One of the many incredible creative masterpieces of evolution is the spare penis.
Not in humans, of course, but in an insect species called Euborellia plebeja, known to many laypeople as earwigs.
For a while after its discovery, the earwig's second penis was thought to be a useless freak of nature, as it's (obviously) redundant as well as pointing in the wrong direction. However, some researchers at the Tokyo Metropolitan University observed these animals during the act and discovered that the "first" penis is rather fragile, and sometimes breaks off in use. The researchers were able to provoke this mutilation by tapping the insects on the back at an inopportune moment.
The researchers found that the earwigs were not badly incapacitated by their loss of this organ. In fact, within two days, they were successfully mating with other females, using their "spare" penis.
More research showed that there is an appreciable number of males of the species living with only their second penis, and a similar number of females with a broken-off penis lodged in their reproductive organs. It is now believed that the discardable penis may be an evolutionary strategy of competition, as the penis remnants prevent the female from mating with another male.
Of course, the earwig male is in trouble if he loses his second penis as well.