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.

Somehow last night we got on the topic of sexuality and evolution, and a random idea occurred to me that I don't recall seeing mentioned before in the literature. Although there seems to be a decent amount of agreement about testicle size in primates -- that it seems correlated with promiscuity, and humans are somewhere in the middle of the continuum in regard to testicle size/body size ratio, I haven't heard many good explanations for why humans have such ridiculously large penises, for primates. And we do, really. More than twice the size of chimp penises, and around five times larger than gorilla penises. That's a pretty big difference. Why is it such a huge difference? What's up with that? We're not really on any extreme in regards to sexual behavior that explains that to me. However, we're the only truly bipedal primate, and it occurred to me that perhaps it's more about anatomical compatibility than anything else. Our hips, butts, and thighs are radically differently shaped than any of the other primates. You never see a chimp with a bubble butt. It seems likely to me that our vulvas and vaginas are simply harder to reach; there's more muscle and fat in the way that can lose some depth of penetration. If we were more in line with the rest of the primates -- say 2" length or so -- would we have significantly greater problems acheiving PIV sex? Could that have been what pushed the development of the human penis to that sort of length?

One a somewhat related topic, did I ever get around to writing down my similar curiosity about concealed ovulation in humans and whether it could be a result of a "domestication cascade"? That's an idle question based on some ideas that really captivated me from a few sources recently -- the idea that humans may be the "paedomorphic primate" -- that we share traits with many of the other domesticated animals -- and the random note on the fox domestication experiments in Russia that showed the development of that cascade to include, at least for foxes, a radically extended breeding period in a relatively small number of generations solely as a result of selecting for those foxes that were most amenable to socialization (the cascade also affected color markings, bark patterns, and a whole assorted of seemingly unrelated traits in a really fascinating way). Most of the sociobiological explanations for that aspect of human sexual evolution don't sit right with me, and don't seem to match truly cross-cultural data without liberal use of a crowbar -- have we really looked at the idea that concealed ovulation may not have been directly selected, but instead be a "side-effect" (can't remember the correct technical term) of another selection process?

Any, just ideas that've been percolating for me. I'd be interested in other insights, especially from anyone who's more familiar with any aspects of the topic, and can provide specific information or resources.

Comments

[kingtycoon.livejournal.com]

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.livejournal.com]

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.livejournal.com]

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.livejournal.com]

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.livejournal.com]

Duck sex is just disturbing in a "good God, she's surely going to drown" sort of way.

[moominmuppet.livejournal.com]

In fact, that's a real risk.

[kingtycoon.livejournal.com]

I'm probably muddling the argument - it was presented, if not endorsed in a book called "Human Natures" - by a Paul Ehrlich? I think so - I remembered that he offered a bunch of speculation - but idea that we might have evolved for competitive gang-sex was my favorite of the lot. Possibly supported by findings (no reference here - call it NPR) that dudes who watch a lot of porn with only girls have lower sperm counts than those who watch gang-sex pornos - and those individuals have abnormally high sperm counts. I think the most surprising thing about that whole business was that just seeing it in porn could cause the effect - just the visual component was enough - interesting.

[moominmuppet.livejournal.com]

The changes in sperm count are interesting, although I'd be as likely to explain that as some sort of testosterone effect from the virtual presence of other males. Porn causing an effect doesn't surprise me too much -- the entire reason we created it was because it did -- we respond physically to psychological stimulation. However, I'd be very surprised by the competitive gang-sex theory -- extremely so. Even bonobos, who are most sexually similar to us, and have huge amounts of recreational and social sex in all combinations, don't follow that pattern at all, as far as I know, and I've never heard any other arguments for it, in 15 years of ongoing academic curiosity about human sexuality and sexual evolution. I'll have to look up his book and check it out -- I'm not sure, but I think that some of Elisabeth Lloyd's arguments from The Case of the Female Orgasm may also apply as critiques here, even though the topics are slightly different. The book is a bit dry and academic, but it's fascinating, and well-worth reading.

[kingtycoon.livejournal.com]

Well, the thing of Bonobos- it's interesting that we share ancestors with them & that they're one of the very few other species that mate out of season/for recreation - but I think it's too easy to assume that those two things are neccissarrily related. I'm not saying that primatology doesn't inform anthropology - those are obvious and parallel matters - but bonobo promiscuity is very different from the human sort. Oh - and one of the main differences between us & our cousins is what explains the original question - because we're bi-pedal it's harder to get in there - necessitating matters - I think you've basically concluded the same thing somewhere. But yeah- comparisons between bonobo & human sexuality - It's my opinion that the value of those comparisons is over-estimated.

[moominmuppet.livejournal.com]

Well, more than that we share ancestors -- they're our closest living relative, and also have the most similar sexual behaviour (although in general temperament we fall somewhere between the bonobos and the regular chimps). It's very true that we can't directly extrapolate from the animal world to human behaviour, because of the huge influence of cultural evolution, but I do think that's a close enough link to be more worth looking at than most similarities science bases major arguments on.

It's all complicated. I'm both fascinated by the field, and highly suspicious and critical of it, because of the extent to which we've often made our theories about the past fit our theories about our present. Somewhere down the comments I made a comment with several of the books I've read recently that are feeding into my current theorizing, if you're curious.

Oh - and one of the main differences between us & our cousins is what explains the original question - because we're bi-pedal it's harder to get in there - necessitating matters - I think you've basically concluded the same thing somewhere.

Right. That was my initial topic -- that I think there may be a purely anatomical explanation for why human penises are so abnormally large.

[marnanel.livejournal.com]

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.livejournal.com]

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.livejournal.com]

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.livejournal.com]

*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.

[bec76.livejournal.com]

Even though not a primate, it's interesting nonetheless; dolphins mate face-to-face (or belly-to-belly really) and they engage in foreplay! How's that for neat?

[marnanel.livejournal.com]

Dolphins are really rather amazing creatures. Sometimes I think they would probably do a better job of running the world than us.

[moominmuppet.livejournal.com]

*nod* The natural world in general is a lot more diverse than we tend to acknowledge.

For reference (for whomever's reading this post), here are some books that are related to the conversation:

Biological Exuberance: Animal Homosexuality and Natural Diversity, The Case of the Female Orgasm: Bias in the Science of Evolution, The Science of Good and Evil (mostly for a very late chapter that briefly mentions the paedomorphic primate theory, and hits on a few topics of human sexual behaviour evolution), Nature: Dogs That Changed the World (video) (for some of what got me wondering about evolutionary cascades.

[moominmuppet.livejournal.com]

Do the other primates all mate doggy-style

Which reminds me -- have you seen the Eddie Izzard bit about mating patterns, and almost every species getting doggy style except for the Salmon, whom God really, really fucking hates?

[marnanel.livejournal.com]

Ha, no I haven't, but I'd love to.

Jared Diamond

[inonit.livejournal.com]

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

Re: Jared Diamond

[moominmuppet.livejournal.com]

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