Does size matter...or shape..Yes we're talking about the penis

An older study brought to my attention recently (subscription required, though I also found this link).

This is strictly to avoid a dong
being the lead in picture on FoS

The authors want to test some ideas regarding behaviors and adaptations human males may have to ensure paternity. As I heard about this research, my skeptic meter went off and I whipped out my critical thinking tool box to look at this problem and research more closely. The colleague telling me about this research seemed to think I was being a bit of an asshole, but Im used to it.


First, we have to consider the central hypothesis that there is a fitness cost associated with human males not being sure of paternity. Obviously, raising children takes a bazillion resources so from that perspective a dad doesn't want to waste his time and energy raising Chet's kid. That explanation jives with our sense of selves in our current society, but what about 4000 years ago? Prehistorically (and historically for most of the planet) people lived as hunter-gatherers, in family units. Not being an expert in anthropology (maybe Dr. Laden will comment if he sees this post), but my understanding is that a given male in the tribe spends less time raising one specific child. Resources are acquired and distributed amongst the members, fucking socialists. So under most of human evolution, what were the selective pressures that would promote the behaviors the authors take as givens?


Even today, while there are reasons to want to ensure paternity, if paternity were a major part of male behavioral make up, presumably we would all have a dozen mistresses on the side to maximize our chances to get some paternity. Of course, while we were visiting our mistresses, our mates could use that time to help the local studmuffin with his paternity issues.


Basically, I don't like these behavioral rationales because it is trivial to argue them any way you like. We could look at the animal kingdom to gain some potential insights, but that's a clusterfuck as well. Chimpanzees, gorillas, and bonobos all have extremely different sexual behaviors. Lions kill their step children, eagles mate for life, cuckoos force surrogates, seahorse males exclusively care for the young, penguins mate for a season raise a chick then trade up or at least out. So whatever human behavior you want to rationalize, there will be an example in the animal kingdom you can use.

Modified from paper

Ok, so let's take the behavioral rationales off the table and simply look at what the authors did. One thing they wanted to test was the semen displacement hypothesis. What? You don't know what the semen displacement hypothesis is? Well, its the idea that the shape of the penis allows it to act like a scoop to remove another dudes semen if said other dude recently had sex with the woman you were currently copulating with.





The authors acquired some artificial penises (commonly known as dildos) and an artificial vagina (commonly referred to as damned if I know) and made artificial semen (cornstarch and water). The authors put the "semen" in the "vagina" and then "copulated" using the "penis". (If ever a sentence needed scare-quotes.) Then the amount of "semen" remaining was then measured to come up with the following data set.

These are the phallus types

So both dildo penis B and D, which have scoops displaced 85-90% of the cornstarch. Whereas  VibroMaxxx penis C displaced 30-40% of the cornstarch. It looks like the hypothesis is supported. However, I have some concerns. First, cornstarch isnt semen. For one thing, semen contains spermatozoa that swim, like towards an egg cell. The authors removed that variable by making the "vagina" a closed orifice. By sealing one end, there's no place for anything else to go but stay still or be pulled out. In the real world, maybe the act of secondary copulation pushes more semen and sperm of the first guy deeper into the vagina closer to the cervix and the ultimate prize the egg. That was not an option here.

Second, where have all the good sperm gone? During ejaculation, the sperm come out with a fair bit of thrust (yes, I know). While much of the semen remains within the vaginal cavity, I wonder how quickly sperm that most likely fertilize an egg leave the vagina? The vaginal cavity is not a hospitable place for sperm. It's essentially a lethal acid bath, not to mention there's mucus that you can get hung up in, and immunological cells acting like you don't belong there. Getting out of Dodge is a good thing. If we take the scooping results presented as solid, the question becomes, in real life what's being scooped out: competing healthy sperm of your best friend or the dead residuals of said ex-best friend?

Third, timing. For this hypothesis to work, it seems like a woman would have to find another mating partner in a reasonably quick period of time. How common is a rapid turn around time? If it isn't that common, there's probably little to no selective advantage because we have to factor in the chance the first guy wins anyway as well as all the times copulations fail to produce a pregnancy.

There was a lot more in the paper, some surveys (YAY!!) and what not. You should read the paper to get all the details. But color me not convinced. The idea the human penis shape plays a role in sexual competition is interesting, but there seems to be too much hand waving to justify the research. If we take these 3-5 things as givens, then.... Well, I'm not a fan of taking things as givens.

I am convinced of one thing though. If during sex, I recall that my penis may have evolved a scoop-like shape to help remove semen from a competitor, it'll probably be time to call it quits.

Interesting aside. In all of this work and study, the woman is completely neutral just a bystander in the process. If there was a penal shape competition going on, I would fully expect that there was also a role for the female in this process, she would not want a sub-par partner ruining her perfectly good plans. Women may promote successful copulations with the primary stud of interest by changing position to allow deeper penetrations during climax for one partner but not the other, for example.

Gallup, G. (2003). The human penis as a semen displacement device Evolution and Human Behavior, 24 (4), 277-289 DOI: 10.1016/S1090-5138(03)00016-3