If you have lots of spikes on your private parts and they look something like this:
chances are that
a) you're not human, and
b) you're involved in a kind of evolutionary arms race between the sexes of your species, known as sexually antagonistic coevolution.
The idea is that the spikes anchor the penis in the female genital tract, blocking the way for any other candidates. As the females are actually injured by the spikes, their evolutionary respond is to produce more connective tissue in that area to protect themselves.
Hard (as it were) evidence for this kind of arms race has been scarce so far, but this week Johanna Rönn, Mari Katvala, and Göran Arnqvist from the University of Uppsala, Sweden, report a systematic study of penis spikiness vs. vagina resistance in seven species of seed beetles and find that there is a clear correlation between the two. The work is due to be published in PNAS Early View this week:
http://www.pnas.org/cgi/doi/10.1073/pnas.0701170104
The best bit is how the authors measured the spikiness of the genitals:
we presented a set of scanning electron micrographs
of the genitalia of each species for 20 persons who were asked
to rank the seven species according to harmfulness of the genital
structures. The persons ranking genitalia, all of whom were
professional biologists nai¨ve to this particular taxa and problem,
were given a standardized written instruction stating only that
male genitalia cause internal injury to females during copulation
in these beetles.