sciencehabit writes: Most male mammals wield a penis covered with spines made of keratin, the same material that forms fingernails, to sweep out competitors' sperm and irritate a female into ovulating. Even chimpanzees, our closest relatives, have penile spines. So why don't men? A new study suggests that this feature disappeared due to a chunk of DNA that went missing after our evolutionary divergence from chimps. The researchers have identified another DNA deletion that may have contributed to humans' bigger brains.