In my first installment of Planet of the Apes, I interviewed a biologist who found that humans carry a mutation, a missing piece of DNA, that freed our species of penis spines. He described these spines as little whiskery projections that may increase sensitivity in the male. Apparently they adorn the male genitalia of chimps and many other mammals.
Penn State's Philip Reno, the biologist in question, said the penis spines on chimps are pretty benign, being only about a millimeter long. The scariest ones belong to, who else, the porcupine.
Porcupine penises are covered with spines the size of fingernails - as if the poor creatures didn't have enough issues already. Not surprisingly, porcupines are known as slow breeders. They're relatively rare animals, which is why some conservationists object to PA's decision to allow people to shoot a limited number of porcupines.