Why does hair on our heads keep growing, but hair on the rest of our bodies grows to a certain length and then stops?

peasypod

Without going in to too much technical detail, I can explain that the hair follicle/growth cycle goes through various stages, that is anagen (growing stage), catagen (transitional stage) and telogen (resting stage). The hair on our scalp has an anagen stage of 2-7 years depending on the individual's diet, age, and health in general. Other parts of our bodies (eyebrows, lashes, underarms etc) have a shorter anagen stage. For example, eyebrows are around 4 months. I could go further into the medical science of it all, but Wiki has enough to get you through to your next haircut. http://en.wikipedia.org/wiki/Hair_follicle Jun 19 06, 6:35 AM

Gnomon

Hair on every part of your body grows to a certain length and then falls out. On most of your body the natural length is a few millimetres to a few centimetres. But on your head the natural length is about 80 cm. So the hairs on your head will grow until they reach 80cm, then they will fall out. Since you have so many hairs, only a few are falling out at any time. Jun 19 06, 6:37 AM

zbeckabee

Genes also play a role as some folks may top out at Gnomon's 80cm (or before) and then there are people like Crystal Gayle... http://www.edcug.org/news/Crystal-Gayle-Hair.jpg Jun 19 06, 9:16 AM

davejacobs

The answers above seem to be restating the point in the question, albeit more technically, but do not attempt to answer the basic WHY? Jun 19 06, 12:40 PM

zbeckabee

Here...this is a little more basic. Q. Why doesn't my eyebrow hair grow as long as my head hair? A. The growing cycle of your hair as well as being genetically determined is different depending on where on your body the hair is located. The hairs in your eyebrows just have a shorter growing cycle than those on your head. Scientists aren't completely sure why this is but natural selection probably has something to do with it: waist length eyebrow hair would have been a distinct disadvantage for our ancestors. http://www.theanswerbank.co.uk/Article2155.html Jun 19 06, 2:20 PM

Gnomon

If you want to know why humans have long hair on their heads and short hair on their bodies, that's a different question. Jun 19 06, 3:50 PM

Baloo55th

It's more normal in the Mammalia for hair (or fur) to grow to a certain length only. The continued growth of human hair is a bit of a freak (as is that of domesticated animals like the poodle that have to be clipped). For animals on all fours, this is obviously important. Tripping up over your hair isn't a good thing in evolutionary terms. Humans being upright (posture-wise - not referring to morals), the hair can grow without becoming a trip hazard. OK, it can get long. My elder small cousin (11 now and not quite so small) has hair she can sit on (but there's a financial reason for that - if she doesn't get it cut before she's 18 she gets £20 off her father!) but it isn't going to get any much longer than that. The reason for it growing like that in humans is probably that it isn't a danger, and that it was selected for by marital choice. No 'fittest to survive' in it, just aesthetic choice. And if you want to know why it should be thus, ask a peahen why she prefers a peacock with a ridiculously large tail. Jun 19 06, 5:18 PM

peasypod

The 'why' is because of the differing anagen stages at various points on the body, as we've stated. Now, why are anagen stages different for certain follicle areas? As Baloo has now tuned into the Caveman Journal of Dermatology, he's perfectly secure in stating that the eye brows, facial hairs, scalp hair etc all have a distinct pre-determined length through evolution, as very long hairs on these parts would cause an impediment to seeing, smelling and other functions. It therefore serves as a survival mechanism for the species. Jun 19 06, 5:43 PM