Why does head hair grow indefinitely while body hair stops growing?

[matthyaouw]

Why does the hair on our bodies (much like the hair on animals) grow to a certain point, then fall out/stop, but the hair on our heads seems to grow indefinately? Do any other wild animals have hair that can grow forever? I'm not only interested in why it grows in physical terms, but evolutionary terms as well. It doesn't seem to pose much of an advantage that I can think of (actually, now that I'm writing this, shade springs to mind...)
Thanks.

 

[Reality_Patrol]

Scalp hair doesn't grow forever, there's a definite cycle to it. In women the growth phase can last several years, in men it's typically briefer. However, in humans the shedding of scalp hair is desynchronized, unlike "moulting" in animals where it's all shed at once. This desynchronized shedding can make it appear as though human scalp hair grows constantly, it doesn't. Interestingly there does appear to be a mild human "moult" that occurs seasonally.

Scalp hair has several evolutionary advantages: photoprotection and thermal insulation are the primary ones. I also think (personal opinion here) that scalp hair promotes lymph circulation in the scalp in response to wind exposure.

 

[matthyaouw]

Interesting, thanks.