Fur
Natural selection

Some mammals naturally have reduced amounts of fur. Some semiaquatic or aquatic mammals such as cetaceans, pinnipeds and hippopotamuses have evolved hairlessness, presumably to reduce resistance through water. The naked mole-rat has evolved hairlessness, perhaps as an adaptation to their subterranean lifestyle. Two of the largest extant terrestrial mammals, the elephant and the rhinoceros, are largely hairless. The hairless bat is mostly hairless but does have short bristly hairs around its neck, on its front toes, and around the throat sac, along with fine hairs on the head and tail membrane. Most hairless animals cannot go in the sun for long periods of time, or stay in the cold for too long. Marsupials are born hairless and grow out fur later in development.

Humans are the only primate species that have undergone significant hair loss. The hairlessness of humans compared to related species may be due to loss of functionality in the pseudogene KRTHAP1 (which helps produce keratin) Although the researchers dated the mutation to 240 000 ya, both the Altai Neandertal and Denisovan peoples possessed the loss-of-function mutation, indicating it is much older. Mutations in the gene HR can lead to complete hair loss, though this is not typical in humans.
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