# Penis size and evolution
**URL:** https://boards.straightdope.com/t/penis-size-and-evolution/90728
**Category:** Great Debates
**Created:** [October 31, 2001, 4:13pm UTC](https://boards.straightdope.com/t/penis-size-and-evolution/90728 "2001-10-31T16:13:31Z")
**Posts on this page:** 20
**Page:** 1
### Author:  [@London\_Calling](https://boards.straightdope.com/u/London_Calling)
#### Post date: [October 31, 2001, 4:13pm UTC](https://boards.straightdope.com/t/penis-size-and-evolution/90728/1 "2001-10-31T16:13:31Z")
This cropped up a week or so ago (sorry Matron, it’s nature) as a passing comment by someone in another thread (IIRC, about the evolution of women in relation to birthing) and I can’t fathom it.
Human men have, I _think_ the largest club in the jungle. The observation was that this occurred because, until our species knew better, size was equated with ability to impregnate i.e. Big Boy got to perform often while little guy took a walk. Yes but…
Problem I can’t resolve is:
- If that is correct, why wouldn’t other mammals/animals think similarly (for example: Do other species think in terms of strength/speed/provider alone and why didn’t we) ?, and
- Is the thinking outlined above generally accepted by evolutionists ?
If so, could the answer be that this demonstrates we had the ability to behave _selfishly_ beyond that of other species – humans, perhaps, having a sense of survival through genes (the manifestation of fertility) _as well as_ via strength, etc., of partners) - but just happened to get that particular relationship (size = fertility) wrong ?
Yep, I’m confused about this.
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### Author:  [@Duck\_Duck\_Goose](https://boards.straightdope.com/u/Duck_Duck_Goose)
#### Post date: [October 31, 2001, 4:30pm UTC](https://boards.straightdope.com/t/penis-size-and-evolution/90728/2 "2001-10-31T16:30:35Z")
Well, just off the top of my head, 'cause I’m on my way out the door, but…
“They” tell us that males’ characteristics are a function of female choice. The peacock’s tail, etc. So I’m assuming that somewhere in the distant past, female hominids decided that a big club was more fun than a gorilla’s inch-long peepee.
Just read a Van De Waal book on chimps and hominids. Chimps have ENORMOUS testicles in relation to body size, humans have average. Chimps have HUGE sperm counts, humans have below-average–somebody somewhere once said, “If this human sperm sample were from a bull, the bull would have been put down as inferior breeding stock.”
OTOH, humans have ENORMOUS penises in relation to body size, chimps have average. So penis size is nothing to do with ability to impregnate, it’s solely for display.
And fun. Sorry, Matron. 😃
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### Author:  [@Mandelstam](https://boards.straightdope.com/u/Mandelstam)
#### Post date: [October 31, 2001, 4:38pm UTC](https://boards.straightdope.com/t/penis-size-and-evolution/90728/3 "2001-10-31T16:38:02Z")
_Human men have, I think the largest club in the jungle._
Not, IIRC, in proportion to their body size. That honor falls, I think, to some species of monkey. I have heard this discussed within a sperm competition framework. The considered group was, I recall, primates rather than the entire jungle. The well-endowed monkey was thought to be so b/c he ventured far from home and, therefore, couldn’t keep an eye on his female partner(s). The larger apparatus, in other words, would give each sex act a boost in terms of potential fertilization. The gorilla, I think, had the smallest ratio of any primate–presumably because he stays close to home and therefore doesn’t anticipate much cuckolding. Human men were said to be dead center between the two: implying that human women are less likely to stray than were those cheeky little female monkeys, but more likely to stray than the average gorilla-ess.
This all came from a documentary on UK Channel 4. I’d love to get a hold of a copy if anyone knows of it and knows how.
Aside: I’ve seen a male horse in the act of, um, perpetuating the species and its club was like nothing I’d ever seen in my neck of the jungle ;).
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### Author:  [@lieu](https://boards.straightdope.com/u/lieu)
#### Post date: [October 31, 2001, 4:42pm UTC](https://boards.straightdope.com/t/penis-size-and-evolution/90728/4 "2001-10-31T16:42:50Z")
And, therefore, we have “Hung like a horse”
instead of “Hung like a spider monkey.”
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### Author:  [@Corrvin](https://boards.straightdope.com/u/Corrvin)
#### Post date: [October 31, 2001, 5:02pm UTC](https://boards.straightdope.com/t/penis-size-and-evolution/90728/5 "2001-10-31T17:02:43Z")
Does it maybe have something to do with the ampleness of female buttocks? From what I understand it’s pretty usual for people to have sex with the man behind the woman (doggie style) and in that position, having a short penis means he’s more likely to fall out or not get his semen very far in.
Of course, this is all a theory based on my own experience; perhaps if men had smaller penises they’d be a bit more adept at steering them where they want them to go, and it wouldn’t be a problem. For the survival of the species, one would hope so…
Corr
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### Author:  [@tracer](https://boards.straightdope.com/u/tracer)
#### Post date: [October 31, 2001, 5:15pm UTC](https://boards.straightdope.com/t/penis-size-and-evolution/90728/6 "2001-10-31T17:15:33Z")
Duck Duck Goose wrote:
> [@](#):
>
> So penis size is nothing to do with ability to impregnate, it’s solely for display.
Actually, there **is** a reason why a large penis will tend to impregnate a human woman better than a small penis will.
Humans are a “soft plug” species. A man’s semen will coagulate inside the vagina and produce a soft plug against the cervix, a barrier to prevent **other** men’s sperm from getting through. A soft plug can last long enough that it can even prevent the **same** male from impregnating the female via a later ejaculation (if the first ejaculation happened while the female wasn’t ovulating). In soft-plug species, the favored form of sexual intercourse involves “pistoning” the penis in and out so as to create a low-pressure zone that **sucks** any other male’s soft-plug off of the female’s cervix.
But this **only works** if there is a **snug fit** between the penis and the vaginal walls. If there is an air gap, the low-pressure zone cannot be created, and any soft plug on the cervix will not be pulled off. Thus, the penis must be **thick enough** , and for that matter, **long enough** , to produce a snug fit that lasts across the entire length of a “piston” stroke.
Now, in other Great Ape species, this doesn’t require a very large penis, because their females’ birth canals are rather narrow. But human females have an **enormous** birth canal. They have to, in order to be able to give birth, because human babies have such large skulls. Thus, the human male’s penis has to be larger as well, in order to achieve a good sort-of-airtight seal with the female’s wider vagina during coitus.
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### Author:  [@jesse\_morrison](https://boards.straightdope.com/u/jesse_morrison)
#### Post date: [October 31, 2001, 11:22pm UTC](https://boards.straightdope.com/t/penis-size-and-evolution/90728/7 "2001-10-31T23:22:35Z")
Originally posted by tracer
Humans are a “soft plug” species. A man’s semen will coagulate inside the vagina and produce a soft plug against the cervix, a barrier to prevent other men’s sperm from getting through. A soft plug can last long enough that it can even prevent the same male from impregnating the female via a later ejaculation (if the first ejaculation happened while the female wasn’t ovulating). In soft-plug species, the favored form of sexual intercourse involves “pistoning” the penis in and out so as to create a low-pressure zone that sucks any other male’s soft-plug off of the female’s cervix.
But this only works if there is a snug fit between the penis and the vaginal walls. If there is an air gap, the low-pressure zone cannot be created, and any soft plug on the cervix will not be pulled off. Thus, the penis must be thick enough, and for that matter, long enough, to produce a snug fit that lasts across the entire length of a “piston” stroke.
* * *
Excellent post tracer,
Do you recall if there is a minimum number of piston strokes for removing the soft plug from the cervix?
I’m not sure how the low-pressure zone is created by pistoning.
Here is how I handle my sperm competitors:
1. I do what ever it takes to get the female vaginally lubricated.
2. I do what ever it takes to get my piston erect.
3. I piston the female until I achieve full penetration.
4. During my pistoning I remove my piston completely, leaving the vagina open temporarily. I am guessing that at this point in time that outside air will enter the vaginal canal. The thicker the piston, the more air that can enter the vaginal canal (piston cylinder).
5. Repeatng the above for several repetitions and sets, I am guessing that my piston is compressing the air in the piston cylinder.
6. After a certain number of repetitions and sets, the air pressure inside the cylinder will build up sufficient pressure to expell the compressed air, piston, and soft plug.
7. At this point I make a decision as to whether I wish to have my piston deliver sperm on the cervix.
8. If my sex partner tells me that I am a piston loser, in comparison to my sperm competitors, then I do not ejaculate. If my sex partner does not give me any statistical numbers about my competitors, then I play the odds and ejaculate, since my piston is above equal.
tracer,
I have stolen your “pistoning” word, and have placed it on various clothing, for resale.
jesse
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### Author:  [@London\_Calling](https://boards.straightdope.com/u/London_Calling)
#### Post date: [November 1, 2001, 2:28pm UTC](https://boards.straightdope.com/t/penis-size-and-evolution/90728/8 "2001-11-01T14:28:39Z")
Another thing that struck me is how humans have grown over the past couple of hundred years (diet). If the club’s relatively big now, what the heck was it when we were 5’ and 150lb’s ? – it makes the contrast with other apes even more stark. Anyhoo…
**Goosey** : “Male characteristics are a function of female choice” - that’s a beautifully concise phrase, isn’t it ?
Now I can see that in a modern context but what if this particular evolution dates back to our knuckle-dragging era. What I mean is: If me look at, for example, much of the ape species, selection doesn’t work in that way because the biggest and strongest ape services the females of the troop i.e. a near absence of female choice (unless my understanding is wrong). Perhaps something kicked in as we got into the bipod thing ?
**Mandelstam** – Wish I’d seen that particular documentary. Do you recall a name ?
The On-The-Road Penis Theory makes sense, to me, if a bigger club increases the chance of fertilisation. My understanding is that it’s a question of sperm count but maybe it’s different for the little monkey’s ? Sounds like a good arrangement, though. Big club, do the job and absent yourself from the nagging.
**Corrvin** – a lot of things “have something to do with the ampleness of female buttocks” but I’m not sure this is one of them. I understand where you’re coming from…okay, I’ll stop now.
**tracer** – that is interesting. Big Boy can suck any prior indiscretion off the cervix and then record a bulls-eye but little guy can’t. It makes sense (I assume you’re right about the process) but it’s only relevant, if I understand you correctly, if the female is a bit of a Dockyard Doris i.e. she’s been around the block. So I guess I come back to (as per Goosie, above): When did we pair off rather than do the dominant ape/troop thing ?
**jesse morrison** – I think it’s all very well marching to the beat of a different drummer but I’m not sure you’re quite in step with yourself, let alone anyone else.
Unfortunately, I’m still having trouble getting my head around this (said the actress to the Bishop). It _seems_ like the Chimps (with their large testes) got it right and we didn’t - although **tracer’s** Suction Theory does counter that (but it assumes the females did put themselves around a bit - as if 🙂 )…?
Thanks for all your responses.
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### Author:  [@IzzyR](https://boards.straightdope.com/u/IzzyR)
#### Post date: [November 1, 2001, 5:08pm UTC](https://boards.straightdope.com/t/penis-size-and-evolution/90728/9 "2001-11-01T17:08:46Z")
> [@](#):
>
> \*Originally posted by London\_Calling \*
> Problem I can’t resolve is:
>
> - If that is correct, why wouldn’t other mammals/animals think similarly (for example: Do other species think in terms of strength/speed/provider alone and why didn’t we) ?
This seems like a funadamental mistake. No animal makes a concious decision to do anything in the belief that it will increase their offspring. A characteristic that offers an actual survival or reproduction will tend to increase the proportion of it’s bearers in the population. But if the advantage is not actually there the proportion will remain constant. Humans, by contrast, can make decisions, which may be wrong.
(This is not to say that I accept the theory to begin with.)
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### Author:  [@tracer](https://boards.straightdope.com/u/tracer)
#### Post date: [November 1, 2001, 9:36pm UTC](https://boards.straightdope.com/t/penis-size-and-evolution/90728/10 "2001-11-01T21:36:37Z")
London\_Calling wrote:
> [@](#):
>
> **tracer** – that is interesting. Big Boy can suck any prior indiscretion off the cervix and then record a bulls-eye but little guy can’t. It makes sense (I assume you’re right about the process) but it’s only relevant, if I understand you correctly, if the female is a bit of a Dockyard Doris i.e. she’s been around the block. So I guess I come back to (as per Goosie, above): When did we pair off rather than do the dominant ape/troop thing ?
>
> […] It _seems_ like the Chimps (with their large testes) got it right and we didn’t - although **tracer’s** Suction Theory does counter that (but it assumes the females did put themselves around a bit - as if 🙂 )…?
You are assuming that the pair-bond a female feels for her mate is sufficient to stop **all** female promiscuity. It is not – neither in our own species, or in other pair-bonding species.
For example, chickadees (the little birds) pair-bond like humans do. The male stays around the nest to help take care of his mate’s eggs. This is advantageous to the female’s genes in all circumstances – the female **knows** that she has made a genetic contribution to the eggs she laid – but is only advantageous to the male’s genes if he is indeed the eggs’ father. As it turns out, around 50 percent of chickadee eggs share **no** genetic material with their male nest-keeper – the real biological father is one of those stud-muffin male chickadees down the street. The female “cheats” on her nest-mate by getting pregnant from a different male. If her nest-mate finds out about this, he abandons her and her eggs, so it is in the female’s best interest to hide any “cheating” she has done from her nest-mate.
Evidence that monogamy is **not** absolute in humans comes primarily from the size of men’s testes. Sure, we don’t have the grapefruit-sized sperm factories of the chimp or the bonobo, but our testes **are** considerably larger than those of a gorilla. (Gorillas are an alpha male society, in which the strongest male, the “alpha male,” gets all the chicks. Presumably, the strongest male also has the best genes, and so is also the most attractive mate for the females. Female gorillas rarely do the horizontal lambada with male gorillas other than the alpha male.)
Plus, consider the mating behavior of an adolescent girl. She usually has the urge to “try out” many of the attractive young men before she “settles down” with one long-term mate. In the wild, where every mating can mean a pregnancy, the males that she has “tried out” which can pull off the other males’ soft plugs most effectively and which have the most aggressive and numerous sperm will have the best chance of impregnating her. BUT, under that circumstance, since no one knows who the father is, no one is going to want to help take care of her baby with her and she will most likely be “shunned” as one of those good-for-nothing single moms. (Ow! Stop hitting!) Even so, the
baby **can** still survive. The pre-civilization human population would have been a constant mix of such “bastard children” with good genes but only one parent to support them, and the “nuclear family kids” with so-so genes but two parents to support them.
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### Author:  [@dude](https://boards.straightdope.com/u/dude)
#### Post date: [November 2, 2001, 7:59am UTC](https://boards.straightdope.com/t/penis-size-and-evolution/90728/11 "2001-11-02T07:59:07Z")
A big errect penis is a sign of good health and hence their will be an advantage for females who prefer ‘big ones’.
Women also like tall and strong men so that leads to a link with big penis size.
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### Author:  [@jesse\_morrison](https://boards.straightdope.com/u/jesse_morrison)
#### Post date: [November 2, 2001, 8:56am UTC](https://boards.straightdope.com/t/penis-size-and-evolution/90728/12 "2001-11-02T08:56:24Z")
I read about some recent research, where the cheating human female mate, is more likely to cheat during her fertile part of the month. I think that the theory is that she is looking for sexy genes (sexy son theory). Sexy sons are more likely to spread sperm to more females, thus giving the cheating wife more grandchildren, thus placing more of the cheating wife’s genes in the gene pool.
The cheating wife marries the fat cat, gets pregnant by the sexy cat and the world is better off thru eugenics.
Whether these sexy cats have longer, thicker pistons in comparison to the fat cats would be interesting research, maybe it has already been measured.
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### Author:  [@MandaJo](https://boards.straightdope.com/u/MandaJo)
#### Post date: [November 2, 2001, 11:57am UTC](https://boards.straightdope.com/t/penis-size-and-evolution/90728/13 "2001-11-02T11:57:40Z")
London Calling, I think it was my post that inspired the OP. I am about to walk out the door, but the article I read that inspired that post can be found at Discover Magazine. You can’t link directly to the article, but if you go to the index here: [http://www.discover.com/archive/index.html](http://www.discover.com/archive/index.html) , and plug in “The Best Ways to Sell Sex” by Jared Diamond (of _Guns, Germs, and Steel_ fame) you will find the article.
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### Author:  [@Yumanite](https://boards.straightdope.com/u/Yumanite)
#### Post date: [November 2, 2001, 3:54pm UTC](https://boards.straightdope.com/t/penis-size-and-evolution/90728/14 "2001-11-02T15:54:49Z")
> [@](#):
>
> The well-endowed monkey was
> thought to be so b/c he ventured far from home and, therefore, couldn’t keep an eye on his female partner(s). The larger apparatus, in
> other words, would give each sex act a boost in terms of potential fertilization. The gorilla, I think, had the smallest ratio of any
> primate–presumably because he stays close to home and therefore doesn’t anticipate much cuckolding.
Let’s be careful not to get _penis_ size confused with _testicular_ size. Testicle size has to do directly with sperm competition, and I think that is what the above paragraph is about.
Also, let’s consider that large penis size does not have to be an adaption. It could be the corresponding physiological change to large birth canals in women. It would be fairly easy for a large vaginal size phenotype to come from a more generic genotype, where, say, a fetus that has just started forming sexual organ tissues (but has not become physiologically male or female yet) allocated a bit more “down there”. If a person with this trait passed it along to her offspring, both males and females would be affected.
IANA developmental biologist, so I don’t really know how plausible that exact scenario is, but I mean it as an example of the fact that large penis size does not necessarily need to be actively selected for to develop.
PS: If my scenario is reasonable, it means that humans females would have larger clitorises (clitori? How’s about just clits) than any other primate. Is this the case?
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### Author:  [@The\_Flying\_Dutchman](https://boards.straightdope.com/u/The_Flying_Dutchman)
#### Post date: [November 3, 2001, 12:00am UTC](https://boards.straightdope.com/t/penis-size-and-evolution/90728/15 "2001-11-03T00:00:13Z")
In mammals , the diameter of the tumescent penis is correlated to the size of the species head, and the length of the erect penis correlates to the length of the trunk of the female.
On the first point I would suggest that the size of the fetal head, particularly large in the human species when compared to apes influences (in an evolutionary context) the diameter of the vaginal canal. The size of the vaginal canal subsequently influences the diameter (in an evolutionary context) of the penis.
Now these hypotheses are just that. They are based on limited observations in the animal kingdom, Whale, Elephant, horse, human, dog.
What really sets the human penis apart however is the lack of a baculum. I think all the other mammals have one.
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### Author:  [@tracer](https://boards.straightdope.com/u/tracer)
#### Post date: [November 3, 2001, 6:10am UTC](https://boards.straightdope.com/t/penis-size-and-evolution/90728/16 "2001-11-03T06:10:14Z")
Bacu-wha?
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### Author:  [@Ice\_Wolf](https://boards.straightdope.com/u/Ice_Wolf)
#### Post date: [November 3, 2001, 12:05pm UTC](https://boards.straightdope.com/t/penis-size-and-evolution/90728/17 "2001-11-03T12:05:13Z")
From [this site](http://www.skullsunlimited.com/reprotuctive-tools.htm):
> [@](#):
>
> Baculum or os penis, is the term used to describe a rather odd bone in mammalian anatomy. These unusual bones are found within many mammalian penises and give them rigidness to aid in copulation.
I await the continuation of this thread with interest.
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### Author:  [@Icerigger](https://boards.straightdope.com/u/Icerigger)
#### Post date: [November 3, 2001, 12:31pm UTC](https://boards.straightdope.com/t/penis-size-and-evolution/90728/18 "2001-11-03T12:31:20Z")
I would just like to add that the woman can control this process as well. If a woman has an orgasm the uterus has contractions and the cervix can disrupt the soft plug and dip into the sperm of her current lover. Since a woman’s mood has a great deal to do with her ability to climax she can lessen her mates chances of fathering a child by not having an orgasm and increase her lovers chances by liking the guy more and having an orgasm to help his sperm. The endless battle of offense and defense, move and counter move. Life is complicated.
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### Author:  [@elucidator](https://boards.straightdope.com/u/elucidator)
#### Post date: [November 3, 2001, 3:31pm UTC](https://boards.straightdope.com/t/penis-size-and-evolution/90728/19 "2001-11-03T15:31:57Z")
I must strongly protest the implication that penis size is an indicator of “evolutionary progress”, as if those of us who are donkey-morphic are somehow primitive throwbacks, and inferior. Note the common belief that this is a characteristic of black men, and the rather suspisious description “hung”.
All we are asking is fair treatment, and pants that fit without cutting off our blood circulation. And perhaps some understanding and compassion, so young women don’t run screaming from the room at the sight of “Moby Dick”.
Is that too much to ask? Is it?
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### Author:  [@London\_Calling](https://boards.straightdope.com/u/London_Calling)
#### Post date: [November 3, 2001, 3:48pm UTC](https://boards.straightdope.com/t/penis-size-and-evolution/90728/20 "2001-11-03T15:48:31Z")
> [@](#):
>
> \*Originally posted by Duck Duck Goose \*
> Well, just off the top of my head, 'cause I’m on my way out the door, but…
> [@](#):
>
> \*Originally posted by Manda OJ \*
> I am about to walk out the door,
Where you all going ??
Okay:
Well, at the moment we seem to be adopting more positions than does Joan Collins after a couple o’ bottles of bubbly.
> [@](#):
>
> \*Originally posted by IzzyR \*
> A characteristic that offers an actual survival or reproduction will tend to increase the proportion of it’s bearers in the population.
**Izzy** – If I read you correctly, your position is at odds with (what I perceive to be) the position of Jared Diamond (see below).
\*\* dude\*\* – Bingo, seems you’re on the Jared Diamond track. Look at the three theories summarised below.
**jesse morrison** – I also think Jared Diamond below develops what you suggest.
**Yumanite** – If you’re saying a female with a deep vagina can, genetically, interpret that depth into a larger penis for her male off-spring (are you ?), then I think we’d have some readily obtainable physical research ? Not sure about that one.
> [@](#):
>
> \*Originally posted by elucidator \*
> And perhaps some understanding and compassion, so young women don’t run screaming from the room at the sight of “Moby Dick”.
I can only empathise. It is, indeed, most distressing.
For convenience, I’ve divided the following into two posts, each post reflecting different _primary_ reasoning (it helped me, anyway). They are:
(Primarily) **Choice**
(Primarily) **Physiological**
– I hope that convenient categorisation is reasonably accurate. Apologies if I misunderstood some posters.
[Next page](https://boards.straightdope.com/t/penis-size-and-evolution/90728.md?page=2)