Duck sperm has antibacterial properties. The ability of the sperm to kill E. coli is higher in ducks with brighter beaks. In humans, E. coli harms sperm. [Source:
Discoblog]
A drake's penis is spiral. It is inverted inside the duck but it takes just 1/3 second to evert it. Instead of blood, lymph fluid is used to extend it. [Source:
Duck Penises]
A duck's vagina is also spiral but it twists the other way! It also has pockets. This may be so that the duck can control which drake fertilises the eggs.
Concerning spiral penises,
pigs
have a penis with a corkscrew on the end. The sow's cervix also is formed as a corkscrew. Unlike the ducks, both have the same orientation (left handed). In order for the boar to be simulated to ejaculate, the penis has to lock into the cervix. This locking prevents backflow of semen.
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