Naked Science Forum
General Science => Question of the Week => Topic started by: thedoc on 01/05/2012 18:00:44
-
I guess that the gene that codes for men having hairy chests and backs is an evolutionary advantage in response to an environmental pressure. But why don't women have beards and hairy chests in response to the same pressures?
Asked by Mugs, Shepherds Bush
Find out more on our podcast page (http://www.thenakedscientists.com/HTML/podcasts/show/2386/)
-
We answered this question on the show...
With the answer is evolutionary expert Professor Robert Foley from Cambridge University....
Robert - Really, the question is why have women lost more hair in the course of our evolution than men have? If we go back to our evolutionary origins and think of the fact that we share ancestry with apes and monkeys, what is unique about humans is that we have generally lost hair. Hence, some of us hear the expression ‘the naked ape’. It’s actually not quite true to say we are hairless or the naked apes because we’ve kept hair in many parts of our body and partly because we haven’t actually lost the hair. It’s just become highly miniaturised, almost certainly because it gave an adaptive advantage in particular, helps keep cool, possibly related to having more upright posture, possibly relating to being much more active and running.
Now the question being asked is why aren’t we all equally hairless? Well to some extent, of course, people across the world vary enormously and then of course there's a big difference between men and women, and the answer most likely is to do with sex or selection. This is the second of Darwin’s great mechanisms, that selection doesn’t just work to fit people in the natural environment. It also comes about because individuals select the traits of the people they want to reproduce with, and probably, it’s the case that loss of hair, a key human characteristic, becomes more important in selecting for women because it makes them more attractive. So while all humans have been selected to lose hair, the process of how you choose a mate has extended much more in women than in men. So the real question perhaps is, why should that be attractive and I'm afraid that's probably something of a mystery.
Hannah - And Clifford K. on the forum suggests that society has adapted to preference of youthful looking women for longer healthy child-bearing years and having a hairless chin may make females look younger, providing the possible reason for the sexual selection.
-
This will be most interesting to see a response.
Beards (and hair on the head) can provide some amount of protection from the elements, although not great. So, if men retain a "hunter-gatherer" role, then the facial hair may provide a benefit in the winter. Perhaps women benefit less from the beard.
Society has adapted to a preference of youthful looking women, and distinguished looking men, perhaps because younger women are less likely to be attached, and have the potential for longer healthy child bearing years. Established men, however, may be a benefit of supporting a family.
In men, a beard doesn't become full until the late teens (that might be a good question on why the facial hair comes so late too).
So, not having a beard in women might help the illusion of maintaining a youthful face, and thus could be a benefit for mate selection.
-
I'm visualizing parallels of sexual dimorphism in the animal world ... turkeys, pheasants, chickens, cardinals, elephant seals, cattle, deer, gorillas, etc but especially male lions and their manes.
I think the idea is that adult males have beards, not adult females or young males, so it's a sign of maleness and maturity combined. So the question could be: Why don't women and young males have beards?
And the answer ........ maybe ........ is for females to recognize/distinguish adult males (at a distance) and to facilitate competitive non-lethal fighting among adult males by protecting the face and neck?? (But what about the obvious lack of protection caused by baldness?)
-
Two aspects of the answer:
1. Different areas of the body are "seeded" by different progenitor cells, which have different genetic switches enabled, accounting for different areas of the body which have different density of hair follicles; different cycles of hair growth and resting (which affects hair length) and different hair pigmentation.
These different areas of skin also respond differently to signals such as levels of sex hormones, and these levels change at puberty in boys and menopause in women.
It is also affected by race, so the gender difference in facial hair is generally less pronounced in Asian populations. So at the genetic level, it all comes down to genetic switches.
2. Fashion. In some societies, dark facial hair is actively removed by women, and actively grown by men, which exaggerates the genetic differences. In other societies, the differences are less.
-
Is anyone here attracted to Woman with beards, I'd call this one on natural selection. :)
-
Is anyone here attracted to Woman with beards, I'd call this one on natural selection. :)
That still doesn't answer why, as there may in fact be some innate qualities to natural selection.
Certainly some people insist on shaving legs, and others don't, and many people don't consider it to be an important issue.
If one looks at the primates, chimps, gorillas, orangutans, monkeys, etc, parts of their faces appear to be hair-free. However, I don't see a significant distinction between facial hair on males vs females. So, while a male lion may have a heavier mane than a female lion, such a distinction doesn't seem to be as significant with primates.
-
I think the fundamental error being made in the question is to ignore the fact that sexual selection plays a role in evolution, alongside environmental factors. If females tend to prefer to mate with hairy males and males tend to prefer hairless females, that would be enough to start a trend, without there being any survival advantage to the change. The peacock's tail would be another excellent example of this, and in fact shows how sexual selection can even lead to physical characteristics which would otherwise seem to have a negative impact on survival.
-
I think the fundamental error being made in the question is to ignore the fact that sexual selection plays a role in evolution, alongside environmental factors. If females tend to prefer to mate with hairy males and males tend to prefer hairless females, that would be enough to start a trend
The question is why would women choose to mate with men who look like Grizzly Adams,
But, men would choose not to mate with women who look like Grizzly Adams.
-
The question is why would women choose to mate with men who look like Grizzly Adams,
But, men would choose not to mate with women who look like Grizzly Adams.
No, the question was why women don't have hairy chests as a result of the same environmental pressure that causes men to have them, and I pointed out that not all evolutionary change is driven by environmental factors. Why sexual selection would tend to favour hairy blokes and hairless women is one for psychologists maybe, but as an obvious display of testosterone levels it might be a factor.
-
You would have thought it would be better for woman to have hairy chests because then the baby has something to hold onto and the mother still has both hands free. This is very evident in primates and in most cases a hairless primate would be catastrophic for the young.
-
Women are better insulated with fat ...
The percentage of essential fat is 3%–5% in men, and 8–12% in women
https://en.wikipedia.org/wiki/Body_fat_percentage
... so are less in need of a fur coat than men.
-
Why don't women have beards?
It seems that the parts of beards that most often show on women are the sideburns and mustache, which seem to be the upper parts of the entire beard. Could beards be composed of parts that can be turned on/off selectively? I've never seen someone with a natural "lower beard" (jaw and neck) without the upper parts (sideburns and mustache). So the beard seems to emanate from the scalp: scalp/eyebrows → sideburns/mustache → jaw/neck.
-
You would have thought it would be better for woman to have hairy chests because then the baby has something to hold onto and the mother still has both hands free. This is very evident in primates and in most cases a hairless primate would be catastrophic for the young.
Either that, or human women lost their beards because they disliked children pulling their hair.
Hair and fur is often thought of increasing tactile sensations, but bare human skin, especially that on the face is extremely touch sensitive, so a bare face might in fact increase the tactile feedback between a woman and her child, or a woman and her mate.
-
It might just be as simple as having a high testosterone level tends to increase body hair growth, and also is liable to lead to increased fertility and therefore more offspring.
-
In response to Aaron_Thomas; babies are too weak to be able to grasp onto anything, let alone hold themselves up. Chest hair would just get in the way, they'd get hairballs, or something
-
But adult males in some ethnic groups have very sparse facial hair, such as Asians and the Inuit, even though they live in cold climates. And if its a sexual preference of females, it must be only certain females. It is an interesting question.
As for hair/fur loss in general, one article I read theorized that the advantage was fewer parasites.
-
You haven't met the wife, have you.
-
But adult males in some ethnic groups have very sparse facial hair, such as Asians and the Inuit, even though they live in cold climates. And if its a sexual preference of females, it must be only certain females. It is an interesting question.
As for hair/fur loss in general, one article I read theorized that the advantage was fewer parasites.
makes sense=beard parasites would affect breastfeeding babes as well as chest hairs
-
The question is easier to answer the other way round. Why do men have beards? After all men have lost hair from the same places as women except for the face.
My belief is, men were historically the hunters and we were ambush hunters. Our eyes are not on stalks so to peer out of the bushes our faces also become visible. A clean shaven face is visible as its clearly defined outline is very obvious especially with Caucasian faces. Facial hair breaks up the outline of the face and makes it blend in with its leafy surroundings. The man that regularly came home with meat would have been the best man to mate with and the most likely to have provided the nutrition and skins for warmth for the offspring to survive to maturity. Small advantages over time can have big effects.
In my opinion, women do not need beards as they were the gatherers and did not need to hide to find there quota of food.
-
How many women like facial hair anyway? Very annoying to kiss a man with a beard. Is it not that both sexes strive for youthfulness? In most (ancient)art representations a man without a beard is considered to be the standard of beauty. Also, I recall that most cultures did not allow their women much choice in picking mates because it was the patriarch/father/men who would choose for them. A woman, a lot of times, had no say in the matter. Then again, we have no idea of how primitive humans lived. For all we know the women helped with/did the hunting (which wasn't so uncommon as usually thought and there are still tribes where women do the hunting)so the insulation theory falls flat + our ancestors did not all live in the cold. Also a beard is a liability in fights. Alexander the great commanded all his men to shave their faces so that the enemies would not yank on them. A lion's mane might protect him against claws and teeth but a human beard will not protect you against a human's greatest weapon; his hands. Also it is unhygienic and high maintenance. Certainly back in the days when basic hygiene was lacking. Humans seem to be quite filthy creatures by nature. Maybe that is why we lost our hair or vice versa. The most agreeable theory is that we lost our hair to beat the parasites. Oh yeah...that reminds me. Beards = flies.
Also we have forgotten a very important note.
We are assuming that all human races have beards. The more we go to towards Asian countries the less hair males have. Pure native Indians from south and north America have no facial hair at all. Also full-blooded Africans don't have facial hair either. Caucasian is the only race that is truly hairy hairy. It is thought that maybe this is due to interbreeding with neanderthals? Something that is exclusive to Caucasians and Mongoloids? Who knows.
-
edit:I meant fleas not flies
-
andywattbulb You don't like beards much do you?
-
I prefer men with beards, and long hair, so the sexual selection idea works for me.
-
Does this mean that women will not have leg and armpit hair?
If women shave for sexual, beauty and hygienic reasons, will women have less hair in a couple of generations?
If men shave, will those men have less amount of hair-if continued by each generation?
-
Women don't have hairy chests for the same reason they evolved larger than normal breasts (most mammals don't have such large mammary glands). Also, I have read before that women are selected for attractiveness based on the amount of unconsious emotional communication in their faces, ie rosey cheeks, red lips, fair skin, and that this can relate to how a girl looks in orgasm. This is parallel to the idea of a female looking youthful and thus more able to birth a child. Men didn't have these same pressures and didn't lose the hair as fast as females did during our species sexual natural selection process.
-
There are a lot more women with beards than one thinks. Usually they get rid of them. I have only ever encountered one woman with a proper beard. It was very dark and silky - and not so very long. But I have seen many women with evidence of hair removal on their faces.
-
Also something to consider, many women who were relatively hairy as post pubescent adults, when nearing menopause, often experience thinning or loss of body hair except on the face, where it often starts to grow, especially on the chin, even if they did not have hair on their chin previously.