No. 45: May-Jun 1986
"A polar bear's hairs are completely transparent. The bear appears white because visible light reflects from the rough inner surface of each hollow hair. However, the hairs are designed to trap ultraviolet light. Like light within an optical fiber, the radiation is conducted along the hairs to the skin. This summertime energy supplement provides up to a quarter of the bear's needs. Thus, even while actively pursuing prey, the bear can still concentrate on building up its blubber layers in preparation for winter."
In other words, the bear's fur lets heat in but not out -- in effect a thermal diode.
(Anonymous; "Solar Bear Technology," Science News, 129:153, 1986.)
Comment. How come polar bears are favored with this "marvelous adaptation" while the arctic foxes and other mammals shiver?