Jesse Bering
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What a Plant Knows
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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 psychologist’—“hcollins2009”).
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 below.
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.
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33 Comments
Add Comment"READERS: If the penis really evolved to displace semen, then why wouldnt other promiscuous primate species, namely chimpanzees, have evolved similarly-designed penises with the coronal ridge?"
Reply | Report Abuse | Link to thisThis question reminds me - I once had a friend suggest that evolution isn't true because the example of a fish evolving in size to ward off predators would only lead to all fish doing the same, and we'd end up with a bunch of really large fish. He (and these readers) miss the point that (1) there are multiple solutions to a problem and (2) for evolution to use any one of them, it must randomly stumble upon them in the form of genetic mutations.
Another way to explain evolution in this sense is to give the example of "silent" rattlesnakes in Eastern Kentucky.
Reply | Report Abuse | Link to thisThere used to be a rumor that rattlesnakes were learning to not rattle and thusly were able to attack more easily but what the real case was that rattlesnakes that rattled loudly were being killed off by the tons so the only snakes that survived in the area were the ones with less tendency/ability to rattle. They in turn bred "silent" rattlers and so "natural selection" worked perfectly.
In the case of displacement, it(obviously) would have appeared randomly and then since the trait helped prevent other sperm from competing, the trait was passed on to the next generation. Each time it was passed on, it got to become even more dominant of a trait in the population(not dominant in the same sense as brown/blue eyes, etc.).
Seems pretty clear cut to me.
It's an interesting adaptationist explanation for the oddities of the human penis. I do wonder how the form of the organ and the behaviours associated with it get ganged together so selection can work on the genes involved. How do genes end up as behaviours linked to organs? The information storage in genes is pretty limited, so obviously a compression has occurred, but just how would make for a fascinating line of research.
Reply | Report Abuse | Link to thisI think that the zebra has a penis with a simalar adaptation or function.
Reply | Report Abuse | Link to thisThis is a pretty interesting argument, but can never be fully proven (like many evolution theories).
Reply | Report Abuse | Link to thisHow do we know that the sperm displacement isn't just a side effect of the penile adaptation for another reason?
Example: When a female has an orgasm, the cervix will "dip" into the seminal fluid deposited into the vagina. Perhaps larger coronal ridges provide more stimulation of the vagina and are therefore more likely to produce orgasm in the woman, cervical "dipping" and spread of that male's genetic material.
The expunging of a rival's seed could be secondary to that.
We will never truly know the reason for the penile changes, but other animals utilize penises for stimulation of ovulation of the female. Take cats for example. Their penises are actually barbed (in the unneutered male) and irritate the walls of the vagina of the queen, causing her to ovulate.
Obviously, human females are not induced ovulators, but the fact remains that we cannot overlook the woman's role in the selection of which penises "survive" and which fail to thrive.
Excellent. Thanks for answering those questions. The foreskin question was a real hole in the original article.
Reply | Report Abuse | Link to this"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"
Reply | Report Abuse | Link to thisPossibly, but you were saying that sperm competition *explains* why the human penis differs from other apes'. Clearly it doesn't, since all ape species face sperm competition. Random chance (pre-adaption) might explain it.
But in fact I doubt that too. Given that sperm competition and basic penis architecture are shared among all apes (and in fact all mammals), then what are the chances of only humans evolving this adaptation? Sure some human ancestor must've had a slightly more knobbly willy but then did no other ape ever have that? If it worked as billed, then it would be very unusual for only humans to have implemented it since it seems to be a fairly simple evolutionary path. As explanation, it faces a high improbability barrier.
There are a couple of other difficult problems this theory needs to overcome. Was multiple copulations with different partners in a 24 h period really so frequent, or is it more common for multiple copulations in this time period to be with the same partner? If the latter, then the 'sperm scoop' will impede fitness, not improve it.
Also, if the theory were true then you would expect after a separation period that first intercourse would be lengthy (to thorouhly remove any competitor sperm) and subsequent intercourse would be short and functional. In fact the opposite is usually the case (in my experience at least!).
The probability barrier would be reduced if there was something special about human sexual behaviour - and in fact there is something special about it! Human pair bonding is cognitively complex, and sensory stimulation (hyperstimulation of reward) plays a key part in it. Sex has an additional function in humans that separates us from other apes (even bonobos, where sex is a social tool but prolonged sex to promote specific pair bonds do not a feature).
Given the importance of sensory stimulation, you might expect the human penis to be large (for the benefit of the female), and the sensory area to be especially increased. Which is, of course, exactly what we see.
The sperm competition theory is plausible but there is no actual evidence that it has adaptive utility in the real world. And alternative theories exist that tie the unique human physiology to unique human behaviour, and so don't face such a large improbability barrier.
tomrees, good arguments, but the logic is flawed.
Reply | Report Abuse | Link to this"Was multiple copulations with different partners in a 24 h period really so frequent, or is it more common for multiple copulations in this time period to be with the same partner? If the latter, then the 'sperm scoop' will impede fitness, not improve it."
Incorrect. Scooping out your own sperm and replacing it with your own sperm often = no advantage, no disadvantage. Scooping out your competitor's sperm and replacing it with your own sperm occasionally = large advantage.
"Also, if the theory were true then you would expect after a separation period that first intercourse would be lengthy (to thorouhly remove any competitor sperm) and subsequent intercourse would be short and functional. In fact the opposite is usually the case (in my experience at least!)."
Men who are very confident and have sex often tend to have sex for long periods of time. Men who are less confident or who haven't had sex in a long time tend to ejaculate quickly. This makes sense, speaking from a selection standpoint.
Men who are confident are typically dominant men, secure in their woman / women and status. They can spend more time scooping out other men's sperm. This is also probably the reason women take time to orgasm -- it allows them a powerful bonding tool with powerful, dominant men (determined by the length of the sex, the skill of the lover, and the emotions he creates in her).
Men who are unconfident or haven't had sex in a while should follow the process of rapid ejaculation, for the reason that sex could easily be interrupted (by a more powerful, stronger competitor) and the best strategy is to quickly ejaculate and inseminate the woman.
"Given the importance of sensory stimulation, you might expect the human penis to be large (for the benefit of the female), and the sensory area to be especially increased. Which is, of course, exactly what we see."
Incorrect. The g-spot is located within the first third of the vagina. The back wall to the cervix IS the most sensitive part of the vagina and gives the most powerful orgasms, but a man does not need a particularly large penis to hit it in an average-length vagina.
"Scooping out your own sperm and replacing it with your own sperm often = no advantage, no disadvantage"
Reply | Report Abuse | Link to thisI think it is a disadvantage - because adding to the amount of sperm is better. Plus subsequent ejaculations carry fewer sperm.
Re: sex duration. I think the theory makes a clear prediction that it not fulfilled. Now, that might be because there are all sorts of competing pressures. But that doesn't change the fact that the prediction is not fulfilled.
Re: penis size. Not really my area of expertise :) but it's popularly considered to be important for a whole range of reasons (including sexual satisfaction). Whether it is or not in practise is irrelevant for the purposes of selection (perhaps the penis acts as peacocks tail!)
Just to reiterate: If a bigger penis is all it takes to decrease sperm competition then I find it very hard to understand why this feature is unique to humans. I've proposed one alternative explanation, there may well be others.
But the real point is that you can invent speculative explanations for penis size until the cows come home. The semen scoop hypothesis is one amnong many (possibly a partial but definitely an incomplete one, in my opinion)
While I can see that this is a well thought-out theory that very well may make sense, I'd like to offer another idea.
Reply | Report Abuse | Link to thisThe foreskin houses 20,000-70,000 specialized erogenous Meissner's corpuscles (nerve endings like those that make the fingertips so sensitive), which are housed in the ridged band (identified by Canadian pathologist, Dr. John Taylor), encircling the opening of the foreskin. These are the nerve endings that allow a man to know what his penis is feeling. They also provide the feedback to the brain that allow a male to ride the wave to orgasm. Without these "accelerator" nerve endings, a male is left with an "off-on" switch. The coronoa of the glans (a "dumb" organ by comparison) normally has what Ritter claimed are neurovascular end organs (bumps on the corona, the widest part of the glans) that stimulate the Meissner's corpuscles in the ridged band of the foreskin as it glides across them. The foreskin allows for smooth, easy, gentle penetration and also provides, along with the labia, the seal needed to keep in the "juices" of a lubricated sexual experience.
So, with penetration, the foreskin is stimulated by the glans, then the everted foreskin comes in contact with the vaginal walls and is stimulated by them and, upon withdrawal, the foreskin is stimulated by the glans again.
An intact male needs short strokes to stimulate himself, which keeps his body in contact with a woman's body, stimulating her to orgasm as well.
A circumcised male, on the other hand, needs long strokes for stimulation and drags a woman's lubrication from her body, causing her disconnect,discomfort, chafing, and pain with intercourse.
The number one sexual complaint of circumcised American males is premature ejaculation. No wonder, without all those specialized nerve endings, the circumcised male goes "Ooh, ooh, oops, honey! I'm sorry, it's because I'm so sensitive. It's not because he's so sensitive, its because he is missing the pleasurable sensation feedback to his brain!
The penis is a wondrous hydraulic mechanism that functions to create pleasure to both the male and his partner. Why? Because Mother Nature made no mistake with the organ of pleasure and procreation. She ensured survival of the species. A survival that is all the more pleasurable when the genitals are intact. Otherwise, it helps to understand that sexual mechanics are disturbed when genitals have been cut and movements during sexual relations must be adjusted to compensate for genital alteration.
There are a lot of comments here that presume that
Reply | Report Abuse | Link to thisa) the penis and vagina are and were and always will be in their current form,
b) an organ can have one and only one purpose.
These assumptions lead to some fascinating vitriol and strange arguments.
On the part of (a), I had a fascinating journey through the Museum of Sex in NYC recently. If I learned anything, it was that everything we think of as a human sexual oddity or behaviour is prevalent in our near relatives - this despite their dramatically alien sexual organs. This would indicate that there <i>just might be</i> more than one way to adapt to the same evolutionary pressure.
On the part of (b), just remember that this theory can be right - and <i>so can yours</i>. Foreskins may increase pleasure <i>and</i> aid in protecting the glans <i>and</i> be an aid to penetration. I can attest to the last two, but have no basis for comparison on the third. Our bodies have many examples of organs that serve many functions, including display, survival, amd reproduction.
Fascinating article with a fun tone - I laughed out loud more than once.
I really liked your thoughts. I think your argument precisely pinpoints the interesting part of science: looking at a phenomenon from all possible angles. Rivaling explanations are not only unavoidable but a must! I suppose that we can answer the question of whether female selection is an important factor of penis shape by testing if a certain shape (i.e. the one important for the semen displacement theory) actually increases the likelyhood of a female orgasm.
Reply | Report Abuse | Link to thisBeautiful Rina79. Just what I was thinking. The penis that can pleasure the female the most is also the penis most likely to pleasure the most females. Women talk. They have for a long time. Bingo! Semen displacement is surely a secondary function.
Reply | Report Abuse | Link to thisThis system is very similar in dragonflies and damselflies in which multiple males often mate with the same female. The female holds the sperm in a specialized organ that then allows for selection on the male genitalia to "scoop" out the sperm of a competitor. This has been extensively supported with experimental data and I think there is an excellent case to be made for human genitalia as well.
Reply | Report Abuse | Link to thisAmputating the foreskin is as disgusting as amputating the female clitoris. If a man wants it amputated, then he can do it on hes own accord when he is of legal age. Same goes for women.
Reply | Report Abuse | Link to thisAmputating the foreskin is as disgusting as amputating the female clitoris. If a man wants it amputated, then he can do it on hes own accord when he is of legal age. Same goes for women.
Reply | Report Abuse | Link to this"what are the chances of only humans evolving this adaptation?"
Reply | Report Abuse | Link to thisWhat are the chances that humans alone would develop advanced cognitive power? Or the odds that only a select few species would develop the use of sonar? Though many species may face the same obstacles (as in that all ape species face sperm competition) it does not follow that they should develop similar characteristics to overcome them. As stated in the article, there are many possible solutions to one problem, and evolution and natural selection are incredibly complex processes involving multiple variables at the level of the individual. There is no reason to suppose that an adaptation should occur in more than one species even if multiple species face similar (or in this case the same) barriers to fitness.
But on another note, excellent point about multiple copulations with the same partner over a given time period. And it is a very interesting question; would the potential occasionally of scooping out your own sperm and then replacing it be a disadvantage in light of the fact that you would also occasionally be scooping out a competitors sperm and replacing it with your own?
I'm sorry, but you don't know what the hell you're talking about with regard to the sensitivity comment. Unless you have a penis (considering your female username I'm assuming not) you don't know what areas are or aren't more or less sensitive, which is probably why you got it SO wrong. Foreskin REDUCES sensitivity. Foreskin is NOT necessary for pleasure. The "corona" is often (per other males AND myself) considered the most sensitive area and is left exposed with circumcision, increasing sensitivity. Premature ejaculation is NOT a problem for all of us [who are circumcised] and also affect uncircumcised men.
Reply | Report Abuse | Link to thisI'm sorry, but you don't know what the hell you're talking about with regard to the sensitivity comment. Unless you have a penis (considering your female username I'm assuming not) you don't know what areas are or aren't more or less sensitive, which is probably why you got it SO wrong. Foreskin REDUCES sensitivity. Foreskin is NOT necessary for pleasure. The "corona" is often (per other males AND myself) considered the most sensitive area and is left exposed with circumcision, increasing sensitivity. Premature ejaculation is NOT a problem for all of us [who are circumcised] and also affect uncircumcised men.
Reply | Report Abuse | Link to thisThere are some good points made here and what I believe to be a few false assumptions and contradicting arguments:
Reply | Report Abuse | Link to thisFirst on my mind is the assertion that If the scoop function was indeed one of the most successful traits that all primates or mammals would have it..
That is a false assertion for two big reasons, the first being that the trait has to come about as a random mutation over thousands of generations (man does not evolve perfectly over one generation). Its statistically unlikely that any more than one would develop that series of mutations to get the modern male penis.
Additionally, the gesture that the scooping function is arbitrary because humans are monogamous is a false assumption based on modern human culture (which is not even absolute). Naturally primates INCLUDING HUMANS are not truly monogamous.
As anyone but the odd high school honeys can attest, humans psychologically connect and copulate with multiple partners throughout their lives. That ladies and gents, is not culture but human nature. Do not take this as being religiously or legally polygamous.
third on list is that the male penis most likely evolved to function in conjunction with the human vagina, and while the core design of a retractable limb to implant semen into a receptacle remains almost universal between vertebrates, male/female reproductive structure differs significantly between genus and to a lesser degree species. Thus one could say that through evolution it is adapted to work best with the human vagina,
Basically the solution for humans may not be efficient enough for other primates even if they randomly stumbled upon the same mutation series that made the modern human penis.
And last but not least, the assertion that the human penis is adapted to stimulate females does not seem to have any natural benefit to the purpose of copulation. The male can impregnate the female even if she is never brought to orgasm.
As far as behavioral influences on the matter, males never had a need to be concerned about the sexual fulfillment of women because up until the last couple hundred years (not long enough to evolve) women never had a real choice as to who they would be paired with. Males chose their women in accordance with social standing and even bartered amongst men in trade. So in short modern motives for fulfilling women sexually cant be applied here.
I agree circumcision increases male sensitivity and makes for more efficient breeding. And the suggestion that dominant males have longer stamina, while unproven seems to be logical
That isnt evolution, it is teleological thinking...
Reply | Report Abuse | Link to thisActually, there is an alternative evolutionary hypothesis: The penile head is larger than the shaft is so that the male hand doesn't fall off during masturbation, or the female's hand during foreplay. Masturbation reduces prostate disease, and increases human health. Foreplay increases partners. Is this a testable hypothesis?
Reply | Report Abuse | Link to thisThe evolution of fatherhood might support sperm scooping. Are not human males the only primates that recognize their own children? If all males support the babies of the group's females, there is no need to remove "your brother's" sperm.
Reply | Report Abuse | Link to thisAs to the size of the phallus, the pleasure principle doesn't apply, neither for the male orgasm nor the female's (especially given that the clitoris is outside the vagina). The confidence factor is interesting but a subordinate male needs only the confidence of time, and dominate males are still famous for their brutish wham bam thankya mam insensitivity. As far as the female's timeline, several men will assure her enough time for her orgasm(s) and scooping or no scooping, plenty of sperm for conception, (the best things 'cum' in threes, sorry, couldn't resist the pun). Bonding time, especially for the more "emotional female", can be accomplished by the male by not withdrawing after orgasm, and since so many of us guys enjoy "going limp" even so far as snoring, not romantic, but back to plunge and cap, sperm swim and once past the cervix no amount of scooping matters. There will be no "smoking gun" or single explanation as to the evolution of the shape of the phallus, and/or the evolution of the sexual response in the human female. One thing IS for sure, you can't explain one without the other.
Random Mutations?
Reply | Report Abuse | Link to thisI may be misunderstanding this, but how will anybody believe that it can be "random" mutations.
Without doubt, most of these "random" mutations seem very designed, I do not know of anybody who will argue against it. You may do so about its implications, but the design is very clear to see.
Anyway, there are so many of these "random" mutations, and so many of them (I would estimate about 98%) seem designed that it is getting really hard to believe they are "random."
Does anybody agree?
Thank you.
"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..."
Reply | Report Abuse | Link to thisThe title of this article could not be more apropos because Gallup clearly misunderstands how a natural penis functions: the foreskin is mobile during coitus. It does not roll back and stay in one place. More correctly, the foreskin stays in one place relative to the female while the shaft and glans move within the foreskin and vagina.
If this hypothesis is to have any validity, it would have to be tested with healthy people who have had no genital modifications. Further, the experiment would have to control for variations in sexual behaviour related to social influence (as opposed to instinct).
The penis continues to be misunderstood.
Like others, my initial reaction to Gallup's elegant study is the circumcision problem. I find his response to this criticism surprising, that the displacement effect is accentuated with circumcision. Foreskin seems like such an easily modified structure evolutionarily, I find it hard to believe natural selection wouldn't have "found" the circumcised benefits long before we did. Moreover, isn't the elaborate foreskin another interesting difference between the penises of humans and other apes? If this is true, then an adaptionist explanation for foreskin would seem just as telling as corona size.
Reply | Report Abuse | Link to thisI think it is possible the foreskin acts as a collection device for the displaced semen following ejaculation. This also explains the "refractory period," which would shrink the size of the head, pushing back displaced semen into the "container" that is the enveloping foreskin. Loss of tumescence would therefore close the foreskin sack containing foreign sperm, and encourage withdrawal from the vagina to prevent foreign sperm from exiting the "foreskin sack." Therefore, in Gallup's experiment, I would expect more cuckoldry in men who are circumcised men than uncircumcised.
Your theory of evolution is flawed. On top of it, you look at something beautiful and purposeful as sex, and conclude that the penis evolved...you have to be an educated fool to buy into such belief. Why can't people open their eyes and remove the deception...The Almight God, King of kings and Lord of lords made us all in His Glorious Image. He is the Creator of the almighty penis
Reply | Report Abuse | Link to thisAnd here I thought I possessed the only penis on earth.
Reply | Report Abuse | Link to thisChimps males have several mates and do not have sex with all of them regularly so this penis displacing function would not be effective. It is effective only if the male has a good chance having sex with the female in a few hours after her unfaithfulness. Which works only if most males have only one attached female. This suggests that the human penis adapted to a society consisting of occasionally unfaithful married people. It would not work in promiscuous cases, it would not be needed if there were no unfaithfulness.
Reply | Report Abuse | Link to thisIt’s been awhile since I read the original article; however I think the factor that's missing here is the evolution of the vagina on the same time line.
Reply | Report Abuse | Link to thisMy hypothesis is that due to the increased social aspects of humans lead to the missionary position with emphasis on clitoral stimulation, and deep thrusting.
In contrast to doggy-style G-spot female stimulation (I've read only 1/3 of women have a G-Spot), an increased penis head would hit the spot more often.
It may be that men are behind the times on the evolutionary trail; if the penis evolved as a scoop I would think there would be a large difference in the size of the scoop as well.
Tests/surveys have shown that the larger the coronal ridge, the more uncomfortable sex is for the female. This is because the glans not only displaces semen, but it also displaces vaginal lubricants. In surveys women report that sex with an uncircumcised male is more pleasurable than sex with a circumcised male. Therefore, I believe that displacement would be a greater benefit than vaginal stimulation.
Reply | Report Abuse | Link to this"...you have to be an educated fool to buy into such belief. " yea you tell those stupid educated people truthseeker.
Reply | Report Abuse | Link to thisSemen displacement theory is amazing.
Reply | Report Abuse | Link to this