Why do we need a long beard?

[Moreno]

Among all the mammals humans have the longest hair and beard. The apes are claimed to be closest to humans physiologically. But they have scarce hair on their faces. 

A bonobos

A Chimpanzee

A male gorilla

A gibbon

A

An orangutan male

At least non of them have such thing as a fully-developed mustache.

Someone can argue that non of the apes live in a cold climate in difference from humans. But here are some representatives of a nations who always lived in a hot climate.

g

Edited September 5, 2018 by Moreno

[Phi for All]

Iirc, beards are one way human males rate each other. Women aren't especially turned on by them.

They show other males how experienced and aggressive a man is. Big beard, big man, stay away from his stuff.

[Moontanman]

It should be noted that not all humans tend toward thick beards, native americans have sparse facial hair, I have very little at 63 years old and had none until i was in my 30s. 

[CharonY]

It should also be noted that not everything in biology has necessarily a direct reason. Sometimes they are leftovers or consequences from other developments.

[Phi for All]

4 minutes ago, CharonY said:

It should also be noted that not everything in biology has necessarily a direct reason. Sometimes they are leftovers or consequences from other developments.

Long beards do make a man look aggressive. They also hide facial expressions. I remember reading that women don't find them particularly sexy except for the implication of virility. Do long beards imply a man has more testosterone?

[CharonY]

Issue with sexual selection in humans is that it does seem to have significant cultural overlay and is therefore extremely variable. Long beards in itself is not an indicator of testosterone levels from what I recall. Though the overall connection is a bit more complex than that.

Individual follicles react to testosterone and it can stimulate hair growth. However, not everyone has the same densities of follicles. Moreover, length depends on linear growth after initiation, which correlates more with DHT (a conversion product of testosterone) levels. So, instead of beard length or density, the growth rate might correlate better. 

Edited September 5, 2018 by CharonY

[Phi for All]

26 minutes ago, CharonY said:

Issue with sexual selection in humans is that it does seem to have significant cultural overlay and is therefore extremely variable. Long beards in itself is not an indicator of testosterone levels from what I recall. Though the overall connection is a bit more complex than that.

Individual follicles react to testosterone and it can stimulate hair growth. However, not everyone has the same densities of follicles. Moreover, length depends on linear growth after initiation, which correlates more with DHT (a conversion product of testosterone) levels. So, instead of beard length or density, the growth rate might correlate better. 

And growth rate is different depending on where the hair is, correct? Head and facial hair can grow much longer than the hair on arms and legs. 

Was the growth rate of our early ancestor's head and facial hair affected at all by inventing tools that could cut the hair? 

[CharonY]

54 minutes ago, Phi for All said:

And growth rate is different depending on where the hair is, correct?

Yes, facial hair has some of the highest growth rates, and I believe it is due to the follicles themselves, but I may be misremembering. Also note that growth rate is not the same as potential length.

55 minutes ago, Phi for All said:

Was the growth rate of our early ancestor's head and facial hair affected at all by inventing tools that could cut the hair? 

I don't think we have any way to assess changes in growth rates.

[StringJunky]

14 minutes ago, CharonY said:

Yes, facial hair has some of the highest growth rates, and I believe it is due to the follicles themselves, but I may be misremembering. Also note that growth rate is not the same as potential length.

I don't think we have any way to assess changes in growth rates.

Head hair has three growth phases and in one of those the hair narrows and breaks off under much less tension than in the other phases. I think the maximum general length is 3ft of growth then it narrows.

[CharonY]

5 minutes ago, StringJunky said:

Head hair has three growth phases and in one of those the hair narrows and breaks off under much less tension than in the other phases. I think the maximum general length is 3ft of growth then it narrows.

From memory, I recall to main states of the follicle with telogen being the resting state and anagen the growth state and I believe there were a number of factors determining the transition including direct hormone sensing but also secondary event that would influence it, including changes vascularization. I.e. mechanistically it is probably not the length itself is necessarily the limiting factor (though at some length it may just break/fall off) but the signaling involved in transitioning from on to the other state. 

But again, my recollection is murky on the details and I may be off.

 

[StringJunky]

2 minutes ago, CharonY said:

From memory, I recall to main states of the follicle with telogen being the resting state and anagen the growth state and I believe there were a number of factors determining the transition including direct hormone sensing but also secondary event that would influence it, including changes vascularization. I.e. mechanistically it is probably not the length itself is necessarily the limiting factor (though at some length it may just break/fall off) but the signaling involved in transitioning from on to the other state. 

But again, my recollection is murky on the details and I may be off.

 

Yes, my memory is of it is also not lucid. I wasn't suggesting there was a fixed preset length but I assume that is probably an average when it flits between the phases. I also imagine there will be lots of  bodily factors that will influence when these phases occur in any individual.

[CharonY]

32 minutes ago, StringJunky said:

Yes, my memory is of it is also not lucid. I wasn't suggesting there was a fixed preset length but I assume that is probably an average when it flits between the phases. I also imagine there will be lots of  bodily factors that will influence when these phases occur in any individual.

Makes sense.

[mistermack]

We humans have a longer and thinner neck than other apes. And because of our upright stance, the throat is more visible and more vulnerable to attack. So maybe the beard is a visible disguise, and a physical protection, to a very vulnerable part of the body. Adult males need to defend themselves from others, and also fight off any predators that try an attack. The beard might also help a little in making the males look more intimidating from a distance, like a lion's mane.

The reason that juvenile males don't have beards and pubic and chest hair might be that they would invite aggression from mature males, before they were strong enough to defend themselves, or quick enough to escape.