Polar bears are superbly insulated by up to of adipose tissue, their hide and their fur. 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. Two genes that are known to influence melanin production, LYST and AIM1, are both mutated in polar bears, possibly leading to the absence on this pigment in their fur. The guard hair is over most of the body. Polar bears gradually moult from May to August, but, unlike other Arctic mammals, they do not shed their coat for a darker shade to provide camouflage in 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 hypothesis was disproved by a study in 1998.