Is arm length sexually selected?

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Bird on a Fire
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Re: Is arm length sexually selected?

Post by Bird on a Fire » Thu Nov 05, 2020 10:01 pm

Yes, for sure, and of course grannies etc. can look after babies anyway.

I think the fighting hypothesis advanced in the paper is probably more plausible than hunting. Would be interesting to know if arm length is sexually dimorphic in other primates.

Just found this paper, for instance, where arm length was sexually dimorphic in chimps but not bonobos:
Objectives

Bonobos and chimpanzees are known to differ in various morphological traits, a dichotomy that is sometimes used as an analogy for evolutionary splits during human evolution. The aim of our study was to measure the forearm length of immature and adult bonobos and adult chimpanzees to assess the extent of age–related changes of forearm length in bonobos and sex–dimorphism in bonobos and chimpanzees.
Materials and methods

As a proxy of somatic growth we measured forearm length of captive bonobos and chimpanzees ranging in age from 1 to 55 years. Measures were taken from subjects inserting their arms into a transparent Plexiglas® tube, a novel technique facilitating repeated measures of nonanesthetized apes in captivity.
Results

Measures from adult females (>12 years) showed that bonobos exceed chimpanzees in terms of forearm length and that sexual dimorphism in forearm length is pronounced in chimpanzees, but not in bonobos. Forearm length increased significantly with chronological age in bonobos. Validation tests revealed that the device generates useful data on morphometric dimensions.
Discussion

In most primates, sexual dimorphism in body size is male‐biased and the differences in forearm length in chimpanzees follow this trend. Given that males of the two species did not differ in forearm length, the absence/presence of sexual dimorphism of this trait must be due to differences in somatic growth in females. Our novel method offers an alternative to obtain morphometric measures and facilitates longitudinal studies on somatic growth.
Linky https://onlinelibrary.wiley.com/doi/10.1002/ajpa.23004 (pdf http://kar.kent.ac.uk/56827)

ETA they don't really speculate on evolutionary causes AFAICT, though they do note that in general chimps show sexual dimorphism (as do humans) and bonobos tend not to, so the arm length result wasn't massively surprising. Anyway, this does rather suggest that the explanation we're looking for could be for the common ancestor of all hominins, in which case tool use is probably not much of a factor and I don't know how much we know about hunting.
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Re: Is arm length sexually selected?

Post by Woodchopper » Fri Nov 06, 2020 7:39 pm

Bird on a Fire wrote:
Thu Nov 05, 2020 8:27 am
This might be of interest https://en.m.wikipedia.org/wiki/Sexual_ ... n_primates

So, what are some alternative explanations for sexual dimorphism in arm length in humans? If not for fighting each other... Hunting?
I was school champion at the javelin and discus. One thing long arms are really useful for is throwing things. Which would be an advantage when hunting.

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Re: Is arm length sexually selected?

Post by Bird on a Fire » Fri Nov 06, 2020 7:55 pm

Woodchopper wrote:
Fri Nov 06, 2020 7:39 pm
Bird on a Fire wrote:
Thu Nov 05, 2020 8:27 am
This might be of interest https://en.m.wikipedia.org/wiki/Sexual_ ... n_primates

So, what are some alternative explanations for sexual dimorphism in arm length in humans? If not for fighting each other... Hunting?
I was school champion at the javelin and discus. One thing long arms are really useful for is throwing things. Which would be an advantage when hunting.
Did your javelin and discus prowess also make you a big hit with teh laydeez?
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Re: Is arm length sexually selected?

Post by Martin Y » Fri Nov 06, 2020 8:13 pm

What if it's completely the other way around? Maybe women evolved slightly shorter arms; perhaps an adaptation to having hopelessly small pockets in women's clothes so there's no need for long arms to reach your keys at the bottom?

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Re: Is arm length sexually selected?

Post by Woodchopper » Fri Nov 06, 2020 8:47 pm

Bird on a Fire wrote:
Fri Nov 06, 2020 7:55 pm
Woodchopper wrote:
Fri Nov 06, 2020 7:39 pm
Bird on a Fire wrote:
Thu Nov 05, 2020 8:27 am
This might be of interest https://en.m.wikipedia.org/wiki/Sexual_ ... n_primates

So, what are some alternative explanations for sexual dimorphism in arm length in humans? If not for fighting each other... Hunting?
I was school champion at the javelin and discus. One thing long arms are really useful for is throwing things. Which would be an advantage when hunting.
Did your javelin and discus prowess also make you a big hit with teh laydeez?
Nope. But social status and consistently bring home food might have done 50 000 years ago.

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Re: Is arm length sexually selected?

Post by shpalman » Fri Nov 06, 2020 8:56 pm

<something about selecting me to remain at arm's length sexually or whatever>
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Re: Is arm length sexually selected?

Post by Woodchopper » Fri Nov 06, 2020 8:58 pm

Woodchopper wrote:
Fri Nov 06, 2020 8:47 pm
Bird on a Fire wrote:
Fri Nov 06, 2020 7:55 pm
Woodchopper wrote:
Fri Nov 06, 2020 7:39 pm


I was school champion at the javelin and discus. One thing long arms are really useful for is throwing things. Which would be an advantage when hunting.
Did your javelin and discus prowess also make you a big hit with teh laydeez?
Nope. But social status and consistently bring home food might have done 50 000 years ago.
Actually, if the evolutionary advantage is due to being slightly better at hunting then it isn’t necessary for the knuckle dragger to be more popular with the laydeez. The long armed blokes just need to have a few more surviving children.

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Re: Is arm length sexually selected?

Post by Bird on a Fire » Sat Nov 07, 2020 12:42 am

True dat.

Another idea: maybe the long arms are for cuddling. Everybody likes a good cuddle.
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Re: Is arm length sexually selected?

Post by dyqik » Sat Nov 07, 2020 3:31 am

Bird on a Fire wrote:
Sat Nov 07, 2020 12:42 am
True dat.

Another idea: maybe the long arms are for cuddling. Everybody likes a good cuddle.
Clearly it helps children survive as long arms are better for cuddling and more able to grab them before they wander off a cliff. </Just So>

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Re: Is arm length sexually selected?

Post by bjn » Sat Nov 07, 2020 6:50 am

If long arms are selected for by whatever mechanism that is a general survival trait, like throwing rocks at bears and spears at beasts, why would it be sexually selected if it is useful in both sexes?

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Re: Is arm length sexually selected?

Post by dyqik » Sat Nov 07, 2020 1:51 pm

bjn wrote:
Sat Nov 07, 2020 6:50 am
If long arms are selected for by whatever mechanism that is a general survival trait, like throwing rocks at bears and spears at beasts, why would it be sexually selected if it is useful in both sexes?
Human pelvises are different between sexes because childbearing adds constraints to females. That's a small possibility in other areas if there are significant energy costs involved.

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Re: Is arm length sexually selected?

Post by Bird on a Fire » Sat Nov 07, 2020 2:14 pm

bjn wrote:
Sat Nov 07, 2020 6:50 am
If long arms are selected for by whatever mechanism that is a general survival trait, like throwing rocks at bears and spears at beasts, why would it be sexually selected if it is useful in both sexes?
Yes, exactly - that's why they're looking at traits like aggression that are known to be expressed more in males than females and that are known to be under sexual selection. Both sexes hunt, but males are more likely than females to fight each other.
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Re: Is arm length sexually selected?

Post by Tessa K » Sat Nov 07, 2020 3:47 pm

Martin Y wrote:
Fri Nov 06, 2020 8:13 pm
What if it's completely the other way around? Maybe women evolved slightly shorter arms; perhaps an adaptation to having hopelessly small pockets in women's clothes so there's no need for long arms to reach your keys at the bottom?
I was going to flip it round too. Shorter arms are better for carrying babies and stuff. As a tall, long-armed woman I find carrying harder than women I know with shorter arms (and don't need help with high shelves, thanks). It's something to do with length v bulk/density of muscles.

I'd put my money on hunting rather than fighting determining male build. Fighting is as much about using your brain as hitting, both using resources (muscles, teeth, claws, weapons) and forming alliances. Females like males who bring home the bacon (protein) and who stick around to help rear the offspring.

The Ape Index suggests that longer arms are beneficial in climbing, swimming and goalkeeping but a disadvantage when bench pressing. According to Wiki: 'research has shown that the ape index has no influence on who wins or loses MMA bouts, an individual's divisional ranking, or whether or not they are successful in their technique use'.

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Re: Is arm length sexually selected?

Post by monkey » Sat Nov 07, 2020 6:00 pm

dyqik wrote:
Sat Nov 07, 2020 1:51 pm
bjn wrote:
Sat Nov 07, 2020 6:50 am
If long arms are selected for by whatever mechanism that is a general survival trait, like throwing rocks at bears and spears at beasts, why would it be sexually selected if it is useful in both sexes?
Human pelvises are different between sexes because childbearing adds constraints to females. That's a small possibility in other areas if there are significant energy costs involved.
I assume that gives women a different gait. Maybe the arm length is related to that.

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Re: Is arm length sexually selected?

Post by dyqik » Sat Nov 07, 2020 6:14 pm

monkey wrote:
Sat Nov 07, 2020 6:00 pm
dyqik wrote:
Sat Nov 07, 2020 1:51 pm
bjn wrote:
Sat Nov 07, 2020 6:50 am
If long arms are selected for by whatever mechanism that is a general survival trait, like throwing rocks at bears and spears at beasts, why would it be sexually selected if it is useful in both sexes?
Human pelvises are different between sexes because childbearing adds constraints to females. That's a small possibility in other areas if there are significant energy costs involved.
I assume that gives women a different gait. Maybe the arm length is related to that.
Certainly not impossible, but probably very marginal.

There's a general assumption that large numbers over long timespans overwhelm noise in these systems, but I'm not so sure about that. Not all noise integrates down.

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Re: Is arm length sexually selected?

Post by Millennie Al » Sun Nov 08, 2020 3:45 am

Bird on a Fire wrote:
Thu Nov 05, 2020 10:01 pm
Just found this paper, for instance, where arm length was sexually dimorphic in chimps but not bonobos:
So what do chimps and humans have in common that bonobos don't? One answer is in the area of aggression: in chimps and humans, the female is considerably aggressive than the male, while bonobos overall are less aggressive, females are more aggressive than chimp females. So I'd say that arm length is caused by sucess in aggression.
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Re: Is arm length sexually selected?

Post by Tessa K » Sun Nov 08, 2020 11:25 am

Millennie Al wrote:
Sun Nov 08, 2020 3:45 am
Bird on a Fire wrote:
Thu Nov 05, 2020 10:01 pm
Just found this paper, for instance, where arm length was sexually dimorphic in chimps but not bonobos:
So what do chimps and humans have in common that bonobos don't? One answer is in the area of aggression: in chimps and humans, the female is considerably aggressive than the male, while bonobos overall are less aggressive, females are more aggressive than chimp females. So I'd say that arm length is caused by sucess in aggression.
Or it could be related to hunting in chimps. Both male and female bonobos hunt whereas only male chimps do. This would provide a larger group of potential hunters in bonobos so there would be less need for the males to have hunting adaptations that females don't.

https://www.livescience.com/9601-bonobo ... mates.html

https://www.newscientist.com/article/dn ... dark-side/

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Re: Is arm length sexually selected?

Post by Millennie Al » Mon Nov 09, 2020 3:22 am

It seems that the lack of sexual dimorphism in bonobos is due to the females having longer arms rather than the males having shorter ones (based on the abstract from https://onlinelibrary.wiley.com/doi/ful ... ajpa.23004), so long arms are caused by something that female bonobos do but only male humans and chimpanzees do. I'm not sure that makes it any clearer as we're still stuck unable to distinguish between aggression between individuals and aggression used in hunting. Though maybe they're the same thing - if you use a technique to kill prey why not use the same technique to fight others of your species?
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Re: Is arm length sexually selected?

Post by Tessa K » Mon Nov 09, 2020 9:17 am

Millennie Al wrote:
Mon Nov 09, 2020 3:22 am
It seems that the lack of sexual dimorphism in bonobos is due to the females having longer arms rather than the males having shorter ones (based on the abstract from https://onlinelibrary.wiley.com/doi/ful ... ajpa.23004), so long arms are caused by something that female bonobos do but only male humans and chimpanzees do. I'm not sure that makes it any clearer as we're still stuck unable to distinguish between aggression between individuals and aggression used in hunting. Though maybe they're the same thing - if you use a technique to kill prey why not use the same technique to fight others of your species?
Although there are obvious similarities between hunting and fighting, prey animals are a different species from the hunters, usually smaller and with a different set of defences/weapons. Maybe longer arms are useful to keep the hunter away from teeth/horns/claws.

Which do chimps do more of - hunting or fighting? While there's a fair emount of aggression in chimp troupes, most of it doesn't result in a full-on fight as the cost can be heavy, even for the victor. It's the same in many other species, a lot of sound and fury to demonstrate superior strength at which point one opponent backs off.

If you're going to hit someone, speed, arm strength and a solid body to put some heft behind the blow is important. Longer arms may make it easier to get a blow in but not necessarily to take them out.

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Re: Is arm length sexually selected?

Post by Boustrophedon » Wed Nov 11, 2020 5:21 pm

Bird on a Fire wrote:
Thu Nov 05, 2020 10:01 pm
Yes, for sure, and of course grannies etc. can look after babies anyway.

I think the fighting hypothesis advanced in the paper is probably more plausible than hunting.
I think the fighting hypothesis accounts for a lot of the world's ills: There is a fraction of males in any society who have possibly grown up using physical means to subdue their peers (Particularly Peers at Eton.) or have been taught that such means are legitimate, who now find themselves frustrated that such means are actively discouraged by law. This possibly accounts for the shows of strength by the right wing in Trump's America.
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Re: Is arm length sexually selected?

Post by Tessa K » Thu Nov 12, 2020 9:33 am

Boustrophedon wrote:
Wed Nov 11, 2020 5:21 pm
Bird on a Fire wrote:
Thu Nov 05, 2020 10:01 pm
Yes, for sure, and of course grannies etc. can look after babies anyway.

I think the fighting hypothesis advanced in the paper is probably more plausible than hunting.
I think the fighting hypothesis accounts for a lot of the world's ills: There is a fraction of males in any society who have possibly grown up using physical means to subdue their peers (Particularly Peers at Eton.) or have been taught that such means are legitimate, who now find themselves frustrated that such means are actively discouraged by law. This possibly accounts for the shows of strength by the right wing in Trump's America.
It has been said that our view of what is 'natural' or 'evolved' male behaviour would be very different if we'd discovered bonobos before chimps.

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Re: Is arm length sexually selected?

Post by JQH » Thu Nov 12, 2020 9:56 am

Not so sure. How many people actually know how bonobos and chimps behave in the wild?
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Re: Is arm length sexually selected?

Post by shpalman » Thu Nov 12, 2020 9:57 am

Do you think men would be less fighty if women were as promiscuous as bonobo females?
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Re: Is arm length sexually selected?

Post by Tessa K » Thu Nov 12, 2020 10:04 am

shpalman wrote:
Thu Nov 12, 2020 9:57 am
Do you think men would be less fighty if women were as promiscuous as bonobo females?
Why just women? Especially as female bonobos shag each other as much as they shag males. You may not mean it this way (I assume you don't) but it does sound like an incel defence for male violence.

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Re: Is arm length sexually selected?

Post by shpalman » Thu Nov 12, 2020 10:12 am

Tessa K wrote:
Thu Nov 12, 2020 10:04 am
shpalman wrote:
Thu Nov 12, 2020 9:57 am
Do you think men would be less fighty if women were as promiscuous as bonobo females?
Why just women? Especially as female bonobos shag each other as much as they shag males. You may not mean it this way (I assume you don't) but it does sound like an incel defence for male violence.
Ok so how do you think our view of what is 'natural' or 'evolved' male behaviour would be very different if we'd discovered bonobos before chimps?
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