Polar bears are superbly insulated by up to 10 cm (3.9 in) of blubber,[37] their hide and their fur; they overheat at temperatures above 10 °C (50 °F), and are nearly invisible under infrared photography.[41] Polar bear fur consists of a layer of dense underfur and an outer layer of guard hairs, which appear white to tan but are actually transparent.[37] The guard hair is 5–15 cm (2.0–5.9 in) over most of the body.[42] Polar bears gradually moult from May to August,[43] but, unlike other Arctic mammals, they do not shed their coat for a darker shade to camouflage themselves in the summer conditions. The hollow guard hairs of a polar bear coat were once thought to act as fiber-optic tubes to conduct light to its black skin, where it could be absorbed; however, this theory was disproved by r