In snakes and lizards, the external genitals get a little funkier. These reptiles have paired external genitals, even though they only use one at a time during mating.

But all of these genitals spring out of the same embryonic structure, called the cloaca. This structure sends out signals to the cells around it in the embryo, telling them to turn into genitals.

Scientists have wondered why these structures — which are triggered to grow by the same genetic mechanisms during embryonic development, and have the same function in adulthood — develop so differently.