Friday, October 23, 2015

Why is the human vagina so big?

We are obsessed with penis and testicle size. Yet, we can barely say "vagina" and when we do we're usually talking about the vulva.

Everyone's come across some article somewhere on-line that is thrilled to share how big human penises really are, for primates, and to explain why they evolved to be so big. It's not really the length, but the girth. Alan Dixson is your go-to on this. He's conservative in his assessment of the literature on penis size and even he concedes that human penis "circumference is unusual when compared to the penes of other hominoids (apes)" (p. 65 in Sexual Selection and the Origins of Human Mating Systems).

A favorite explanation for the big phallus is female mate choice, that females selectively make babies with males who have larger and, presumably, more pleasurable semen delivery devices. This is backed up by studies. When life size projections of naked men are shown to female subjects, they say they find the ones with bigger ones to be more attractive. [This is exactly how mate choice works where I live, how about you?]

Other explanations include male competition. If you can deliver your package to the front yard but the other guy can deliver to the front door, his is more likely to be carried inside the house first. Or, if he can steal away what you just delivered, then, again, his package has yours beat. Thanks to his big penis he's more likely to pass on his winning penis genes than you are to pass on your loser penis genes. Loser.

All this is just terribly fun to write about and I'm not even going nuts (gah) like they do. And they do. They really do. And all over the Internet they do: "Evolution of human penis" gets 53,000 hits just on alone, and about 832,000 on Google.

But doesn't it make sense that for a penis to be somewhat useful it has to be somewhat correlated to vagina size?

I'm talking about all penises in the universe and all vaginas too. Sure there's variation, but a penis can't be too wide. It helps to be long, probably, but it can't be too long.

So neither pleasure nor psychology need matter at all, just function associated with some sort of fit. Pleasure and psychology are never invoked to explain penis morphology in other animals. If anything, it's the cornucopia of horrifying, not pleasing, animal penises that begs for evolutionary explanations.

Wouldn't you explain the size and shape of the key by the size and shape of the lock? So wouldn't it be a little more scientifically sound to hypothesize that the human penis is sized and shaped like that because it fits well into the human vagina?

Sure, it gets chicken-and-eggy or turtles-all-the-way-downy, but c'mon. Isn't it a bit obvious that the privates that fit inside the other privates are probably correlated? You'd think that even the people who have never had intercourse would default to this explanation for the evolution of the human penis.

Figure 2.  Examples of genital covariation in waterfowl.
Figure 2. Examples of genital covariation in waterfowl.
(A) Harlequin duck (Histrionicus histrionicus) and (B) African goose (Anser cygnoides), two species with a short phallus and no forced copulations, in which females have simple vaginas as in Fig 1a. (C) Long-tailed duck (Clangula hyemalis), and (D) MallardAnas platyrhynchos two species with a long phallus and high levels of forced copulations, in which females have very elaborate vaginas (size bars = 2 cm). ] = Phallus, * = Testis, ★ = Muscular base of the male phallus, ▹ = upper and lower limits of the vagina.

But we're rarely, if ever, told that human penises are relatively girthy because human vaginas are. It's always about male competition or female preference.

Sure, we may be a little weird compared to our close relatives for not having a baculum (penis bone), and maybe that's the sort of thing you want to explain for whatever reason, but does human penis size and shape need a uniquely human story?

Assuming it's correlated to the vagina like it probably is in many other species,* then no it doesn't... unless the size and shape of the human vagina has an exceptional story.

Does it? We wouldn't know. There are zero (look!) articles titled "Why is the human vagina so big?"

Until right now.

Here we go. If we were going to answer it the same way we've long explained the human penis, and other animal penis shapes, then we've got a few ideas...

Because walking upright made the vagina conspicuous and males thought a bigger vagina was better. Because big vaginas outcompete small ones at catching sperm. Because of male pleasure from coitus with a big vagina. Because of heat dissipation or thermoregulation. Because of a tradeoff with brain size.

And of course, we'd need to demonstrate that the human vagina is in fact larger, relative to body size, than the vaginas of other primates. Regardless, a sound answer to the question of vagina size and shape focuses on childbirth, wouldn't you say? She's got to be big enough to push out a baby and, for humans, it's a great big baby. 

So if there's an exceptionally human story for the great big human penis, that exceptional story originates not in a woman's orgasms, not in her pornographic thoughts or her lustful eyes, but in her decidedly unsexy "birth canal."

And I dug up a nice little note to explain this to us all written by Dr. Bowman, a gynecologist, back in 2008 for the Archives of Sexual Behavior

That note is magnificent. It starts out giving the only vagina-size-based, not to mention childbirth-based, explanation for human penises that I can find in the literature (which is thankfully cited by Dixson in his book mentioned above). But it still manages to bring the explanation beyond the vagina and onto another proud triumph: "In sum, man’s larger penis is a consequence of his larger brain."

After you clean up the coffee you just spat onto your computer screen, you can read it all for yourself up there in the figure.

Guess who didn't read it? That study in PNAS, mentioned above, that showed women naked penises, got a high attractive score for the big ones, and thinks that's evidence for mate choice now, today, let alone back when (I'm going to speculate that) women had a tiny bit less of it.

Point is, the literature rages on with the special explanations for the big penis with nary a big vagina in sight.

But you heard it here, at least.

Childbirth is why the human vagina is so big and, consequently, why the male penis is so big. It's pretty straightforward. Yet we're still left scratching our heads as to why the penis question endures.

Is evolutionary science averse to big vaginas?

Does nobody love a big vagina?

Because that's just ridiculous. Everybody came from one.

*Unfortunately a few searches led me to find no cross-species comparisons of mammalian vagina lengths or any vaginal measures. It may be out there, but I haven' t found it. I found some measures for bitches... DOGS! And some heifers... COWS! So I've got to compile some data if I'm to do this properly. Baby size might be a way to do this.

**UPDATE. p. 73 in Dixson has Figure 4.3 with nine primate species' penile and vaginal lengths plotted. Thanks Patrick C for reminding me where I'd seen something like this and where to point readers!


Peter said...

"If you can deliver your package to the front yard but the other guy can deliver to the front door, his is more likely to be carried inside the house first."

I'm glad you mentioned this one, because it's one of my favourite rants about ignorant evolutionary just-so stories. The "longer penis allows delivery closer to the ovum" theory is self-evident horseshit that could only have been invented by someone quite unfamiliar with actual sex, for two very simple reasons.

First... how to put this delicately? Copulation is not a static event. It involves a certain degree of rhythmic activity. To-ing and fro-ing, shall we say. The absolute length of a penis, and in particular the kind of small incremental changes in penis size conferred by individual alleles[1] cannot be functionally relevant in the context of real sex, involving real motion.

[1] If there were single large-effect genes that conferred measureable changes in penis length, we'd know about it by now.

Second... sperm don't emerge from the penis and just stop. Ejaculation propels the semen a significant distance - several inches upwards against gravity, or a couple of feet horizontally. A few millimetres of penis length again cannot have any effect on the final placement of sperm within the vagina.

The "delivery to the doorstep" hypothesis only makes sense in the context of a species with copulatory strategy involving minimal movement of the penis during intromission, and in which the sperm are deposited at low velocity. Without in any way wishing to brag... this does not accord with experience.

Holly Dunsworth said...

(Let's try this again. My first attempt got lost, maybe because in the reCaptcha process I identified deep dish pizza as a "pie"?)

Peter, your comment is one of the absolute best I've ever attracted. Must be the size of my post!

I agree with what's implied in your articulate thoughts, that this deserves serious and thoughtful consideration. I merely dashed this off yesterday afternoon after two viral stories this week highlight male competition to explain human traits: the ""Dead Man Punching story and the ""Calls vs. Balls one that's written so well it's almost stupid of me to link it so close to my post!

Holly Dunsworth said...

(whoops, html fail!)

Holly Dunsworth said...

Of the many things I didn't cover explicitly, one needs to be mentioned here in the comments:

As far as I know, no demonstration of large human penis size, among primates, has ever been made relative to vagina size. So one could argue (and I certainly do) that if indeed these data aren't out there, then we still don't know if human penises deserve to be held apart as girthier than expected for a primate, especially for an ape. All these claims that human penises are large could be dead wrong, and so all this text dedicated to explaining the absent phenomenon could be even more irrelevant than my I suggest in my post.

Jerusha Achterberg said...

Your point about vulva vs. vagina struck a chord with me this morning, based on a recent interaction I had with an ultrasound tech. Warning: I'm going to refer to my own vagina--don't freak out.

I had an abdominal and vaginal ultrasound. Women, many of you know this deal right? So you have the abdominal one, then go pee because they made you drink an ocean of water an hour ahead of the appointment, then they told you that the previous appointment was late and so you have to keep your knees together for another 45 minutes. Or maybe that was just me. Anyhoo...moving onto the vaginal part of the appointment...

I'm on the table, feet are stirrup-ed, bum scootched, and the tech attempts to insert the wand. She does this by prodding, seemingly at random, my vulva.
Tech: Try to relax. I'm having trouble with the insertion.
JTA: That's not my vagina.
Tech: [puzzled face]
JTA: That's. Not. My. Vagina.

I mean come on. She's RIGHT THERE. Her face is staring into the abyss of my womanhood, and she can't find my vagina? Worse, she doesn't realize she hasn't found it. People, this is her ACTUAL JOB--she wasn't even prodding on the center line! I really hope she's not gay, because I would feel *so* bad for her partner.


Rick said...


Even giving the thrusting nature of much (certainly not all) heterosexual copulation, presumably a longer penis would have a deeper average (measured between the shallowest and deepest points of the back-and-forth) penetration, so an advantage could still be conferred.

> Ejaculation propels the semen a significant distance - several inches upwards against gravity, or a couple of feet horizontally.

That's just flat false left as a universal claim about human male ejaculations, and it's not even all that accurate as a general claim. Even men who typically "spurt" don't always spurt, and many men typically don't spurt, either. (Dan Savage uses "spurters" and "dribblers".) So semen really isn't necessarily propelled "a significant difference" in any given ejaculation, and it may very well not be propelled at all, in which case being placed "at the front door" may be an advantage.

Additionally, and here comes (heh) the truly wild speculation, perhaps there are advantages of sperm viability that correlate with time spent in the urethra (brought to mind by the rifling of a barrel). I have no particular reason to think this is so, of course, but it seems at least possible (and perhaps even plausible, given the relative inhospitability of the vagina).

I don't think there's an obvious advantage to penis length, but I think it's obvious that there isn't obviously no advantage.

Anonymous said...

"Wouldn't you explain the size and shape of the key by the size and shape of the lock? So wouldn't it be a little more scientifically sound to hypothesize that the human penis is sized and shaped like that because it fits well into the human vagina?"

This is almost certainly the case, in my experience the penis and vagina (or in most species penis and female cloaca) are almost always described in terms of co-evolution and sexual conflict. I think the specific case of humans the scientific literature may have been invaded by the male ego!

Whilst it may well not be true in humans (any more) don't discount the "delivery to the doorstep" hypothesis entirely, it's one of the better explanations for how the penis evolved at all in the first place. If you wanna see this taken to its ultimate extreme google "Traumatic insemination" and be prepared never to look at sex quite the same way again. This also resulted in probably the most memorable line in a scientific paper I've ever read: "Insemination by proxy via homosexual traumatic insemination"

Holly Dunsworth said...

Thanks everyone! This is a good day.

Now, alright... who wants to add some more comments about vaginas so that the comment thread doesn't mirror the problem the post exposes?

Rick said...

Holly, this was actually my original question: why is "fit" between penis and vagina a necessary source of selective pressure on the penis? I think it's quite plausible that the primary pressure was on the vagina being able to accommodate birth, but I'm not sure why the penis would necessarily follow. If we're discounting "front-door" explanations, and I assume we'll also discount semen-leak explanations, then it seems the penis could simply have stayed the same size. A big vagina presumably does not require a big penis.

But, here's a thought (sort of following Lisa Lloyd on orgasm here): perhaps the same developmental processes which make a big vagina make a big penis, and the big penis is a (useful?) byproduct of a big vagina.

Ken Weiss said...

Great, interesting, thoughtful post (as usual)!
As some comments reflect, and I think a related but generic way to put the points, is that biology is rife with what are often aptly called evolutionary Just-So stories. They sometimes equate plausibility with truth, or current function for selective reason for being. There is too little willingness to acknowledge that selection is generally very very slow, complex, changing and so on, and that our reconstructions after the fact often over-simplify or condense time, and simply do not have enough theoretical or biological constraints. Physics and cosmology have problems with wild speculations about what is not known or, sometimes, even not even testable (e.g., some multiverse theories), but at least they have the benefit of more rigorous methods of relating ideas to facts and of gaining cogent facts to test the ideas.

When the subject comes to humans and, of course, to sex, the speculation can get pretty far out of hand. And of course, current culture, egos and personal experiences or predilections can play havoc with any idea of constraints on speculation. One might say, for example, that since a percentage of people are homosexual, and their fitness is as a result zero or, given cultural conditions, at least less than that of heterosexuals, any genetically based sex-organ-relevant traits that relate to homosexual activities would be selected against, and what organ traits we see today is mainly just what's not been selected out in that way! Even if the fraction of homosexual people was only a percent or so, that would be very strong selection relative to estimates of fitness differences found empirically by population ecologists. In other words, one can cook up plausibility arguments whose conclusion is that what we have today has nothing to do with how it works, or worked in the past.......

Anonymous said...

REally interesting article.

One point snagged my attention; "Because walking upright made the vagina conspicuous...." I'm a bit bemused by that. As a woman, it just didn't ring true.

In the interests of science, I had a look at what google thinks are images of 'naked woman standing', and am pretty convinced that unless your viewpoint is near a woman's knees, or the woman is leaning over the back of a table or sofa, you can't see anything like a vagina. Tables, sofas, etc. are kind of rare before recent history. But if you look at images of 'naked woman sitting' you get poses which, if it were not for the nicely positioned legs, would reveal a lot more of the general vaginal area. Googling 'ape, sitting' suggests that apes sit in fairly similar ways - if anything, more often in more revealing squat poses (but this may be because I have a filter on google which means that I don't see pornographic images of women ...).

The main difference between women and apes, as far as I can see, vis a vis genital visibility is nothing to do with walking: it's body hair. Ape vaginas are shadowy affairs, human ones less so (maybe we have better lighting), and have a visual 'hey, look at me!'.

And talking of 'hey, look at me' - want to see a vagina on show: google images of 'monkey on heat'.

Anonymous said...

"Dixson" is his real name? Awesome.

Holly Dunsworth said...

Thanks- Bipedalism is a fave explanation for the big human penis. That's what that was about. Turning the tables.

Anonymous said...

""Dixson" is his real name? Awesome."

Reminds me of the most warmongering NATO general named Breedlove.

Holly Dunsworth said...

As someone pointed out on Facebook, I should have titled this, "Why is Man's Vagina So Big?"

Anonymous said...

I remember being teach at the University that the penis of Chimpanzee had evolved in accordance with the "size" of females' swealing... Seems that idea was not further explored...

Holly Dunsworth said...

Yes, thank you for mentioning this. I'm familiar with that but it's rare that the same explanation is applied to chimps' closest relatives - why not?

pudendum said...

This point got my attention as well. Since the vagina is internal, when a woman is bipedal, the only part that is visible is the mons pubis, and possibly the labia majora.

Holly Dunsworth said...

Conspicuous ness due to Bipedalism is a fave explanation for the large penis. In that paragraph I'm doing many things, one is turning the story around but with vaginas. It's not supposed to be serious or scientific.

Anonymous said...

It's an interesting issue. One thing that came to mind is hyenas giving birth through their pseudopenis. I look forward to the longer article.

Holly Dunsworth said...

Exceptions are marvelous! I believe they only have to line-up holes to enable fertilization! Much like cloacal kissing in many animals, I suppose.

Holly Dunsworth said...

And the point of your comment slipped by me at first. Yeah, that strange name for the birth canal of a hyena just because field researchers mistook it for a penis.

Anonymous said...

Here's Martin's scatterplot of Dixson

His thoughts on girth & width

I (hyena commenter) considered if there were a lot of wiggle room (no pun intended), the high penis-vagina correlation may simply be an artifact of inter-species allometry. But looking at chimps, gorillas, humans suggests that this is not the case - given their respective sexual dimorphism, data not consistent with simple allometry (in which case all highly dimorphic species would be below regression line) and more consistent with your adaptive lock & key model. Nevertheless, high degree of intra-individual variation in size of human genitalia suggests either multiple strategies or simply relatively low selection within functional size range (probably latter - so a little wiggle room).

re: Bowman: Aren't shoulders, not the head, the big problem during birth?

Holly Dunsworth said...

Thanks to all for the comments and eye-opening information.

To the question about shoulders above, they certainly are a major problem but I don't know if anyone can say they are "the" problem over another, like head or pelvis or overall fetal size, or any number of issues with the uterus or cervix, or a surplus of energy, or a greedy placenta, ...

Regarding some of the feedback here and also on social media: Many people are taking my post literally in ways that were not intended. And that, I think, speaks to how the ideas that I'm commenting on have become so predominant.

Further, I can be bold with my claims based on mostly my intuition alone because that's the example that's set by the science, and its dissemination, that I'm commenting on.

Also, I have a key to a particular lock that jiggles quite a bit around yet it still does the trick.

Stacy said...

First, this post was fantastic. I think the primary takeaway is that science has been dominated by men for so long and theories of genital evolution reflected that.

Second, hasn't it been proven false that penis length is positively correlated with fertility?
Also, when we say "big vagina" does that mean length or girth? If we're adhering to the argument that a longer penis is better for getting the sperm to their destination, then wouldn't a shorter vagina be better (unless we're more like ducks)? A wide, shallow vagina seems biomechanically better for childbirth, unless sliding through increases infant-vaginal surface contact and promotes transference of necessary microbes.

Holly Dunsworth said...

There is either a language barrier or a humor barrier or something going on above.

Holly Dunsworth said...

Actually, that comment reads like a native English speaker wrote it and, whether or not it was sparked by poor reading comprehension and the inability to detect satire, it is awful.

Stacy said...

Perhaps the winky face emoji has been left off.

Holly Dunsworth said...

Ah! You mean it is I who has failed at detecting humor?! Sorry, but the tone is way off for my taste. I do hope it was meant in jest.

Stacy said...

No, just that this Anonymous forgot to wink, which would indicate that he can't possibly be serious. (I never understand people who are actually serious and continue to read to the end of articles they disagree with.) Maybe I needed a winky face on my comment too!

Anonymous said...

What is meant by a "large" vagina? Most sit around in a flattened, shortened state. They don't sit there being "large." They *accommodate* a penis. A vagina is very stretchy. You can have a baby come out of it and then it'll snap back. A lady with a shorter vagina can still accommodate a very large penis, because given time, the vagina can stretch, and the uterus and other organs can be pushed (slowly, please) out of the way, with no pain. A penis has a maximum length; a vagina can handle a baby's head and comically large sex toys. So I think measuring the "size" of a vagina would be hard.

Unknown said...

I am not sure whether this is directly relevant to actual size of genitals, but we published a paper some years ago that looked at variation in clitoral size, vaginal depth and penis length with the idea that less variability would suggest less selection pressure on that structure. What we found using published samples of men's and women's genitals was that of the three, the clitoris was the most variable with a CV ratio of about 3 compared both to penile length variability and to vaginal depth variability. By contrast the CV ratio for penile length to vaginal depth was about 1, or comparable. We suggested that these data are consistent with clitoral size not being under strong selective pressure, whereas Penile length and vaginal depth reflected comparable selective pressure raising the possibility that their dimensions co-evolved. Since I didn't collect the actual data I can't vouch for how much measurement influenced the findings, but I thought they give some food for thought.

Wallen, K., Lloyd, E.A. (2008). Clitoral variability compared to penile variability supports nonadaptation of female orgasm. Evolution and Development, 10(1): 1-2.

Holly Dunsworth said...

Thanks so much Kim. I'll definitely check out your paper. I'm interested for many obvious reasons but one is also because I thought the norm among evolutionary scientists was to explain less variability with *more* selection pressure, not less, as you explain.

Thanks again.

Holly Dunsworth said...

P.S. I just re-read your comment and I think you simply mistyped the part at the beginning that says "less variability would suggest less selection pressure on that structure" given everything that follows as well as the title of the paper, which I can see now that I'm looking at the comment in full view (not behind the scenes where it was cut off). Thanks again!

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