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The Misunderstood Penis

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    April 30, 2009 The Misunderstood Penis Gordon Gallup sets the record straight on the semen displacement theory By Jesse Bering So, it seems people have some
    Message 1 of 1 , May 4, 2009
      April 30, 2009
      The Misunderstood Penis
      Gordon Gallup sets the record straight on the semen displacement theory

      By Jesse Bering

      So, it seems people have some pretty strong feelings about penises.
      Reactions to my last post—“Secrets of the Phallus: Why Does the Penis
      Look Like That?”— ranged from the incredulous (are you seriously
      suggesting that chimpanzees aren’t promiscuous?—“tomrees”), to the
      imaginative (penises! they're so cute, you just want to pinch their
      cheeks and give them cookies—“montavilla”), to the rather irritable
      (stupid, biased thinking again from an ‘evolutionary

      For some reason, when it comes to asking whether human beings have
      evolved some specialized trait over the past several million years
      I’ve found that people tend to get weirdly worked up about it. I once
      had the most unpleasant conversation with a very unlikable
      ornithologist while dining at a Japanese steakhouse in Binghamton. I
      think I was a hairsbreadth away from being strangled by this
      scientist who took considerable offense to my suggestion that magpie
      intelligence isn’t as humanlike as it may appear. But the subject of
      human penis evolution appears to have touched a special nerve.

      Therefore, I decided to speak with evolutionary psychologist Gordon
      Gallup of the State University of New York at Albany directly, whose
      controversial “semen displacement theory,” after all, was the one
      that struck up such a fierce (and I must say, entertaining) brouhaha
      regarding the adaptive functioning of this enigmatic organ. I
      explained to him that there appeared to be a bit of confusion in the
      reading audience concerning some of the central evolutionary tenets
      of his position, and perhaps he might offer us a few more details
      regarding the theory to lay any recurring misunderstandings to rest.

      In looking over the varied responses to the earlier post here at
      Scientific American and elsewhere on the Internet, Gordon and I
      noticed several conceptually flawed themes cropping up in people’s
      interpretations of his argument. Since it would be impossible for him
      to address every rejoinder to his semen displacement theory (and,
      frankly, some of them were so bizarre that I couldn’t make much sense
      of them anyway), I’ve translated a handful of these “core” questions

      READERS: The latex genitalia study wasn't terribly convincing because
      the models were circumcised, and in real life the foreskin would
      interfere with the semen-displacing functions of the coronal ridge.
      So, does the foreskin pose a problem for the semen displacement theory?

      GALLUP: The length of the foreskin is one of the most variable
      features of the human penis. When most uncircumcised males achieve an
      erection it pulls the foreskin back over the glans and back down the
      shaft of the penis, enabling the coronal ridge to do its business and
      scoop rival males’ semen away from the woman’s cervix. Because
      circumcision reduces the diameter of the shaft immediately behind the
      glans and accentuates the coronal ridge, we’ve speculated that the
      practice of circumcision may have unwittingly modified the penis in
      ways that enable it to function as a more effective semen
      displacement device. Armchair speculation? No. The idea could be
      tested by comparing the incidence of non-paternity between
      circumcised and intact males. My prediction would be that circumcised
      males ought to experience a lower incidence of being cuckolded.

      READERS: So why did human penises evolve to have foreskin at all then?

      GALLUP: Evolution does not occur by design. The best way to think
      about most adaptations is in terms of cost/benefit ratios. I suspect
      that the foreskin provided protection of the glans and what you see
      is the result of a statistical compromise of sorts.

      READERS: If the penis really evolved to displace semen, then why
      wouldn’t other promiscuous primate species, namely chimpanzees, have
      evolved similarly-designed penises with the coronal ridge?

      GALLUP: Again, evolution doesn’t occur by design. It occurs by
      selection, and the raw material for such selection consists of
      nothing more than random genetic accidents (mutations). Embedded in
      the evolutionary history of human genital design were some penis
      shape mutations, not present in other species, that led to a device
      that could be used to compete with other males for paternity. Other
      promiscuous primates such as chimpanzees have solved the problem
      through sperm competition. Male chimpanzees have testicles that are
      three times the size of humans and differences in sperm count are on
      the same order of magnitude. Chimpanzees compete among one another
      for paternity by leaving the largest and most potent volume of semen
      in the female reproductive tract. When it comes to selection based on
      genetic accidents, there are a number of ways to skin the adaptive cat.

      Bering here. I should say in closing that those who’ve been
      intellectually conditioned to recoil in disagreement at the mere
      mention of evolutionary explanations of human behavior are likely to
      scream “Just-So Story!” no matter how convincing the argument and
      well-supported it is by empirical evidence. Some evolutionary
      theories indeed leave a lot to be desired. But, in the present case,
      the semen displacement theory just so happens to make a lot of
      adaptationist sense—and I suspect that once you know what you’re
      dealing with down there, you’ll probably never look at a penis quite
      the same way again.
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