Scrutable

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.

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?

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.

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?

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.

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.

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?

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?

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?

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.

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.

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:

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.

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?

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.

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.

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?

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.