t actually white—it just looks that way. Each hair shaft is pigment-free and transparent with a hollow core that scatters and reflects visible light, much like what happens with ice and snow.

Polar bears look whitest when they are clean and in high-angle sunlight, especially just after the molt period, which usually begins in spring and is complete by late summer. Before molting, accumulated oils in their fur from the seals they eat can make them look yellow.