Birds tend to have very small and simple genitalia, but ducks are a big exception. The male's phallus is sometimes as long as the its body and is twisted clockwise, like a corkscrew (right). Females have long tubular vaginas, like this one (left) of a Pekin duck, which may have coevolved to prevent forced copulation by unwanted mates, according to a report this week in PLoS ONE. The female oviduct has many out-pockets or "dead ends" and is also spiraled—not an easy fit for a male duck.