Humans make vitamin D by a reaction of UV light from the sun with certain chemicals in the skin. How does this work with furred animals? I see a few possibilities:
Fur is transparent to UV light. Pretty sure this in not the case. Human hair is pretty much the same as fur and UV does not penetrate human hair. Otherwise you’d get sunburned and tanned on your scalp under your hair, which does not happen.
Furred animals don’t need vitamin D. Possibly, although you’d think that at least some do.
Furred animals have another way of generating vit D. I think this is the most likely answer.
Something else I’m not thinking of?