Why snakes grow two penises, but men only one: research breakthrough

Researchers may have figured out why different species develop different genitalia. In snakes and reptiles, the genitals grow to mimic leg buds - producing twin organs. In humans, the genitals grow to mimic a tail bud - so the penis is a single structure.

In a study published in Nature, researchers focused on the difference between squamates (snakes and lizards) and amniotes (birds and mammals). In humans and other amniotes, there's only one external genital structure.

In snakes and lizards, the external genitals get a little funkier. These reptiles have paired external genitals, 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.

According to this new research, the location of the cloaca might be the key. This embryonic trigger (which eventually turns into the urinary and gut tracks) sends out a molecular call to the cells around it. "Genitals", it says, "make me some genitals".