The answer is they all have about the same number of hair follicles. Although people generally think of humans as only having hair on the scalp and some on the body, almost all our skin is covered in hair follicles with the exception of the palms and soles. We have little follicles, called vellus hair follicles, over our face and our bodies, that only produce tiny, non-pigmented hair fibers. Because they are not so easy to see, we tend to forget about them, but they are still hair follicles. A horse doesn't have many more hair follicles than a human - it is just that the hair follicles are bigger and spread further apart in the skin. At the other end of the scale, mice have also have about the same number of hair follicles, but the follicles are much smaller and closer together in the skin. In total, a human has about 5 million hair follicles. Of those one million are on the head and of these around 100,000 actually cover the scalp area. A human hair fiber on the scalp is typically around 80 micrometers thick. a horse hair fiber is typically 120 micrometers thick (though it varies in different strains of horse) while a mouse hair fiber is typically around 5 micrometers thick.
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