Friday Weird Science: Penises are Funny Looking!

Feb 04 2011 Published by under Friday Weird Science

This one comes courtesy of NCBI ROFL, who presented the abstract last week. Can’t beat their fantastic penis-shaped light fixture, either.

I’d like to begin today’s Friday Weird Science with a brief story about SciMom. SciMom is a brilliant mom. She’s encouraging and supportive and smart, and also really REALLY funny. Mostly because she’s prone to say really funny things. And begin telling jokes by saying the punchline. But one day we were with my college roommate over spring break (we went to each other’s houses over spring break. Way to geeky and poor to go to the beach, I can tell you. But it was still good times) and had to stop by the grocery store. I recall we were in the bean aisle, when suddenly my mom turns around and says “you know, penises are really funny looking”.

Now you know where I get it from. 🙂
(Inserting fold due to extensively NSFW pictures...)

And penises ARE funny looking. I mean, look at this!


And this!

And we cannot forget this.


And the human penis is funny looking as well. You probably don’t think so, but that’s just cause you’re used to it. After all, ducks think that is TOTALLY normal.

On a previous Friday Weird Science, we got into a discussion in the comments of testicle and penis size in humans in comparison to other primates. In many species, testicle size in particular correlates with how many partners the female of the species has. Thus, animals like rats and right whales have HUGE balls because the females tend to have many partners (sometimes more than one at a time!). It makes sense, if your lady is going to have multiple partners, to have large balls, and produce lots of SPERM, in the hopes that you’ll outcompete your fellow males to get to be the babydaddy.

In some species of primate, this follows too, gorillas have small balls, chimps have larger ones. But what about humans? It turns out that humans fall at the higher end of the primate spectrum in terms of testicle size, but they REALLY fall at the high end in terms of penis size, with thicker penises than other primates.

Now, is this just because the males of the species need to compete? Or is it something else? After all, ball size is one thing, but penis size is another. And what about SHAPE? What’s with that big bulbous glans?

Well, this guy has a theory.

EA Bowman. “An explanation for the shape of the human penis” Archives of Sexual Behavior, 2010.

So first, some basic penis anatomy.


The shaft is at the top, and at the bottom is the bulbous head that is surrounded by a thin membraneous and loose skin which is what gets removed in male circumcision. But the question is, why is the glans bigger? Why is it that bulbous kind of shape? Scientists have previously hypothesized that the glans is shaped this was in order to kind of scoop out the semen of other males. It’s not peculiar to humans, other animals like whales have it, too.

Bowman says that this is unlikely, due to the extremely acidic pH of the vagina, which he says would immediately destroy any sperm that didn’t make it up to the uterus. I’m not too sure about this, the pH of the vagina is between 3.8 and 4.2, and sperm are well capable of surviving in there for a little while, in fact, the mucosa favors sperm survival. Another hypothesis (which isn’t addressed here) is the bulbousness of the glans as a way to increase a pressure differential during intercourse, acting like a plunging mechanism to get their sperm out and your sperm in. But Bowman thinks that scooping or plunging isn’t the point at all.

He’s got a different theory. And he thinks it’s got to do with the foreskin. He points out that the foreskin is a very absorbent membrane (and cites for this the increased vulnerability to HIV infection in uncircumcised men, though there’s no ACTUAL citation, and I’ll have to look this up). If this membrane is absorbent, what’s it absorbent FOR? His hypothesis? Vaginal secretions. Women have secretions in the vagina (duh) containing various compounds including hormones like oxytocin and vasopressin. And so Bowman hypothesizes that women secrete these hormones in their vaginas, and men absorb them with the foreskin, to promote things like pair bonding and serve as a deep evolutionary level of communication. He also notes that this means the best position is “primate rear approach” (that’s doggy style) in order to hit the posterior wall of the vagina where he says a puddle of vaginal secretion is likely to be (would love to see the citation for that one).

Sci’s got to say I don’t really buy this. There are no references cited (though I can cite a few for some of them), and…where is the evidence? Does this mean that couples who have sex with a condom (thus covering the glans and preventing absorption) exhibit less pair bonding than those who have sex without? What about the effects of hormonal contraceptives (which drastically change the vaginal mucosa) on pair bonding? And what about the effects of, you know, having a relationship with someone? Wouldn’t that produce more pair bonding?

I don’t buy it til we test it. Going to have to have a condom and condomless condition, and a contraceptive and non-contraceptive condition (if we’re not dealing with couples trying to get pregnant, could use a cap or other contraceptive device). But now we run into a problem. Should we test already pair bonded couples, and ask the man about emotional closeness before and after sex? Or should we test totally random pairings (tough to get THAT past IRB approval) and see how people feel before and after sex? And before we even get THERE, is there any proof that the foreskin can absorb hormones at all? Anyone ever soaked a foreskin in vaginal mucosa to look at uptake? Perhaps we may want to begin in vitro.

So does a penis have a bulbous glans because of a plunging mechanism? Because of a scooping mechanism? Or because of a hormone mechanism? Or is it all three? Or none of the above?

Regardless, they’re still pretty funny looking.

Bowman EA (2010). An explanation for the shape of the human penis. Archives of sexual behavior, 39 (2) PMID: 19851854

25 responses so far

  • Jason Dick says:

    Seems to me that checking for various hormone levels over time would probably be better able to show whether or not there is some sort of effect like this.

    • scicurious says:

      See I think you'd have to get penile samples, and you'd have to get samples OF the foreskin, because otherwise you're just sampling the lady's vaginal secretions, and not what he's absorbing. Somehow I doubt there would be volunteers...

      I really don't think the absorpotion levels would ever be high enough to detect in the blood, though perhaps, if it's going to affect behavior...

  • Kele says:

    What about his idea about a larger birth canal causing increasing thickness in human penises? Is that evidenced at all or a just-so story too?

    • scicurious says:

      Don't know about evidence for that one, but the thickness would go along with the plunging hypothesis as well as the larger birth canal (gotta have a good seal to create suction, amirite?).

  • seth says:

    Plug ? Scoop? Always understood it was to stop your hand slipping off.

  • prochoice says:

    that´s one of the influencing factors (although I do not know how to measure that in fossils).
    The other question - I remember something about that in the early days of photography, but no statistics - is display.
    Males acquiring higher ranks - and chances for fertility - if they can show a big penis to other males.
    And while testicles may become too burdensome, or temperature for ripening sperm difficult to maintain if they are too small or too big for a species and its lifestyle (words in search machines give 0 for and lots of ads for google), the penis can grow very visual with the only price of aging of heavy-duty tissue, and in evolution that does not count.

    • scicurious says:

      See I don't know about display, personally. You'd think if display of penises in the flaccid state (for other males) was a big deal, we'd have more showers than growers in the population. Though there's no data on that, I suspect it's not the case. Now, if early humans pranced around waving their ERECTIONS around, that's a different thing.

  • Matt says:

    You forgot the Echidna! Monotremes are fetishists of the highest order 😉

  • Maia Szalavitz says:

    Re: condoms v. condomless sex and bonding. This study found a relationship between semen exposure via condomless sex and less depression FWIW.

    • scicurious says:

      Yeah, people also found one with oral sex and less depression. LOTS of confounds in that study, though the data are interesting.

  • rknop says:

    And what about the effects of, you know, having a relationship with someone? Wouldn’t that produce more pair bonding?


  • brooksphd says:

    After circumcision woulnd't the glans "dry out" and become less absorbant?

    How would that factor in? Would it all?

    Has there been enough time in circumcising populations to note any difference between, ahem, cut and uncut males/females in bnding and fertility?

    I'd look in the literature, but I think our IT monitors might get suspicious if I do it at work...

  • Jon says:

    I'm not sure I buy any of these explanations. Having males evolve to accept hormones from the female in order to facilitate bonding seems odd - two traits evolve in each sex separately, i.e. evolutionary cooperation between the sexes, instead of the more usual warfare? If it's that important to bond as a result of sex, it'd be simpler to tie things into ejaculation. For that matter, why is the male receiving something facilitate pair bonding, but not the female? The pair bonding idea would seem to require both individuals to have the bonding facilitated - otherwise you'll just end up with unrequited love. Of course, maybe that's where the sexual conflict comes in. Even so, if it's conflict, why would males actively facilitate being taken advantage of? This whole thing just seems too convoluted and to require a bit of group selection.

    The other two theories seem relatively simple to test - just grab some relatively realistic models, put them together, and see what happens. Who doesn't want to spend grant money on sex toys?

  • jim says:

    The plunger hypothesis seems likely. I've never had "sloppy seconds" so have no direct experience, but the term itself seems to imply the evacuation of a considerable quantity of body fluids.

  • Whoa! Duck penises are AWESOME!

  • beebeeo says:

    I think I read ages ago, somewhere, that the purpose of the foreskin may be to faciliate rape.
    I am not sure whether this is the original where I first read it, but here is a quote from a Medical Hypotheses article:
    "Preliminary measurements of the force required for intromission with and without the benefit of the foreskin have been made with a simple device. The device consisted of a Styrofoam cup the bottom of which was cut to make an artificial introitus. The cup was mounted on diet scales so that the force being applied could be noted (Fig. 1). Twelve measurements were made with the glans penis alternately exposed or covered by the foreskin. Intromission with the glans exposed required a minimum of 500 g of force and was uncomfortable. In contrast only 40 to 50 g were required for complete insertion of the glans when it was covered, and there was no discomfort. Thus, intiating intromission with the foreskin covering the glans resulted in a greater than 10-fold reduction in the force needed to penetrate an `introitus' and make an otherwise uncomfortable intromission comfortable."

  • SciMom says:

    Re erect penis waving: The ancient Greeks were all over that. Some very funny short plays were written to follow a full day of tragedy so people wouldn't feel like going home and slitting their wrists (bad Medea). These short plays are referred to as Satyr Plays, and seemed to largely consist of actors prancing around wearing goatskin leggings and giant inflatable penises. So giant were they, that the actors had to push the penises down so they could speak...and it was quite the hoot to see them bang themselves in the face with the big guys after they let them go.

    And yes, I still think penises are funny looking and apparently I am not alone. Go Greeks.

    • scicurious says:

      Why did you not tell me this when I was a kid!?1?! AWESOME. And I might have sat through Medea if I knew there were huge inflatable penises (penii?) at the end.

  • E. Manhattan says:

    At orgasm, almost all men's glans swell - some to almost twice the size of their just-before-orgasming size. In action, this performs a lot like a plunger - but assuming that a plunger is to remove other men's semen is not particularly evidence-based. The plugging/plunging could serve several functions, including a) preventing semen loss and b) clearing any cervical obstructions and c) forcing semen through tight openings.

  • Bug says:

    This is fascinating. Everyone here is discussing the idea that the shape of the human penis is designed to help sperm compete with other sperm by scooping, etc. What if the shape of the human penis was designed to facilitate orgasm in the female, and therefore solidify bonding and hopefully beat out competitors from THAT perspective? When my sweetie's glans swells to almost twice the size, it has a tendency to send me over the moon.

    I'm just sayin...

  • J Fairfield says:

    The scooping theory could only have been conceived by circumcised men. If you're uncut, the entire sulcus is taken up on the outstroke by your bunched-up foreskin, also known in this position as the "ridged band" (yup, that's a medical term. look it up). The corona is unlikely to be scooping out anything.

    My experience suggests to me that the glans stimulates both partners: it engorges to help expand the vaginal walls, and it has a corona to help stimulate the man's own foreskin when it passes over. After all, the foreskin has thousands more, and far more varied and complex, nerve endings than the glans. The glans is virtually insensitive to fine touch, which doesn't mean what most people think it does. It simply means that the type of nerve endings it contains are few in number and cannot discern heat, cold or the type of pressure being applied. It is sensitive, but in a very "dumb" way relative to other structures of the penis. This makes perfect sense, as the glans is something of a battering ram.

    Every part of the penis has a purpose, including the foreskin. Research has shown (Cold et al) that the human foreskin is far from vestigial. In fact, it has been increasing in length and complexity throughout human history.

    What confounds me is why the curve of the penis seems so relatively unimportant. Some erections curve forcefully upward, some stick straight out, some curve downward (like mine) and some bend to the right or left. How can it not make a difference? Are some shapes more suited to missionary style than rear approach? Is there no "ideal"? Bizarre.

  • Darcy Mullen says:

    I can't really see the duck penis very clearly. Could you please upload a closeup picture.

  • sara says:

    since i was a child i want to know how is rooster penis

Leave a Reply