Fur length alone does not decide how warm a coat is
Fur is the soft, dense hair coat covering nearly all mammals—oily guard hairs over thick underfur. Its jobs include protection, sensation, waterproofing, and camouflage, but thermoregulation comes first. Length is a poor warmth predictor: tropical sloths can match arctic hair length with less insulation.
A full coat can have three layers. At the bottom sits down hair: flat, wavy, short and the most numerous, holding a blanket of dry air against the skin. On top lie guard hairs, longer and coarser with nearly straight shafts; they carry most of the colour and gloss, shed water, block ultraviolet light and soften scratches. Between them come awn hairs, which start out stiff like guards and turn thin and wavy partway up, so their base insulates while the tip repels water. Dogs and cats can raise their guard hairs in a threat display when agitated.
Mammals also grow specialised hairs. Whiskers, or vibrissae, are sensory; porcupine spines are stiffened guard hairs used for defence; a lion's mane is made of bristles meant to be seen; newborns are often wrapped in insulating down called velli; and wool is long, soft and frequently curly. Fossils of Castorocauda, Megaconus and Spinolestes preserve compound follicles with both guard hair and underfur, showing the modern arrangement dates back to docodonts, haramiyidans and eutriconodonts.
Warmth depends on density more than length. Some tropical mammals, sloths among them, have fur as long as arctic species but insulate far less. The muskox pairs 30-centimetre guard hairs with a dense underfur that seals it against minus 40 degrees. Camels use thick fur the other way, as a sunshade: the coat can reach 70 °C in summer while the skin beneath stays near 40 °C. Aquatic mammals trap air in their fur to keep the skin dry and warm.
Colour answers several pressures at once. Arctic foxes, stoats, collared lemmings and snowshoe hares turn from brown to white each winter for camouflage, while the bold black and white of skunks and honey badgers most likely warns predators away. Dark coats absorb more solar heat, which is why some voles darken for winter. The green tint of sloths comes from algae living in their fur, not from the hair itself.
Source: Fur
