15 min's a day is a very small amount of time spent in the sun.make you think how people do become low in vitamin D.everyone seem's happier in the summer.we must be getting a massive dose compared to the winter.if as you say gnasher only 15 min's is ample.
i know Cats and dogs naturally acquire vitamin D nutrient from UVB sunlight. Unlike humans and reptiles, cats and dogs do not make vitamin D in their skin. When UVB rays strike an animal's fur, oils in the fur are activated to produce vitamin D. Cats and dogs consume vitamin D when they lick or groom their coats.