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The human body can synthesize vitamin D in the skin from cholesterol, when sun exposure is adequate. But how can haired mammals do it? Do not they need it?

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At least the vast majority of adult mammals have hair/fur (not sure if you are trying to make a distinction), possibly all (I will have to check on a hairless adult mammal or mammals that made to adulthood without hair). Hair is actually one of the defining characteristics of mammals. So what do you mean by "haired mammals" (humans have hair)? –  Atl LED Jul 19 '13 at 20:01
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@AtlLED, I think the OP means mammals whose skin is apparently inaccessible to the UV required for vitD synthesis, i.e. that don't have any exposed skin. –  Alan Boyd Jul 19 '13 at 20:16
    
@AlanBoyd Right, I was trying to edit my comment and got distracted (and came back too slow). OP: Would a polar bear and a dog be an ok example, then, for how different animals do it? –  Atl LED Jul 19 '13 at 21:00
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According to "Melanocyte biology and skin pigmentation" (Yin and Fisher, 2007), the exposed parts of mammal bodies, such as their noses have melanocytes that can be locations of vitamin D production.

According to this Wikipedia article, the secretions in woolly animals also produce vitamin D.

This is further explored in "Vitamin D(3) synthesis in the entire skin surface of dairy cows despite hair coverage." (Hymoller and Jensen, 2010), where they found that cows synthesise vitamin D all over their body, where they dispute an idea that vitamin D is manufactured in the fur secretions, and ingested via grooming.

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