Re: Why are humans the only animal whose hair keeps growing?

That's a very good question!

As you noticed, humans are the ones that are unusual... because very few other animals have long hair.

Most hair, on most animals, grows to a specific length; whatever length is most useful for the animal. Animals that need to stay warm in cold weather often have longer hair - compare the long fur on an alaskan husky to the short fur on this mexican chuhuahua.

Husky	Chihuahua

In order for hair to grow the right length, most hair follicles are programmed to grow hair only so long - the hair grows to a certain length and then stops. After a while, the hair falls out and the follicle grows a new one. The follicle knows how long the hair should be; it's genetically programmed to stop growing after a while.

Some hairs stay the right length because they are made to be somewhat weak- they break off when the get too long!

So the other question is, why does human hair keep growing? Actually, not all human hair does. The hair on your arms doesn't keep growing, and neither do your eyelashes or eyebrows. They tend to fall out or break off before they get too long.

So why does the hair on your head get so long? It does seem a little strange, because before people invented combs and scissors your hair could get very long and get tangled and dirty, which could be a serious problem. I don't know for sure, but I'll make a couple of guesses.

• Sometimes, animals have features that don't seem very smart because it helps them look prettier to the opposite sex. Male peacocks are the best example: they drag around these humongous, colorful tails that don't seem to serve any purpose except that peahens are attracted to them. Perhaps some early humans found long hair attractive.

• One idea is related to warmth, like the husky and chihuahua above. Though it's not always true, humans native to warmer places, such as Africa or Australia, tend to have hair that doesn't grow as long as humans native to colder places like Europe and North America. It could be that early humans adapting to colder homes (back before we invented cotton clothing) found that long hair was worth the trouble - a lot of long hair can help keep your neck warm when you're surrounded by snow!

So I can't say for certain why many humans grow such long hair, but I hope I've helped explain why most kinds of hair and fur don't keep growing. I've taken a couple of guesses about long human hair, and perhaps you can come up with some more ideas; share them with me if you do!

Evan Dorn