Research published in this week's Nature solves the mystery of why snakes and lizards develop pairs of genitalia while the rest of us don't.
Species lucky enough to have two penises do so because the genitals develop from cells located near the legs, or where the legs would have been.
The genitalia of all species develop from something called the cloaca. The location of this cloaca is the key to whether an animal ends up with two sets of junk or not.
When embryos develop the cloaca sends out a signal to the surrounding cells triggering the growth of genitals.
Patrick Tschopp and his team from Harvard Medical School studied the development of embryos of mice, anoles (a type of lizard), house snakes, and pythons. In the mouse embryo the cloaca is futher back near the cells destined to become a tail, and the mouse ends up with just the one penis. While in the reptiles the cloaca is closer to the potential legs, and they develop two penises.