Adaptationism in the Human Penis

As Scicurious’ mom points out, penises are funny lookin’. As long as humans have been humans, men and women have looked down and thought, “now what could be the possible reason for that?” The question no doubt vexed our early ancestors so much that they simply had to evolve larger brains to think about it more.

We’ve even looked at the penises of other species and pondered their functional anatomy. The chimpanzee penis, for example, is long, skinny, and kind of pointy at the end. Chimpanzee penises are designed to dislodge hard little plugs of semen left in the vaginal canal by a previous male.  When a chimpanzee female comes into estrus, she will mate pretty much non-stop until she comes out of it.  Lines will form at the base of a tree in which she dwells, and the chimpanzee males will simply wait their turn, ascend the tree, and rely on their penis and sperm to do all the competitive work. Their copulatory plugs form and basically seal off the cervix from the incoming sperm of later males, and may also help to keep their own sperm in there, so that when ovulation actually occurs your sperm is right there and ready. Copulatory plugs are pretty common in primates, and if they work, they obviously have a pretty big effect on an individual male’s fitness.  Males who have pluggy semen and pointy penises have sons with pluggy semen and pointy penises, and a pointy little arms race is born.

I suspect that the above hypothesis (Which I call The Crowbar Hypothesis) for chimpanzee penis shape was the inspiration for one of the most-often discussed hypotheses for Human penis shape: The Plunger Hypothesis (TPH). TPH is basically the idea that the bulbous shape of the glans in humans acts as a plunger which scoops a previous male’s sperm out of the vaginal canal, while at the same time delivering his own sperm to an optimal position for insemination.  At least, until the next competitor’s plunger penis scoops it out. Some insects actually do exactly this.

So, what should we expect from such a hypothesis?  First and foremost, we have to have evolved in a society in which there is a lot of sperm competition.  Chimpanzees have tons of it, and since their troops are primarily composed of related males, there is little to no outward signs of competition for females.  They’re certainly not thumping their chests or doing other threat displays to get their brothers to lay off their female.  Instead, they’re politely lining up and waiting their turn.

So do humans fit this criteria? Maybe, maybe not.

We should also expect that the shape of the “plunger” actually affects how much semen is scooped out.  Males with better plungers should have more offspring, and there should be some variation in penis shape, glans to shaft ratio, all that stuff.  Does this criterion apply?  Maybe, but no one’s looked at it.  All we have is the infamous dildo experiment.

And now we have the Glans-as-Hormone-Sponge Hypothesis. The GHSH suggests that the human penis is plunger-shaped in order to scoop, but instead of scooping out the competitor’s sperm, it’s scooping up vaginal secretions filled with love-inducing hormones.  The foreskin, he states, is particularly sponge-like, and will soak up her hormones, be absorbed into his blood stream, and induce the release of more hormones from his brain.

Okay, that’s an interesting idea, too.  And he’s not saying he has evidence for it; he’s just suggesting it.

Here’s the problem with both of them, though: The glans begins life as the genital tubercle.  In males, the base of the tubercle grows out to become the shaft of the penis, while the glans becomes demarcated from the shaft by the coronary sulcus- the ring around the plunger, if you will.

The shaft of the penis gets a little groove down the middle of it which will eventually fold around itself to become the urethra.  This groove does not extend into the glans.  In the glans, a little pit develops at the tip and moves inward until it meets up with the future urethra. The two adult structures are distinct because they’re formed from from distinct embryonic structures.

And here’s the part that will blow your mind:  Females also have a coronary sulcus around their glans (of the clitoris). Her coronary sulcus separates the glans from the rest of the phallus. The main difference between the male glans and the female glans is that the male glans had to extend out from the body so it could get to the vagina, and did so by growing a larger phallus.

So my question is, why do females have vaguely plunger-shaped clitorises?  Or are we perhaps trying to ascribe function to a simple relict of an embryonic event? Do females have a coronary sulcus because males have a coronary sulcus, or might it be the other way around?  Do males have a ledge around their glans for a reason, or is it simply because the clitoral glans is a distinct structure from the rest of it?

ResearchBlogging.orgBowman EA (2010). An explanation for the shape of the human penis. Archives of sexual behavior, 39 (2) PMID: 19851854
BIRKHEAD, T., & HUNTER, F. (1990). Mechanisms of sperm competition Trends in Ecology & Evolution, 5 (2), 48-52 DOI: 10.1016/0169-5347(90)90047-H

H/T to Scicurious, where you can go to see all sorts of penis pictures.

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10 responses to “Adaptationism in the Human Penis

  1. Pingback: Research Roundup: The Penis Edition « Mauka to Makai

  2. Pingback: Adaptationism in the Human Penis (via A Primate of Modern Aspect) | First Praxis

  3. AF February 4, 2011 at 7:35 pm

    Human penises are also relatively large compared to those of chimps and gorillas. If I were to put forth an evolutionary just so story to explain this, I’d say it started happening when female humans got bright enough to start associating size with strength in an abstract sense.

  4. seriousmonkeybusiness February 4, 2011 at 11:28 pm

    Morphology isn’t my strong suit in the least (lack of exposure), but does the baculum in the chimpanzee penis play a role at all? Or does it not matter, hence why it doesn’t exist in humans?

    (I probably oversimplified that a little bit, but I noticed on both posts on the subject–yours and SciCurious’, nothing was mentioned about this and I got curious.)

    • zinjanthropus February 5, 2011 at 12:37 pm

      Humans are actually one of the only species that doesn’t have a baculum (or its associated os clitoris!) so I don’t know if it plays a role in chimpanzee sex other than the normal, “guide it in and keep it there longer” that it does in the other mammals. Chimpanzees aren’t exactly known for their stamina, but it wouldn’t surprise me if something that long and skinny needed a little extra support to hit its target.

      There are lots of hypotheses for why humans lost their baculum (mostly display-related) but I don’t remember any of them being all that convincing… Could just be that selection was relaxed, but I really have no idea!

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  6. megan February 24, 2011 at 5:31 am

    Another thing to think about – did this change in penis shape come about before or after we became bipedal? I’m thinking about the angle of the human vagina on a given day and the fact that the semen runneth out. I know nothing about the angles of chimpanzee vaginas as female chimps walk or sit, or how long it takes a semen plug to coagulate. If humans were quadrapedal would the semen stay in the vagina any better? Could the plunger be trying to spread the semen around so it doesn’t all fall out in one big plop, thereby increasing the chance some of it will enter the cervix?

  7. scilicious March 11, 2011 at 11:10 am

    I’m trying to recall my very distant classes, but don’t male gland and female clitoris from the same embryonic structure? Could your last point be related to that? Mind I’m not sure there, it’s been too many years and ontogenesis never was my strong suit.

    Interesting point on chimps, I’ll keep that in mind!

  8. Pingback: Science online, bright and beautiful edition | Jeremy Yoder

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