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.
They need vitamin D, and they get it the same way, by sun exposure. You need way less UV than needed to sunburn your skin to get your daily required amount of vitamin D.
Think of it this way. The biochemical pathways for many (most?) vitamins are common across species. Humans came late to the animal kingdom (in relationship to others), when most of those pathways were already “set in stone, this way works”. The way we process vitamin D hence is the same way many other animals do. In contrast, though, humans, primates, and few other species have lost the ability to synthesize vitamin C, making it a diet requirement.
In regards to fur and sun exposure, granted this mostly applies for white, nonpigmented skin, but many furry creatures get solar-induced skin changes including squamous cell carcinoma. UV gets through.
And before someone mentions it: Humans and animals can also get pre-formed vitamin D from their diets. This does not diminish the fact that they can, indeed, form it in the skin from precursors. Animals had this set up in place long before humans showed up.
BTW, going back further, the “getting vitamin D from sunlight exposure” setup was already fixed in amphibians and reptiles.
The ratio of how much vitamin D an animal gets through sun exposure vs diet varies by species (and what it is being fed, weather, age, management, etc.). But that pathway exists in furry and non furry species.
The book used to back up this cite was written with humans in mind, not a book about animal physiology. That said, the most recent paper the book uses to cite that part (Carpenter and Zhao, 1999, Forgotten Mysteries in the Early History of Vitamin D, The Journal of Nutrition) is very cautious about making that conclusion. They make suggestions, and mention that the experiments should be repeated. Furthermore, they reject the first part of that quote (about the preening gland being the only way to obtain vitamin D), precisely by mention evidence of vitamin D biosynthesis in the non-feathered bird skin (the legs). BTW, birds do obtain most of the vitamin D from their diets.
Horses are covered with hair, but they can get sunburned. (Even here in Minnesota, where summer sometimes seems pretty short.) So if enough sunlight gets through their hair to sunburn their skin, surely there is enough to supply sufficient vitamin D.
OK, consulted with the main book at work. No mention of preening glands or greasy secretions, but again, skin absorption varies by species.
Some furry animals appear to obtain most of their vitamin D from the skin (mainly herbivores), while others depend on their diets to get most of it (mainly carnivores… and New World primates).
So of the original choices given by the OP, (2) is definitely wrong, and the correct answers are (1) and (3) (diet). Just like humans can get vitamin D from their skin AND diets, so can animals.
I don’t believe it and the physics is very much against it. The square footage of eyes is tiny. Unless the mechanism for making vitamin D is tens of times more efficient than anywhere else on the skin the total would be negligible.
I tried to find some medical backing for this, but Googling just provides the usual hash of self-proclaimed experts. Probably the typical basis for this belief can be found here.
It’s all stuff like like. Or worse, such as this from that notorious quack, Dr. Joseph Mercola:
But if you go to PubMed to search actual medical journals, you’ll find nothing at all.