Why don't humans have spines on their penises? Ask a chimpanzee!

Yes, you read the title correctly. Other primates have spiny penises, including our closest cousins. The question is: why don't we?

Several theories have been proposed to account for penis spines in those species that have them. It is possible (as with the earwigs I wrote about recently) that penis spines serve to remove sperm left by other suitors that previously copulated with a female. Another theory is that the spines injure the female during copulation, ensuring that she will not be in the mood for sex again for a while - long enough, at least, that competitor sperm arrives too late.

Whatever the reason spines grow from the penises of other species, they decidedly do not grow there in humans. No doubt this comes as quite a relief to many of my female readers.

We may not know exactly why our species dropped this peculiar prickly penile adaptation, but thanks to the work of McLean and colleagues at Stanford University (published today in Nature) we know how we dropped it.  We lost some of our DNA.

McLean and colleagues (including Gill Bejerano and David Kingsley) used a novel - and very clever - method of analyzing the DNA of chimpanzees and humans. Rather than looking for similarities (as many studies do), McLean et al. looked at the differences. They found more than 500 regions present in the chimpanzee genome that is missing in humans.

One of the sequences missing in humans is located near the Androgen Receptor gene in chimpanzees.  When the sequence was inserted into mice, embryos developed penis spines.  They also developed sensory whiskers - another trait that our ancestors dropped sometime after we shared a common ancestor with chimpanzees.

The missing sequence in humans appears to be a developmental switch: something that turns on genes for particular traits (such as spines or whiskers). Lose the switch, lose the trait.

The correlation between these two traits is interesting: Perhaps we lost our whiskers because of selection against penis spines? Or did we lose our penis spines because of selection against whiskers?  Some have argued that loss of penis spines accompanied our foray into monogamy. I have always been suspicious of this theory, though, because human cultures are not all socially monogamous and there is plenty of philandering in those that are.

As much as we men like to think everything revolves around our genitals, perhaps it revolves more around our whiskers! (or lack thereof)