More about penis evolution
March 10th, 2011
04:40 PM ET

More about penis evolution

By now you might've read about new research on the genes involved in  the evolution of the human penis. With more than 2,200 comments, our Wednesday story was a talker.

And it wasn't an excuse to say the p-word. Scientists have published a study in the esteemed, peer-reviewed journal Nature that happens to talk about penises. Specifically, they looked at DNA sequences among humans and primate relatives that basically serve as "switches" for other genes.

They focused on two switches that humans lack, but that chimpanzees have. One of these switches, the researchers believe, makes chimpanzees have whiskers and what they call "spines" on the penis. And the other relates to the brain - the absence of a particular DNA strand allowed the human brain to grow larger than the chimpanzee's, the researchers theorize.

But you didn't click on this headline, or the other one, to learn more about brains, now did you!

So, back to what you came here for. It turns out that there's some disagreement about the role of the primate male anatomy discussed in this research. As some of you noted in the comments, there are still many open questions when it comes to exactly how primates, including humans, evolved the way they did.

Yesterday, I had an e-mail chat with Christine Drea at Duke University, who studies mammalian social behavior and reproductive development. She sent me all kinds of detailed photos to illustrate differences in penis anatomy among primates. And there is a lot of variation.

In her view, the word "spines" may not be the right term for the small bumps that chimpanzees have. Other creatures such as cats have the more spike-like protrusions because the females of these species need appropriate stimulation from the male to ovulate. But primates ovulate spontaneously, so we wouldn't need spines for that purpose.

And, as I wrote yesterday, one idea about the function of the spines is that they can help the male pull out the sperm of another male that has previously mated with a female. But in chimpanzees and their relatives, their small bumps wouldn't be able to remove a competitor's semen in the same way, Drea said. In fact, chimps have unusually large testicles that produce large quantities of sperm, which do serve as a means of competition among males to get a female pregnant.

Also, in some primates, such as lemurs, sperm coagulates into "plugs" that can be pulled out with the spines; human semen does not do this, so we wouldn't be expected to have spines to remove it.

So, what are the bumps on chimp penises for? It's all speculation, but those of you (and there were many!) who joked about being "ribbed for her pleasure" may not be far off, as one explanation for the structures in chimps and other primates may be just that. They could be an "internal courtship device," Drea says.

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