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Adaptationism in the Human Penis
February 4, 2011Posted by on
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.
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?
Bowman 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.