The Human Penis as "Anatomical Squeegee"

i-cbb197d5b0d8397493cc8644a9b5a77d-window_squeegee.jpgOh wow.

Oh wow.

This press release is simply astonishing. Maybe it is because it has been a long time -- and as a consequence I have a mind for dirty press releases. Maybe it is just because I quite generally have a dirty mind. However, this is the singularly most amazing press release ever to be released in the history of science. This is not hyperbole. Read for yourself:

In the context of sexual reproduction, natural selection is generally thought of as a pre-copulation mechanism. We are drawn to features of the human body that tell us our partner is healthy and will provide us a fighting opportunity to carry on our genetic lineage. But a new article appearing in the February issue of Current Directions in Psychological Science suggests that the human male has evolved mechanisms to pass on his genes during post-copulation as well, a phenomenon dubbed "sperm competition."

In their article, Todd Shackelford and Aaron Getz at Florida Atlantic University describe sperm competition as "the inevitable consequence of males competing for fertilizations."

For a monogamous species, sperm competition may seem unlikely. But according to the authors, extra pair copulations (i.e. affairs) appear to be a significant part of our ancestral history and could, evolutionarily speaking, spell disaster. Ultimately, a male whose female partner engages in extra-pair copulation is at risk of unwittingly investing his resources into a genetically unrelated offspring.

Competition may also affect sperm count, say the authors. When men spend more time away from their partners (time that their partners could have spent with other males), the number of sperm in their ejaculate increases upon their next copulation. In one study, the authors note, simulated phalluses that most closely resembled the human penis removed an ejaculate-like substance from an artificial vagina. This could potentially signify that that the penis developed its shape to act as an anatomical squeegee.

But sperm competition does not solely take place in the realm of our biological makeup. According to the authors, many sexual behaviors such as deep copulatory thrusting may function to remove rival sperm. Accordingly, sexual partners report that men thrust more deeply and quickly into the vagina following allegations of infidelity. The same periods of separation that increase sperm number in male ejaculates may also help to explain the increasingly lustful feelings human males develop after long periods of time apart from their mate. That is, the human male may want to copulate as soon as possible as insurance against possible extra-pair copulation.

As sperm competition comes to light as yet another facet of sexual adaptation, it raises the question of how many other ways have humans evolved in a world dictated by "survival of the fittest?" "Sexual conflict between males and females," Shackelford and Goetz describe, "produces a coevolutionary arms race between the sexes, in which an advantage gained by one sex selects for counteradaptations in the other sex." Thus future research may move beyond male adaptations and, for example, attempt to identify mechanisms in which females increase retention of sperm inseminated by men with the most favorable genes. (Emphasis mine.)

Where to even begin?

1) I want to know where the funding is coming from for this. If NIH is paying for someone to use a fake penis to have sex with a fake vagina, I want an explanation.

I am no prude. (As is obvious to anyone who has ever read this blog.) I am sure there are some physics-related issues in the mechanics of copulation. I am not unwilling to consider the idea that the penis is indeed shaped appropriately to function as an anatomical squeegee. (This of course begs the question of circumcision, but I digress.)

However, does the investigation of these issues require the construction of artificial penises and vaginas? Are we not all experimenters in this area? I am quite certain that some couple at Florida Atlantic University would be pleased as punch to participate in this study.

2) I am willing to by the increase in sperm count followed by a prolonged absence as adaptive. However, this I am skeptical about:

That is, the human male may want to copulate as soon as possible as insurance against possible extra-pair copulation.

The human male wants to copulate as soon as possible after a long absence because the human male likes sex A LOT. This is the most parsimonious explanation.

I was not under the impression that this required a theoretical framework, nor does it particularly require evolutionary explication.

This is what fascinates me about evolutionary psychology. Not that it is radically speculative, but rather because it tries to explain the incredibly obvious in unnecessarily complicated ways.


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And what about the evolutionary compulsion to say "Mine's Bigger Dude"... even if it's not? (See compensation, over-compensation, convertible sports-car purchase by guys over age 50...)

Did they actually have to "construct" artificial penises and vaginas? Why didn't they just go to their local handy-dandy sex toy shop? And they could do a comparison with differnt dildo shapes.

By natural cynic (not verified) on 12 Feb 2007 #permalink

Imagine that you get google alerts for news with your name to track how often your Web site is being mentioned. Then imagine you get a google alert that attaches your name to this article. Should I be proud of this (mis)association? Or should I permanently erase it from my memory?

Actually they did exactly what you describe and used a collection of sex toys to do the experiment.

You can read the paper at…

Worth reading for the methods alone!

It's not clear to me, however, that it 'begs the question' of circumcision. Are you saying that circumcision would circumvent the squegee function? That doesn't preclude any evolutionary explanation for human penis shape - we alter our bodies in lots of ways now that probably aren't very adaptive in the context of our evolutionary history.

That journal must be really short on submissions. "Speculative" doesn't even begin to describe it. I mean, suggesting an evolutionary explanation for "deep thrusting" among upset and resentful men who suspect their wives of infidelity? I bet in the next issue they'll offer an evolutionary theory of felching.

Hold on, now. Sperm competition is a well-demonstrated aspect of many different organisms' reproductive systems. Many form what are called "copulatory plugs," for example (which cannot be found at the sex shop), so the males have hooks on the end of their johnsons to snag the previous male's plug. It's actually more parsimonious to assume that humans share this trait with chimps and bonobos (our closest relatives by far) than to assume we don't (the standard theory).

I'm writing a book about all this at the moment, so believe me, I could go on and on.

The point is, just cause it seems obvious doesn't mean it is. It's pretty obvious to me that the sun goes around the earth and that drilling a hole in my skull would relieve the pain of my headache...


I have to agree with the researchers to some degree. I can see this as increasing the fitness of males with that trait.

Providing evidence for fitness increasing traits is sometimes very difficult though - it often flirts with mere speculation - as in this case.

In all seriousness, apart from some people's distastefulness for group sexual activity (whether or not for the purpose of research), wouldn't live volunteers be better for this anyway? I'm assuming here that there'd be some way to do population estimates of genetic material from multiple "donors" with a vaginal swab.

Or is human sexual performance too much of a wild card for a scientific study? In any case, it's a guaranteed Ig Nobel Prize.

It's always strange to see scientists in one field dismissing the worthiness of research in another. Even if you take simple size, the fact that H. Sapiens males do indeed have disproportionately large penises relative to their primate counterparts, the question of how this was evolutionarily adaptive is germane.

While one can certainly find volunteers to have sex, the challenges in getting research funded and running with human subjects are large enough that a Neuroscience MD/PhD such as yourself should be able to see the advantages in using surrogate parts. The level of instrumentation possible is also germane; have you had much success using humans in single-neuron monitoring experiments?

Aside from the trite dismissal of their methodology, you put the cart before the horse as regards males' propensity towards mating after time apart. That human males are as interested in non-procreative sex as they are is likely additional evidence for sperm competition, not a counter-argument; if the researchers are right, having frequent sex with a mate, even if you're not going to reproduce, is appropriate if your genitalia help prevent other sperm from fertilization.

The easy test of your dismissal would be to see how eager men are to have sex with a partner they have not seen in some time when they have been having a good deal of sex in the meantime; I would submit that they are still differentially eager.

Good luck getting that study past the review board, however.

Unfortunately, sperm competition as topic in human evolution is so old there was a BBC documentary featuring Desmond Morris (advocate for human pair-bonding... of ironies!) in the mid-90s; Geoffrey Parker laid a firm foundation for the thesis in 1970.

For some really good material, while not explicitly on sperm comptetition but in fact treating the sexual selection pressures that (speculatively) shaped the human penis (biggest among primates!), MUST READ Chapter 7 of Geoffrey Miller's The Mating Mind, in which he nearly argues that humans are exalted among species for the influence the female has wielded on the shaping of her mates' penises for the pleasure they may provide.

Anyway, in the spirit of science, I do sincerely hope you strive to falsify all such theses through your own observations and testing. Independence, peer review and reproduction of results should not be taken lightly as virtues.