Random theory...

[moominmuppet]

Please assume everything in this post is predicated with "iirc" -- this is late-night theorizing based on an assortment of half-remembered information. Corrections in comments are perfectly welcome, as are contradictory ideas

Comments

[kingtycoon]

The theory I like is that because of the promiscuity of female primates big penises are an advantage - because they can 'plunge out' the spermatazoa of competetive males through suction.

[moominmuppet]

I'd have an easier time with that explanation if we saw patterns that reflected it, but it doesn't seem that we do -- the most promiscuous of the primates aren't those with the largest penises (although they are the ones with the largest testicles). Humans are only moderately promiscuous compared to other primate species, yet we're way over the curve on penis size.

[kingtycoon]

Well, but we have secret promiscuity relating to concealed ovulation. We're different from quite a few of our closer relatives in that we came up in a troupe-nomadic way - plains dwelling & not arboreal - so meeting/warring on the other tribes was/is still common enough - so there's a level of secret competition.

This is not the question of why big dicks are a trait of our species but rather why bigger ones are selected.

[moominmuppet]

But the "plunger effect" wouldn't even be theoretically effective in that situation -- too much time delay between partners to make any difference to removal of competitor sperm -- that tactic tends to come up in species where a group of males is mobbing a female at the same time. (witness the bizarre evolution of many duck penises -- good maude!)

[marnanel]

Do the other primates all mate doggy-style-- are humans the only ones who ever do things face-to-face? And if so, is that true across all modern humanity? And if so, when did that start, historically? I am curious.

I hadn't heard the explanation that humans were paedomorphic like domesticated dogs, and I think it's fascinating. [Perhaps I'm not a good person to reason about it, though, because all I remember about the fox domestication experiments is going "omg squee so cute!" :) ]

[moominmuppet]

Nope, humans definitely aren't the only species that does. I know for certain that Bonobos do, I'm almost certain that several other of the primates at least occasionally do as well. In terms of the anatomy, though, I think my point about extra padding applies in either position -- it's simply harder (both because of the padding, and because of the hip changes) for us to 'present' our vulvas in the same ways that primates are able to. I think that'd be easier to explain if I could draw out what I mean, but if you picture a profile of another primate or a human in the same positions, it seems to be that in almost any position, the human vulva is a bit more recessed between the thighs and buttocks (how much would vary hugely by body build, but I think the basic point stands).

[marnanel]

the human vulva is a bit more recessed between the thighs and buttocks

Funnily enough I had always assumed, from what I knew about how common face-to-face coupling was, that the entrance to the vagina would be further to the front than it is (so that the couple's hips would be about parallel). It was a surprise to me when I first actually encountered one and found that it's actually more recessed between the thighs.

[moominmuppet]

*nod* And imagine how tricky that would get with a 2" member. The entire human vulva and vagina are differently arrayed than other primates, too (a topic that came up in the Evolution of Orgasm book) -- possibly for some sexually adaptive reasons, possibly simply because of the physiological changes for bipedal primates with giant heads, and the birth process necessary for us to survive it.

Jared Diamond

[inonit]

I know you like him, and he discusses both of your topics at length: Why Is Sex Fun?.

Re: Jared Diamond

[moominmuppet]

Perfect, thanks! It's been on my "To Read" spreadsheet for ages; I'll need to move it up the list.