Why does (some) human hair grow indefinitely? Cecil kind of answered this question in one of his columns that I’m too lazy to look up right now – his answer was “to put bows in,” the idea being that having long hair allowed humans to make themselves more attractive to potential mates. I’m not buying it, and am looking for some other WAGs, or even facts.
I’ve been in “crunch mode” at work for the past couple of months, and have pretty much abandoned shaving and getting my hair cut to give me more time to sleep, work, and post messages on message boards. And it occurs to me now that I’m even less capable a hunter now than I was before – my hair keeps falling over my eyes, and even more alarming, I’ve got hair growing down over my mouth! When I look at the scruffy caveman in the mirror every morning, I definitely DON’T think, “Ah, there’s a creature who would be more attractive to a potential mate.”
And anyway, wouldn’t having unrestricted sight and food-hole access be more advantageous (sp?) than having something to decorate? I’ve heard it speculated before that humans have thick head hair to help prevent losing too much heat from the top of the head, and that men have beards to help prevent frostbite around the face – if that’s the case, then why don’t we shed our hair and beards at a more manageable length?
(On the plus side, by the way, not having to shave is awesome. I’m starting to think I’ve been living a lie perpetuated by the Gilette company for the past 15 years.)
bibliophage
moderator GQ
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Sexual selection is a very real phenomenon. For example, the peacock’s tail evolved for no reason other than to attract mates. It does not help him gather food or escape predators. In fact, it hinders him in those areas.
Unca Cece was quite astute in noting that no one really knows why some hair just keeps growing and growing. The answer is mixed up in that whole mystery of the evolutionary origins of man.
Solly was also quite astute to point out in his OP that a lot of hair on our body does stop growing at a certain point. Terminal hair such as the type on our legs and arms has a million different purposes - some not relating to trapping heat at all. Think about your nose hair or your eyebrows.
As a kid I remember hearing that 80% of your body heat is lost through your head. That number seems a little high (sounds more like some bullshit figure your mom would make up to get you to wear your hat on a cold day) but without a doubt a decent amount of body heat is lost through our scalps. What with our brain and all that blood up there…
That explains why we have hair up there at all, but why does it grow so long? There are two popular guesses, neither of which are scientifically supportable in the least. One seems more likely to me than the other.
The first is that long hair attracts prospective mates. It’s not that far out when you think about it. A lot of animals on this great green earth grow “manes” to attract the opposite sex or to make themselves appear large during sexual challenges. Even some primates have manes. Tamarins and gorillas come to mind. Long head and facial hair on a male human is sort of reminiscent of a mane.
The other idea relates to the fact that early humans did an awful lot of walking and that human infants needed something to hold onto while being carried by the mothers and the mother suddenly had to use her hands. Long head hair would certainly help in this situation.
As an interesting side note, there is some rare genetic disorder with a very long name where your scalp hair (and face hair if you’re a man) will terminate at a certain length. I call people that suffer with this affliction “Freakish Genetic Throwbacks”.
You’re quite right. If you’re so good-looking that the opposite sex just can’t resist reproducing with you…it’s all right if a few of you gets eaten every now and again. If a bright tail gets you laid more often than it gets you eaten, you’re going to be one successful species.
If long hair is an outcome of sexual selection, then wouldn’t women have an obvious preference for long hair and beards (I am assuming female preference will be what matters since that seems to be the case in many examples of sexual dimorphism in humans)?
There seems to be an awful lot of cultural variance in that kind of preference, though. Does anyone know if the matter has been studied?
After reading everybody’s comments, I guess the “sexual attraction” theory does make a little bit more sense. Men and women both have head hair (for heat retention?) throughout childhood, when their main evolutionary goal is just to survive until puberty. Then, once puberty sets in, males start to grow beards as a visual signal of sexual maturity. Which would lead me to believe that once a man reached puberty, the importance of being quickly identifiable as a possible sexual partner outweighed the convenience of being able to eat (and see) without interference…
So I guess that that kind of answers Yumanite’s question (but I’m just speculating with no real facts to back it up). Even in modern American society, where it seems to me that the norm is for men to be clean-shaven, there’s an undercurrent idea that having a beard is a sign of “manliness.” So I would speculate that you wouldn’t be looking for women to have a <I>preference</I> for long hair and beards, but just that women have a general preference for <I>men</I> (for purposes of procreation anyway), and that beards are a sign of male-ness or virility. Just being identifiable as a certain gender and age range might be more significant than any concept of “attractiveness,” which is socially and culturally defined and more ephemeral.
As for why it grows indefinitely, I guess I’d buy the “mane” theory – longer hair is more identifiable.
Sorry for posting this in the wrong section, by the way – I guess I was assuming that there was a statue of limitations on how long we had to respond to a column before it became a “general question.”
OK, I have no personal experience with beards since I am female, but I have had plenty of experience with long (waist-length) hair. I doubt that long hair would have been that much of a hinderance in hunting. You see, when your hair is growing out, it does get in your eyes, but once it has grown past a certain length, it pretty much stays put when you brush it behind your shoulders. You could also braid it (once braiding is developed) or tie it back with something (piece of long grass, maybe, or a leather cord of some sort once that is developed). If human hair naturally stopped growing at, say, chin length, it would probably be a much bigger PITA than hair that is long enough to go behind your shoulders. If that longer hair also gets you mates, well, it’s the obvious choice, really.
In caveman days, that awkward growing-out period would usually have only happened in childhood, well before the kid is old enough to hunt anyway.
I’m still not sure that the contention that “long hair is sexier” is true. It dosn’t seem to be a strong selective factor these days, and so I’m curious as to how it would have worked in our past.
Firstly not all humans have hair of infinite length. The tight curls of the San bushmen and the Congo pygmies seem to be of determinate and fairly short length. Negro hair may be of indeterminate length, but the curling nature seems to prent it becoming a major obstacle for many people. Bearing in mind that the Khoisan and the Congo pygmies may be similar to the ancestral human type it’s entirely possible that long hair didn’t develop until people had the ability to tie it back, braid it, burn it off, shave it etc. As such long hair would have been an optional fashion accesory as long as it’s existed.
Hair length, and particularly density and thickness, is a good indicator of health in most mammals. Wool thickness and strength is one of the best signs of a sheeps performance in the past, and many animals shed hair in response to parasites, disease etc. On the other hand a dog or horse with a thick, glossy coat is known to be in good condition. This fact probably didn’t go unnoticed by evolution when those factors which became sexually attractive were being selected for. Those individuals turned on by a thick glossy head of hair (after all that’s the only place humans really had hair) were selecting healthy mates, and a genetic attraction to such a trait would become dominant. Mutations that allowed for even longer, glossier hair would have had an advantage as a result of this. We then get into a vicious cycle. Liking long dense hair has an advantage and is selected for. The desire for long hair becomes more common and more powerful. Long dense hair has an even greater reproductive advantage because it is more attractive and is selected for. The hair gets longer. This in turn leads to people having both long hair and a desire for same. The desire for long hair becomes even more common and long hair becomes even more common. The situation rapidly builds into an infinite loop that could end up with something like the peacocks tail.
And long hair is still a fairly major selective factor even today. Not necessarly waist length hair, but most men still don’t find bald women vetry attractive (I know I don’t.) A head of thick hair is still very much a fashion accesory. They don’t make all those shampoos and conditioners because people don’t really consider hair condition to be a sign of physical beauty. Those things sell because we know that people find healthy hair to be very appealing. I suspect the same applies to both men and women, though it appears much less strongly selected for by women.
So if long, dense hair is something that people desire, why is balding among men so prominent?
Sure, the gene that causes baldness resulted from a random mutation (and, from what I’ve read, apparently it provides the added benefit of helping in the increased production of vitamin D). But at some point, for bald men to have become such a significant portion of the human male population, it had to have been a selected trait, right? This would seem, in my mind, to contradict the idea that long hair is a reproductive advantage (and I don’t think the vitamin D production is enough of an edge to promote the balding gene)
First some facts. Traits can become very widespread through random selection even if they aren’t advantageous provided they are neutral. It could be completely random.
So long as baldness appears after you have chosen a mate in a life-bonded society it won’t affect your reproductive success. This was typically done before 20 yo in most societies, so balding probably isn’t a great issue. Second wives in polygamous societies tend to be selected later in life and tend not to be self-selecting based on appearance. Rather they will be betrothed by their own parents or choose to enter a polygamous relationship due to the partner’s material wealth, status etc. By the time a second wife is selected appearance probably isn’t an issue.
1)Unlike males who select for the youngest possible partner of reproductive age, human females select for partners as old as or older than themselves. The reasons for this are complex. One of the signs of sexual maturity in human males is the higher hairline. Baldness tends to be associated with an early receding hairline/widows peak so it’s possible that baldness gives an advantage by making a male more desirable earlier.
2)Baldness may be a cue to other males that you are old and hence not in the mate chasing game. You have a mate to look after and so aren’t a serious threat. Less conflict, less injuries, less widows and orphans.
3)At some stage humans began valuing ond people for their knowledge. In the vast majority of human society age has privileges, particularly for men. Balding could have developed randomly yet become associated with the hormonal changes of old age. With no ages being known individuals who balded early would have appeared older and venerable earlier and been better able to care for their offspring as a result.
4)Baldness could be associated with a huge range of unknown factors aside from vitamin D production that could give it a large enough advanatge to override any possible disadvantages. It could be linked to an increased resistance to polio for example. This doesn’t mean taht baldness helps fight polio, rather that the two responsible genes are closely linkes, or that the gene that does help counteract polio has a side-effect of causing baldness later in life. However no-one has ever cross-referenced the presence of baldness genes to all the epidemiological studies ever done, so there’s no way of knowing.
It’s worth noting that baldness occurs later in life except in extreme cases. It seems to me that baldness is a disadvantage in the selection process, yet somehow becomes and advantage after competing for mates is no longer an issue.
Oh, and possibly the most important point.
As noted above long hair may not be as highly selected for by women as it is by men. However because this isn’t a sex-linked trait both genders will inherit it. So long as men like long hair and women don’t find it objectionable then hairy genes will become widespread.
This is almost certainly true to some extent. While women may have a slight preference for men with hair it doesn’t seem as high on their list of priorities as it is on men’s. Most women will happily go out with bald men or men with shaved or close cropped hair, but most men have an aversion to shorn women. This may be cultural but since it appears in so many cultures I suspect that it is partailly genetic. Long hair in women seems to be considered universally attractive by men, whereas long hair on men seems to be attarctive only to some cultures and based mostly on fashion.
Long glossy hair in women is a long-term indicator of health. If a woman has been healthy long enough to grow that head of hair, then she should be able to stay healthy enough to bear and raise a man’s child.
While woman are drawn to healthy looking hair on men, hair length may not be such an issue because sperm production is not something that requires many months of good health to commence. A man who looks like he’s been healthy for at least the last few months probably has viable sperm too.
This also explains why men prefer young mates - younger women generally have better health and a better chance of surviving long enough to raise a healthy child. Age is not such an issue when it comes to selecting a male because the male does not have to bear the child. Of course, women still express a preference for younger men because that also means the male might be around long enough to help raise the child. Choosing a mate was a balancing act between youth and survivability.