Thursday, March 10, 2011

Penis Spines

Most male mammals have spines on their penis. The spines are small keratinized projections that are connected to sensory receptors. We don't need to get into a discussion of their purpose—other blogs do that. The example shown here is a cat penis from Tom.

Humans don't have penis spines even though most other primates do. Part of the regulatory region of the relevant gene (androgen receptor, AR) has been deleted from our genome at some time after our lineage split from the chimpanzee lineage.

Is the loss of penis spines in humans an adaptation or is it an evolutionary accident? John Hawks discusses this: The real "junk" DNA. Read what he has to say on the matter. If you post comments here I'm sure he will see them.

The relevant paper was just published in Nature, McLean et al. (2011). Here's what the authors say,
Our results show that humans have lost an ancestral penile spine enhancer from the AR locus. Humans also fail to form the penile spines commonly found in other animals, including chimpanzees, macaques and mice (Fig. 2l). Simplified penile morphology tends to be associated with monogamous reproductive strategies in primates. Ablation of spines decreases tactile sensitivity and increases the duration of intromission, indicating their loss in the human lineage may be associated with the longer duration of copulation in our species relative to chimpanzees. This fits with an adaptive suite, including feminization of the male canine dentition, moderate-sized testes with low sperm motility, and concealed ovulation with permanently enlarged mammary glands, that suggests our ancestors evolved numerous morphological characteristics associated with pair-bonding and increased paternal care.

WARNING: I may be a little more selective about allowing comments in this thread. I know it violates Sandwalk policy but for this one time I'm not going to allow adolescent male humor to distract from the science. There's plenty of other opportunities for us to indulge our sense of humor in other postings.

McLean, C.Y., et al. (2011) Human-specific loss of regulatory DNA and the evolution of human-specific traits. Nature 471:216–219. [doi:10.1038/nature09774]


  1. The media coverage is, as expected, pretty poor and uncritical of the more sweeping conclusions of the paper.


    The adaptive story is all that is discussed, despite the criticisms pointing out the poor correlations out there for the monogamy-penis spine loss argument (such as the ones John Hawk's pointed out on his blog).

    Sadly, I'm not surprised. Mix up sex and genomics, and the media is going to see a great big $ story.

  2. Larry says, "I may be a little more selective about allowing comments in this thread."

    Good idea, especially since there are far too many examples of "adolescent male humor" on Monday, March 07, 2011
    "When Did People First Start Knowing the One True God?"

  3. With all the "science" in the world, science important to the wellbing of the inhabitants of this planet, don't you think time might be better used? Excuse my offense, and with "pubescent male humor" aside, but might I suggest this topic rather innane? Furthermore, it is the view of this disturbed individual, that interest in this subject, is indicative of the same veins of thought, with comments varying only because of modesty, or etiquette. Are we in danger somehow from an imiment "reactivation" of the "penile spine" gene? LOL!!!

  4. Knowing what a dirty word "adaptation" can be with you, do you think the loss of penis spines is an adaptation or do you feel the authors conclusions are a "just-so" story.
    I admit this question borders on snarky, but it is also sincere.

  5. MJ asks,

    Knowing what a dirty word "adaptation" can be with you, do you think the loss of penis spines is an adaptation or do you feel the authors conclusions are a "just-so" story.

    My first inclination is to dismiss their explanation as an after-the-fact just-so story. As far as I know there's no evidence that loss of spines is adaptive so it's just as likely that it's an accident.

    Imagine that we could go back in time two million years. At that time all of our ancestors had the spines. A mutation arose in one male individual leading to loss of spines. According to the just-so story, that individual gained a selective advantage over the other dozen-or-so males in his troop leading to the eventual exclusion of all spine-bearing grandchildren and great-grandchildren from mating with the dozen-or-so females of reproductive age.

    One wonders how the young women became so adept at selecting the spineless males over the ones with no spines. Or was it truly the case that the absence of penis spines caused the mutant males to invest a lot more time in raising their children?

    1. Women do like spineless males - now an evolutionary justification!

  6. If you really think that humans don't have penis spines, I have found a picture which will SHOCK THE HELL out of you on this wikipedia page:

    Seriously, look up "penile papules". Women also have them - they're known as vestibular papillomatosis.

    Seriously, some days I wonder if we're living in the Victorian era all over again. What next? Printing pictures of stinkhorns upside-down?