Friday Weird Science: Penis Spines, what are they REALLY?

Mar 11 2011 Published by under Friday Weird Science

I'm sure by now everyone here has heard about the Penis Spine Story. The actual paper involved bigger brains as well as penis spines...but who cares about brains when PENIS SPINES are around?? Yeah I didn't think so.

When I read the coverage of the penis spines, and heard it was the technical term, I was really confused. I thought, isn't the technical term the baculum? The bone in the penis of animals like mice and chimps and dogs, which facilitates erections. Humans don't have them. Penis Spine, baculum, makes sense, right? I was completely mystified as to why people weren't just calling it the baculum and seemed to be talking like there was more than one!!! Surely they didn't mean something like THIS:

And lo and behold, they DID mean ACTUAL PENIS SPINES!!! BE STILL MY HEART. I still didn't believe it, until Eric Michael Johnson (who will be doing another post on the penis spine paper over at Zinjanthropus! Check it out!) mentioned a need for a paper. And the paper...was on penis spines. The actual spines!

Osman Hill. "Note on the male external genitalia of the chimpanzee" Proceedings of the Zoological Society of London, 1944.

Science. You couldn't MAKE this stuff up.

So basically, the author, Dr. Hill, got an opportunity to study a freshly dead chimpanzee. Unlike previous specimens which got preserved in bad solutions, this one was SO freshly dead that they were able to preserve the body in the kind of fluid usually used for human embalming (he doesn't specify, but seeing as it's 1944, I'd bet on some formaldehyde in there). Why Dr. Hill was particularly interested in examining the penis, I don't know. But he drew it VERY carefully.

Count yourself lucky, the original view is a LOT larger. What you're looking at here is the view looking up between the legs, like you're staring at the taint, which is toward the bottom of the picture. The penis sticks up at the top, and you can note the extremely large testicles of the chimp. Testicle size in primates has an inverse correlation with monogamy, so you can see that chimps are VERY NON-monogamous.

What Dr. Hill noted upon examination of the penis was a series of tiny BUMPS on the surface of the glans (the head) of the penis. He wanted to study this in more detail, and took a sample. Yeah, he skinned the penis. He was also able to compare his adult specimen with the specimens of a fetal chimp and two teenage chimps.

In describing the basic form of the penis, it seems just like...a penis. Unlike humans, chimp penises taper toward the end, but they do have a foreskin. Also, it's lightly hairy. But it's when he starts talking about the spines that the true poetry comes out.

The surface of the glans is beset, especially on its lateral surface, with
numerous low, horny elevations, of yellowish-brown colour, sharply distin-
guished from the pale rosy tint of the remainder of the epithelial covering.
These papillae are hard t o the touch and impart a roughness to the finger

You can see there the little dots meant to represent the penis spines. In comparison, the fetal and adolescent chimps had them too, though the fetal chimp penis spines were described as "gelatinous" and relatively clear.

In detail, the spines are on average about 0.35mm wide. He goes into the cellular structure, but it's nothing out of the ordinary, except that the spines are...hard. In comparison to the rest of the skin. He describes them as "cornified", and they appear to be composed kind of like callouses...only tiny spines. Tiny corny spines.

While the author is mostly concerned with the potential evolution of the spines, I'm more concerned with their function. What are they FOR?! Why are they THERE?! One hypothesis is that they are actually more stimulating for the male (ribbed for his pleasure, anyone?), and therefore promote quicker sex. This could be especially important in promiscuous species, where it's important to get it over and done as quickly as possible, and on to the next! This means that humans, who don't have penis spines (thank goodness), find sex LESS stimulating, and it takes them longer. This might be ideal for promoting social bonding in humans, while in chimps social bonding via sex is far less important. But there are other possibilities. It's possible that bumps and ridges on the penises of promiscuous species are good for scooping out the sperm of competitors.

I just wonder what the girl chimps think.

Hill (1944). Note on the male external genitalia of the chimpanzee Proceedings of the Zoological Society of London

27 responses so far

  • Yoder says:

    "I just wonder what the girl chimps think."

    Er ... ribbed for her pleasure?

    Sorry, couldn't resist. I'm up late writing for the second night in a row, and starting to feel it. I'd seen all the Twittering about this without having time to take a closer look, and assumed it was referring to baculae as well—this is decidedly freakier.

    • Xenobio says:

      Cats have penis spines that stimulate ovulation in the female, if I remember correctly (some carnivores ovulate on demand or something like that). So I'm going with the "ribbed for her pleasure" idea. =D If we hadn't lost those genes, there wouldn't be a market for French ticklers.

    • Katie says:

      I would do without the ribbed condoms with you

  • Couldn't the spines just sort of be evolutionary leftovers? They served some sort of purpose in earlier ancestors of chimpanzees but no longer serve a purpose, but also didn't entirely get selected out? I wonder if there is any variation in the density or amount of cornification or something of these spines, among chimps.

  • EMJ says:

    That's a really good question Jason. The trouble is you're looking at the entire body of work on chimpanzee penis spines. I know? How could people just let this topic slide? But Hill is the only study that I've been able to find anywhere in the literature (it's also the only one cited in the Nature study). There are two other studies that look at primate penis morphology and both classify Pan as having no spines along with humans. But I don't want to give too much away, I'll have more in my post later this morning.

    • scicurious says:

      What Eric said. But I bet these days it's a bit easier to find a well preserved chimp penis...people should have another look!

  • Are we sure the spines aren't genital warts?

  • Arthur says:

    Actually I have the spines but not so distinct...
    My girlfriends have always been surprised and curious but tell them it s normal...

  • Almost forgot to send the link to this stream. Here it is: "Penis Spines, Pearly Papules, and Pope Benedict's Balls"

  • Kapitano says:

    If the clitoris is a modified penis (which I read it is), and some men have residual spines called 'papules' some women have them too on their clitorides? Has this ever been studied?

  • Adela says:

    Kapitano, I think the female version are on the labia.

  • Rachel says:

    Um. There's a single (1944) paper on a single (dead) specimen of a single species of a primate describing features that would almost certainly be described as infectious in origin were they to be found on a live animal (as per Peter English's comment above), and we're immediately jumping to what the evolutionary implications of these alleged penile spines are likely to be? Really?

  • scinerd says:

    why hasn't anyone considered that the spines in chimps, apes, etc are not only 'ribbed for her pleasure' but that the reason they don't exist in human males is because females have sexually selected the males to have larger brains instead of 'fun' penises?

    most of us girls would rather have a male who will be conscious of his existence (thereby acknowledging a role as parent) and smart enough to find and keep a home/shelter, rather than one with a ribbed penis. i think our ancestor females could have chosen the male with the grand thoughts over the spiky penis, just like we would do now (well, many of us at least)

    plus, the smarter the man the better the ideas he will have to give her pleasure without the ubiquitous callous spines (how novel and exciting!) disappearance of callous spines may have also forced the male to last longer and give a chance for the couple to bond, but that was as a secondary (and enhancing) effect of the initial sexual selection.

    • Capital says:

      Why not have a male with both a spiny penis and a big brain, given that the spiny penis is adaptive? It's not an either-or scenario, especially considering that penis spines probably wouldn't be very "expensive" to grow, so a maturing dude wouldn't have to lack penis spines in order to have a big brain.

      It's a pretty common misconception about natural selection that non-adaptive traits are "culled" much faster than in reality. Natural selection is pretty good (metaphorically) at stumbling upon and propagating useful traits when given enough time, but there's little survival incentive to get rid of neutral or slightly negative traits. The energy (and maybe breeding :P) penalty of my enormous nose isn't too significant when compared with all the rest of the traits I could pass on to my children.

      So we end up with a lot of neutral or maybe even slightly negative traits, simply because they aren't adaptive or harmful enough to have interacted with natural selection. Some of these are evolutionary holdovers, like Jason was wondering about above, and some are just random bits of chance.

  • Brian Iverson says:

    What about the spines making it harder for the engorged penis to be withdrawn before ejaculation - harder for the female to change her mind?

  • scinerd says:

    I dont have the imagination necessary to visualize how a callosity (or many of them) can make a penis get stuck, particularly in a lubricated vagina (as we know only females chimps in estrus mate and those have quite the lubricant). I think its tempting to imagine a series of unlikely and certainly unselectable traits with a word like 'spines'. Sex couldnt be too hurtful or harmful to a female if it is something that needs to be repeated, and as we know it is. It has also evolved in humans to be highly enjoyable, so likely that it has had a history of enjoyability, and not unnecessary pain. I bet females wouldnt be willing to engage in intercourse if it was incredibly hurtful, as a stuck spiny penis would be. In fact, a hurtful penis could mean extinction.

  • inam says:

    Well, on the end of my organ, down each side in a symetrical row, there are ten small protrusions that have been there all my life. And they stick out more when erect.

  • Nando says:

    I have Penis Spines do's any one know what that means?

  • stephanie says:

    Scinerd, if animals let pain get in the way of reproducing, then I'd imagine it wouldn't the pain of a spiney penis, but that of the labor..... While I know that childbirth hurts like none other, it's not going to stop me from having another child. Sometimes instincts overrule.

  • KScience says:

    The reverse is true, a penis is a modified clitoris as fetuses all start out female and change to male if the chromosomes and hormones dictate. Just thought you'd like to know that all humans were female in the very beginning of life. I suspect that you are male.

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  • Squeak says:

    Has anyone tested to see whether or not similar spines still exist in Bonobos?

    Although they are close relatives, chimps and bonobos have strikingly different social dynamics: chimps society is prone to violence, and bonobos are relatively peaceful. Why? One theory suggests that a small change in the availability of food may have encouraged the evolution of today's chimp and bonobo societies.

    An experiment was conducted; sixteen sexually experienced adult male marmosets were paired with ovariectomized females before, and after, removal of penile spines, and a sham operation. Spine removal resulted in an increased duration of preintromission pelvic thrusting and of intromitted thrusting.

    Three males exhibited partial intromissions during some postspinectomy tests, an effect which had not been observed prior to the operation.

    Sham operations had no behavioural effects. Results indicate that penile spines play a significant (but not indispensible) role in sensory feedback during copulation in this primate species.

    I guess to ensure diversity and to pass on your genes it is possible that sometimes you just have to get the job done quickly. It would be interesting to know if the more docile species, bonobos, still have use for them... or if they are shedding them away with their violent past.

  • Jennie says:

    It's also meant to help keep the male penis in the female as he's having sex with her. So in the case of rape (which happens fairly often in some other species), it prevents the female from escaping.

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