[mrsmith2211 (OP)]

Many animals, dogs, cats etc. fur/hair seems to get to a certain length and stop growing, but the same does not seem to happen to head and men's facial hair but does seem to happen to arm and leg hair. What's the deal with that?

[chris]

Interesting question. Of course, the fur of some animals does continue to grow: take a sheep for example, which continues to grow a longer and thicker coat until the farmer chops it off. But you are right that many animals have uniform coat lengths. This is a consequence of the way that hairs are produced by hair follicles.

Hair follicles are small rings of stem cells embedded within the dermis; they produce a cylindrical filament of the protein keratin, which is the same stuff that makes us nails and claws. Follicles have five phases of activity: an active or growing phase, called the "anagen phase", during which the follicle is active and grows hair. This is followed by a catagen phase when the follicle deactivates and rests; after this is a telogen phase and exogen phase when the hair drops out, and then a kenogen phase prior to re-entry into anagen phase and the formation of a new hair.

The relative lengths of these phases alter the growth dynamics of the hair and therefore its ultimate length. "Long" hairs, like human scalp hair, can have an anagen phase lasting years. An eyelash, on the other hand grows for only a few weeks.

Thus, by varying the anagen phase duration, animals can very their hair length and thickness. And by synchronising the phases they can thicken their coats in winter, if required.