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.

[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!" :) ]

Jared Diamond

[inonit.livejournal.com]

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