PZ Myers. 2005 Feb 26. Penis evolution. <http://pharyngula.org/index/weblog/penis_evolution/>. Accessed 2008 Dec 01.
Posted on M00o93H7pQ09L8X1t49cHY01Z5j4TT91fGfr on Saturday, February 26, 2005
Penis evolution
After the recent struggles trying to keep up with the traffic on this site, you wouldn't think I'd feel compelled to go trolling for more visitors, but isn't that the nature of weblogging? The only point to it all is to rack up a bigger score than the next guy, as if we were playing pinball. So what's a good ploy? As Lauren has cleverly pointed out, sex sells. And while it may be estrogen week, I'm going to buck the trend, since we all know what's really important for weblog popularity: penises.
So I was just browsing through some fun journals (Integrative and Comparative Biology, always good for some unusual stuff) and ran across a paper with a wonderful title: "The Functional Morphology of Penile Erection: Tissue Designs for Increasing and Maintaining Stiffness." If that kind of thing will sell commercial air time during the Super Bowl, it's got to be popular.
As is typical, though, while the promise of salaciousness drew me in, it was the science and the evolutionary story that kept me interested. Amniote penises have had a complex history. They have evolved independently multiple times, and perhaps most troubling to the male ego, they have been secondarily lost at least a few times. And every time they have evolved, they converge on a remarkably similar morphological solution.
That last observation is perhaps not very surprising. The penis has a very simple job to do: to maintain sufficient stiffness to enter an orifice in the mate, and to deliver sperm. That's it. Every amniote settles on a similar solution, forming a hydrostat, a tube containing a pressurized incompressible fluid surrounded by a membrane under tension. It's a kind of glorified water balloon.
Where different amniote lineages differ is in how they build their penis. Mammals have a medial penis containing two inflatable, vascular erectile bodies, and the tissue used for it embryonically is gathered from non-cloacal epithelia and connective tissue. Crocodile and turtle penises contain a single erectile body, and they form their penises from tissues on the ventral cloacal wall. Squamates (lizards and snakes) have paired penises built from lateral cloacal wall tissue. Birds, like crocodiles, assemble a penis (when they have one) from the ventral wall of the cloaca, but they use the lymphatic system as a pump, rather than the blood system.
In these cross-sections through the penises of a turtle, bird, mammal, and snake, you can see that while each is different in its organization, all contain the same kind of functional core: a vascular space (VS) surrounded by a tensile membrane (TM).
Diagrams illustrating transverse sections of amniote intromittent organs. A turtle penis is at the upper left, a bird penis is at the upper right, a mammalian penis is at the lower left, and a snake hemipene is at the lower right. All the structures are hydrostatic: each contains a central vascular space (VS) and surrounding tensile membrane (TM) characteristic of hydrostats.
Similar in function, but different in embryonic origin—this all suggests that these organs evolved independently. There are multiple hypotheses about the exact order and pattern of descent of the penis—the diagrams below illustrate two—but one of the striking things about this pattern is how lineages, such as the birds, can so blithely lose their intromittent organ. Most birds lack penises altogether, and they are found mainly in ratites and ducks, yet both of the cladograms below show that the ancestral bird most likely had one. Think about that… it hasn't been at all uncommon for female vertebrates to be untroubled by the absence of a penis in their mates, and apparently have preferred it that way.
Phylogeny of extant amniotes illustrating alternate hypotheses for the distribution of intromittent organs in amniotes. A. The penis is an amniote synapomorphy. B. The penis as a convergent trait. The position of the Testudines is disputed; alternate hypotheses increase the possible number of independent penile origins, as does the patchy distribution of the organ within Aves.
This paper presents the evolutionary history of the penis as background to its primary focus, which is on the bioengineering of a rigid hydrostat. All of the organisms converge on an exceedingly similar solution. The tensile membrane of the hydrostat consists of alternating layers of collagen fibers, each oriented at 90° to one another. This is the same principle used to give plywood its rigidity, by having fibers oriented to oppose bending along their least extensible axis at every angle. In addition, the fibers in the rest state are crimped, allowing them to stretch during extension, but then become straight and resistent to further extension when the organ is fully inflated.
Diagram of collagen fiber arrangement in the wall tissue of a flaccid mammalian penis. There are two layers forming an axial orthogonal array: an outer layer with fibers at 0° to the long axis of the penis and an inner layer with fibers at 90° to the long axis. Collagen fibers are highly crimped in the flaccid penis, but straighten upon erection.
The paper also describes some experimental measures of stiffness. The author extracts the penis of the nine-banded armadillo, and can then pump it up by inflating it with known volumes of fluid, whele measuring its resistance to bending (reported as values of E, or Young's modulus of elasticity.) I confess to having the odd thought that locker-room bragging ought to supplement reports of inches/centimeters of length with E values in 10-5Nm2. Imagine all the insecure jocks reporting to their local physiologist for a three-point bending test!
Change in the average flexural stiffness of mammalian penile erectile tissue (corpus cavernosum) during inflation, as measured by three-point bending tests in the nine-banded armadillo (Dasypus novemcinctus). Flexural stiffness increases as internal volume increases, and is highest when the corpus cavernosum reaches maximum volume. There is not a statistically significant difference in the flexural stiffness of the corpus cavernosum when the structure is bent laterally or dorsoventrally (n=4; F-ratio=2.70; 0.05 < P < 0.1).
The author concludes from the degree of convergence seen in the species studied that there must be adaptive significance to the arrangement.
What, then, can we conclude if we observe convergence at more than one anatomical or functional level? Multiple levels of convergence could imply that there are more constraints on the system—that there are fewer possible anatomical designs that successfully meet the selective regime. Therefore, if there is only one way to solve the problem imposed by the selective regime, we will see convergence at more levels than if many equally successful anatomies can evolve.
If this hypothesis is true, the evidence from mammals and turtles suggests that the amniotes that have evolved inflatable penises have been subjected to an extremely restrictive selective regime. Penile convergence in mammals and turtles does not stop at gross functional similarity; they have converged on a single anatomical design down to the level of specific collagen fiber arrangements. The differences in penile collagen fiber layering that exist between mammals and turtles do not, as of yet, seem to have any functional effect on penile stiffness. It may be that the way the axial orthogonal array is put together is less critical to the problem of increasing penile flexural stiffness than the presence of the array itself.
I agree in part, but it also seems to me that contingency is equally significant. The reason that all of these animals have converged on the same solution is that they've begun with similar raw materials: a high-pressure circulatory system and a tissue bed rich in the protein collagen.
There was no mention of the adaptive significance of this organ to weblogging or to administering Harvard, to my disappointment.
Kelly DA (2002) The Functional Morphology of Penile Erection: Tissue Designs for Increasing and Maintaining Stiffness. Integ. and Comp. Biol. 42:216–221.
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An excellent and interesting post. Of course an aside about bacula would have afforded your commenters the opportunity to compete for the best Star Trek joke.
According to some proposed arrangements, ratites and anatids are part of a clade that also includes the galliformes (chickens and turkeys and a bunch of others) and tinamiformes. It seems at least some tinamous have penises as well. Chickens and turkeys and (I suspect citelessly) all other galliformes do not. If the cladistic arrangement I've seen is a good one, that means birds have evolved dicklessness at least twice.#: Posted by Chris Clarke on 02/26 at 11:57 AM - Fascinating! Fascinating! You will crash your server again.
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The Amazing Disappearing Penis? You should send this around to some paper publishers -- it's definitely magazine material! ^_^
#: Posted by on 02/26 at 12:43 PM
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So these orthogonally layered collagen fibres, I assume that none of the magic penis extension methods advertised all over the place can actually do anything to increase them or change them in any way to actually make your penis longer?
#: Posted by on 02/26 at 01:05 PM
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"...it hasn't been at all uncommon for female vertebrates to be untroubled by the absence of a penis in their mates, and apparently have preferred it that way."
I don't know if I should be afraid, amused or impressed! Interesting stuff, and I am sure that any rightwingers from powerline that were still lurking about left when they saw the title.#: Posted by on 02/26 at 01:32 PM - Penis evo eh? You are a glutton for punishment brother PZ.
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Identically functioning penis evolved independantly many times? Well this explains how Captain Kirk was able to boff so many alien women.
#: Posted by on 02/26 at 03:50 PM
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Uh...then how do penisless birds have sex, anyways? Is it just messy, or what?
#: Posted by on 02/26 at 05:03 PM
- They rub cloacas together in a beautiful act of avian affection.
- In some birds, the male cloaca produces copious quantities of foam (known around the labs afectionatelly as "Cloaka-Cola") that apparently serves to help the semen pass quickly from his cloaca to her cloaca. Many birds can store semen for a number of days and pick whose semen to use to fertilize her eggs.
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I can't believe I have a serious question to contribute to this.
How common are penis bones and how do they fit into the evolutionary scheme. I know many mammals have penis bones, especially carnivores. The penis bone is kept in the abdomen and, when needed, a set of muscles push it into a sheath in the fleshy part of the penis. I asssume the adaptation is for speed. Sliding a bone into and out of a sheath is much faster than waiting for the hydraulics to kick in and limits the time spent mating, which is, after all, vulnerable position.
I have a small (about 20cm long) oosik (walrus or seal penis) here on my desk, but I have seen some from old walruses that are as big as a human femur. Give me a few minutes and I'll post a picture of my oosik over at <a>archy</a>.#: Posted by John McKay on 02/26 at 06:32 PM -
Oh, so 20cm is small, is it? You're not fooling anyone, you know....
#: Posted by on 02/26 at 07:34 PM
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Speaking of dicks, the NYT education section has a 2-page story on Lawrence Summers. It's sure to elicit groans aplenty. (And not the good kind.)
#: Posted by on 02/26 at 09:27 PM
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Ugh, "groans" and "penis" in the same sentence?
http://sciencepolitics.blogspot.com/2005/02/size-does-matter-or-does-it.html -
I blogged on rodent penises a while back. The baculum has evolved (apparently) independently multiple times. Several species of chipmunk are principally distinguished based on phallic morphology. One theory I've seen is that, rather than speed, the baculum improves sperm transfer (it extends beyond the phallus in the rodents I've seen). It also probably makes it easier for the male to maintain reproductive lock and prevent other males from mating with the same female.
The things we study. -
Ugh, "groans" and "penis" in the same sentence?
Where? Where?#: Posted by on 02/26 at 11:57 PM -
Giant male squids seem to have evolved penises modeled on the hypodermic needle (and up to 1.5 long) rather than on the inflatable water bag. I understand they use it to inject goodlooking passing females. This organ may have adaptive weblogging significance...
#: Posted by on 02/27 at 12:39 AM
- Speaking of losing one's penis, in pulmonate land snails, which are normally hermaphrodites, it is not unusual to have species, & even genera, in which most or all individuals are aphallic, that is, without a penis. Such individuals reproduce either by selfing (if they can still produce semen) or by parthenogenesis. It's been claimed that aphally is more common in the smallest snails (1-2 mm) for economical reasons: whatever doesn't fit inside the tiny body goes if it's not absolutely necessary. Alas, penis seems to be such an organ.
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I've been reading your blog a couple weeks and really enjoy it. Thanks for your (work?); it's refreshing. This read was certainly educational.
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We forget that the mammal penis fulfils a dual function: to transfer sperm to the female, and to drain liquid waste. I cannot imagine a worse design than a penis, as an engineer, I think that each function should have a specialized organ. Is there any animal with two penises, one for sex and another for wastewater removal? Someone should proclaim an open competition for the design of a better human body, the same as architects and engineers submit competing designs for bridges and other structures.
#: Posted by on 02/27 at 10:43 AM
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Jaimito, I think you mean "only a civil engineer would have run a waste disposal pipeline through the middle of a recreational facility."
#: Posted by Chris Clarke on 02/27 at 10:57 AM
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Only a civil engineer would have run a waste disposal pipeline through the middle of a recreational facility.
Yo, jaimito/Chris. Don't disrespect the mysterious Designing prowess of the G-O-D.
Apparently, He [sic] actually wanted it for human males to suffer when they happen to dispose of liquid waste shortly after they ejaculate.
That sly cat... Always testing us - and, uh, whatnot.#: Posted by on 02/27 at 07:28 PM -
As a Biomedical Engineering student, this post was right up my alley (hyuk) . My tissue engineering professor hasn't mentioned the collagen in penii yet, but he did mention that it is used in arteries for the same purpose. Now I'm wondering whether we will find any GAGs in the penis to withstand that pressure. GAG is short for glycosaminoglycan, which is a sugar that holds water and is a key component of many parts of our tissues. (Off to google.)
(Returns)
Sure enough, we do indeed find GAGs in our joysticks.
"Collagen and the GAG concentration in the human penis undergo extensive modifications during development and shortly after birth but from ages 2 to approximately 46 years changes are limited to the proportion of GAG species in TA from older individuals."
http://www.jurology.com/pt/re/juro/abstract.00005392-200503000-00136.htm;jsessionid=CiIjAkz6lyoAuWurtBQ2Jjx3l4NmYZPHW3l53Ew0P2mXM13Y2oBx!1142682874!-949856032!9001!-1
"Our study shows that the different structures of the human penis produce distinct profiles of glycosaminoglycans, which are well suited to the individual functional characteristics of these structures."
http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=11084377&dopt=Abstract
After that last article, I decided to stop. That was enough penis GAGs and penectomies for one night (shudder).#: Posted by on 02/27 at 09:08 PM -
So, being iggorant, I googled 'baculum.' In addition to finding out just what it is, I found these...
Baculum at Hard to Find Items
Baculum Sale
New & used Baculum.
Check out the deals now!
http://www.eBay.com
Sexy Baculum Singles
View photos, personals and hot
profiles of local singles.
http://www.infobert.com
Find Baculum
We have what you're looking for.
Baculum & much more! [at least this one rhymes]
I never knew... -
For those curious about the distribution of bacula among mammals, they are known in placental mamals from the follwing groups:
Primates
Rodentia
Insectivora
Carnivora
Chiroptera
Giving a very handy mnemonic.#: Posted by on 03/01 at 02:14 PM -
Apparently, this has turned into Penis Blogging Week, as a companion to the Estrogen Week:
http://markc1.typepad.com/relentlesslyoptimistic/2005/03/cephaloblogging.html -
According to some proposed arrangements, ratites and anatids are part of a clade that also includes the galliformes (chickens and turkeys and a bunch of others) and tinamiformes. It seems at least some tinamous have penises as well. Chickens and turkeys and (I suspect citelessly) all other galliformes do not. If the cladistic arrangement I've seen is a good one, that means birds have evolved dicklessness at least twice.
That particular proposed arrangement (Sibley & Ahlquist's, at one point) is clearly wrong, as even Sibley & Ahlquist eventually agreed. Instead, paleognaths (ratites and tinamous) form a clade whose sister is all the other birds (neognaths); within neognaths, ducks and chickens (Galloanserae) go together as sister of all remaining birds (Neoaves). Given that topology, and given the assignment of states you make and that I am too lazy to look up, the most parsimonious solution is that the ancestral bird didn't have a penis, and it was evolved twice. If we assume that paleognath and duck penises are homologous (which would require detailed comparisons that I'm also too lazy to look up) then we again get what you say: ancestral bird with a penis (assumed), and two losses. There's even a third equally parsimonious solution: ancestral penis, one loss, in the ancestor of neognaths, and a regain of the character in ducks.
By the way, duck penises have been in the popular press before. The penises of one tribe, Oxyurini, are longer than the rest of their bodies. See McCracken, K. G. The 20-cm spiny penis of the Argentine Lake Duck (Oxyura vittata). Auk 117:820-825. This announcement made one of the tabloids -- Weekly World News, I think.#: Posted by on 03/02 at 01:50 PM -
That particular proposed arrangement (Sibley & Ahlquist's, at one point) is clearly wrong, as even Sibley & Ahlquist eventually agreed. [etc.]
Fascinating. Thanks, John.
By the way, duck penises have been in the popular press before
As long as they stay off the table. The tongues are bad enough.#: Posted by Chris Clarke on 03/02 at 02:11 PM -
The penis is not essencial for survival - it has evolved and un-evolved countless times, and many specia are quite happy without having one. Perhaps females prefer it that way, as they exert control over penis-less males and can control which sperm reaches their eggs. Penis is useful to force copulation (which is why humans may have developed it to such an extraordinary size and with suction pumplike action), and to exhibit sexual prowess. The case of the Argentine lake duck (Oxyura vittata), evolution produced a penis of 42 cm with a brush-like tip to scrub off the sperm of previous mates. All in all, a useful organ.
#: Posted by on 03/02 at 10:58 PM
- Speaking from near-total ignorance here, is it possible that the partly-common genome carries a template for a penoid organ and can turn it on or off depending on circumstances?