One of my fondest days of an undergrad ornithology course was the day we learned about duck genitals. It was funny, clearly, because it’s about penises and we were in undergrad. But equally fascinating evolutionarily: why would a male duck evolve a penis that twists like a screw? And how could a female evolve a vagina that is shaped like a screwhole with opposite threading? (Insert your favorite sexual screwing joke here.)
I cannot do the new research by Patricia Brennan more justice than Carl Zimmer does, so I recommend that you head over to his post on The Loom. He includes video of a duck boner, as well as a duck penetrating glass tubes of different shapes. If you have a duck fetish, I would stay away from these videos at work.