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Comment by ani87 | Hacker News Reader

> Warrener and her colleagues found that wider hips do not increase the cost of locomotion. Indeed, both women and men are equally efficient at walking and running

> I have a number of papers that show that women are great walkers, and in some particular tasks women are better—they don’t use as much energy, they don’t build as much heat, they can carry heavier loads with less of an energetic burden.

I am not a biologist, but those parts just don't click for me; how/why would evolution have jump through all the costly hoops to differentiate the pelvis of males and females so significantly if the result was to give a locomotion system that was overall worse to the individuals who needed it the most for their survival, without any benefit in return ?

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Why is sexual differentiation necessarily costly in every case? There are many sex differences that give an advantage for one gender that would have been an advantage, or no cost, for the other gender, but weren't as strongly required and so don't exist. Sexual differentiation may be less _likely_ for any given trait, but traits tend to pop up on random chromosomes, or the side effects of sex hormones be farther-reaching than necessary and evolution just "didn't bother to correct," and so I'd expect that some traits that don't necessarily need to be dimorphic (but are needed by one gender or the other) might become dimorphic just by chance.

Men have deeper voices than women. What would be the disadvantage for baritone women? You could argue that a deeper voice makes you less easily heard by prey... but then what's the advantage for women to have a higher pitched voice? Why didn't we all continue this trend to speak at the registry of blue whales? I suspect it's one of those things that "just sort of happened" and then it became a positive feedback loop spurred on by sexual attraction as a result of this initial differentiation, and now we're just at a pretty good state spurred on by a combination of random chance, survival advantage, and a little reproductive enforcement.

Increased sexual dimorphism (and a genetic/hormonal configuration that lowers the cost and increases the odds of sexual dimorphism happening for any given feature) is actually a positive thing for some animals depending on their social and reproductive strategies. When sexual dimorphism is high, you tend to see more random differences that pop up between the sexes but that may or may not provide any evolutionary advantage beyond "well, the other sex is just attracted to it now so let's keep this party going." When dimorphism is low, differences generally have to provide a stronger survival advantage.

Evolution isn't always straightforward or "carefully thought out." The hoops it jumps through aren't always costly and don't always have a rhyme or reason. It's often chaotic and and takes weird turns and it's not like humans are at a "final state" anyway. We're evolving.

I'm not _necessarily_ disagreeing with you (although it's probably ultimately easier to look at the physics, not biology, to prove or disprove your point) but I'm just saying that I think the situation is vastly more complicated than "sexual differences are costly." I don't think they're necessarily costly, simply less likely. A subtle, but important difference.

> but then what's the advantage for women to have a higher pitched voice?

Living in a neighborhood with a lot of teenagers, I will dare a guess; this loud, high pitched scream was probably a very efficient call for help.

But I see your point, and I agree that the answer could be just as simple as randomness of genetics. It just feels rather unsatisfactory for a change who must have taken far more than one single mutation to be that common. Feels like someone winning the lottery a dozen times in a row.

Known genes associated with hip bone size in women (but not men) are found on chromosome 2 (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2522269/). But the expression of those genes is likely controlled by estrogen (http://www.sciencemag.org/news/2016/04/woman-s-pelvis-narrow...)

Individuals who have androgen insensitivity syndrome (XY chromosomes, but that don't respond to testosterone and are, for most intents and purposes, sterile females) will have large hips/pelvic bones.

So, yes, hip size takes a few modified base pairs (although probably all within one gene, or pretty close together on the same chromosome), but evolution only needs to "decide" (heavy emphasis on the quotes there) that this physiological change is hormonally triggered. Many skeletal changes are hormonally triggered, so this may have even been the easiest/most likely path.

Mothers preserving energy and being able to carry more directly benefit the survival of offspring (since that's who they're carrying and where their surplus energy is going), whereas an improvement in those areas for fathers doesn't have as direct of an impact so it doesn't get selected for.

>without any benefit in return ?

Isn't the implied benefit that it might allow for an easier birth?

"in some particular tasks women are better"

I assume men's hips are still better for tasks traditionally associated with the male gender role - e.g. being hunters and warriors.

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