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]