Researchers think they have answered the biological mystery of why reptiles have two penises and birds and mammals (with the exception of that guy on Reddit) only have one.
The answer, according to a paper published in Nature yesterday is that it depends on the location of the embryonic cloaca, which eventually develops into the urinary tract.
The cloaca issues signals telling neighbouring cells to form into genitals. In reptiles the cloaca is nearer their legs, or in the case of snakes what used to be paired limbs, so two penises develop (even though only one is used for mating). In mammals the cloaca is nearer the tail bud, so one penis develops. And no we don't know how this research impacts on our understanding of double dick dude.
Lead author Patrick Tschopp of Harvard Medical School's Department of Genetics said: "This might help to explain why limbs and genitalia use such similar gene regulatory programs during development."