The Misunderstood Penis
- 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.