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The animal with the greatest hair density is not the mouse.  Not the persian cat either.  Perhaps unsurprisingly, it is the chinchilla.  With more than 40,000 hairs per square inch chinchillas have the highest fur density of any animal. Their fur is so dense that skin parasites (such as fleas) cannot live on one because they would suffocate. Whereas humans grow one hair from each follicle, a chinchilla produces more than fifty hairs from a single follicle!

Mammalian hair follicles represent one of the most rapidly proliferating tissue systems in a living organism.  In humans, scalp hair typically grows at a rate of 1/2" per month, representing a remarkably robust rate of cell division.  Equally interesting, is the fact that hair goes through several distinct cycles, loosely analogous to the cycles deciduous tree leaves go through as the seasons change.  Like the leaves on a tree, hair follicles go through a growing stage (anagen) a shedding stage (catagen) and a resting stage (telogen).