Most male bird species do not have a penis. Unlike their mammalian counterparts, the reproductive structure in birds is a bit of a sewer called the cloaca. Also found in amphibians, reptiles and egg laying mammals (monotremata), the cloaca is an orifice where intestinal, urinary and reproductive structures terminate. While the majority of birds lack a true penis, 3% have a pseudo-penis. Commonly referred to as a phallus, waterfowl like swans, geese and ducks possess twisted genitalia that can look a bit like a corkscrew. The product of sexual selection, female waterfowl have evolved long and intricate reproductive tracts that enhance their ability to regulate and restrict mating attempts. Here the “Red Queen” hypotheses, which is typically used to describe the evolutionary race between predator and prey, suggests a mechanism for the evolution of a phallus that can be twice as long as the owner’s body. It turns out that in some duck species, the pseudo-penis can be as large as 15”. Well endowed males can successfully navigate the intricate female reproductive tract better than males who might be lacking in this department. Interestingly (as if that wasn’t enough), the female oviduct consists of clockwise spirals, while the male phallus has evolved into counterclockwise twists that will allow him to negotiate the tricky pathway to her eggs. As the phallus gets longer, the oviduct follows suit. This evolutionary arms race provides females with the capacity to select or reject her suiters. Yet the same mechanism that protects females from unwanted mates also selects for males with an increasingly long and complex phallus than can be used to inseminate would be carriers of his genes.
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