Duck mating sessions are serious business -- in fact, they are often extremely aggressive. The uniquely shaped penises and vaginas associated with male and female ducks, respectively, contributing to their one-of-a-kind mating techniques.
Male Sex Organs
As opposed to 97 percent of other bird species, male ducks -- or drakes, as they are sometimes called -- have a penis that is normally housed inside-out, within a sac in its body. During sex, the penis is ejected from this sac, and reaches a full length of approximately 20 centimeters. Drake penises are unique in that they feature a corkscrew design and have ridges and backward pointing spines. Certain aquatic birds, such as drakes, may have a penis to prevent sperm from being washed away in the water.
Female Sex Organs
The sex organs of female ducks are also highly specialized and unique. According to the National Geographic article "Ballistic penises and corkscrew vaginas -- the sexual battles of ducks," female ducks have a long and twisted vagina that features a number of spirals and dead-end pockets. Research in 2009 by Patricia Brennen of Yale University suggests that these elaborate vaginas may serve as a sort of chastity belt, thus limiting the effectiveness of penetration by their male suitors.
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During sex, males mount female ducks and line up their penises with the vaginas of their chosen females. Once positioned properly, the drake's penis explodes outward, into the oviduct -- or vagina -- of the female, a process that takes approximately one-third of a second. Females make their oviduct receptive to intercourse by holding their body level and lifting their tail feathers. Relaxing and contracting the walls of the genital tract also helps the drake reach full penetration once within the oviduct.
When it comes to mating, drakes are quite aggressive creatures -- in fact, as many as one in three duck mating events are rapes. Three or four drakes may attack a female duck simultaneously, resulting in her injury or even death. Fortunately, female ducks do have some countermeasures that can be used to ward off unwanted advances by drakes. Females can, for instance, position their bodies in a way that prevents full penetration and may limit the locale at which sperm is deposited. In nine out of ten "rapes," offending sperm is trapped in a side pocket in the vagina and eliminated, thus preventing unwanted pregnancy.