Male ducks, or drakes, possess a reproductive anatomy that is highly unusual among birds. The unique form and function of this organ are the result of specific evolutionary pressures related to their mating behaviors and environment.
The Corkscrew Anatomy
The drake’s phallus is characterized by its remarkable length and counter-clockwise corkscrew shape. When not in use, the organ is kept inverted inside a phallic sac in the cloaca. Its size can be substantial, with some species having a phallus that can equal their total body length when fully extended. For example, the Argentine lake duck holds the record, with one specimen’s phallus measuring 42.5 cm. This organ grows to its full size during the breeding season and may shrink afterward.
It is composed of a base and an apical, or tip, region which have different surface features. The base may be covered in spines, while the tip can have a softer, brush-like texture. Semen is not transported through an enclosed urethra as in mammals, but rather along an external groove called the sulcus spermaticus. The entire structure is composed of dense collagen fibers, giving it a pale, tentacle-like appearance.
Explosive Eversion and Mating
The erection of the duck phallus is a rapid and explosive event. Unlike mammals, which rely on blood flow for erection, the drake’s phallus becomes erect through a process of lymphatic engorgement. Lymph fluid accumulates in cavities at the base of the cloaca and is forced into a lumen inside the inverted phallus. This pressure causes the organ to evert, or turn inside out, and be projected into the female’s reproductive tract in as little as one-third of a second.
This method of “explosive eversion” is directly linked to the competitive and often coercive nature of duck mating. In many waterfowl species, forced copulations are common. The speed of the process ensures that insemination can occur during very brief cloacal contact between the male and female.
The Coevolutionary Arms Race
Female ducks have developed equally elaborate reproductive tracts in response. The female’s vagina is also shaped like a corkscrew, but it spirals in the opposite, clockwise direction to the male phallus. This anatomical mismatch makes successful insemination difficult without the female’s cooperation.
The female tract is a labyrinth of blind pouches and dead-end sacs that can trap sperm from unwanted males. These features provide the female with a degree of control over fertilization, allowing her to select which male’s sperm reaches her eggs. This dynamic interaction, where male and female genitalia have evolved in response to each other, is a classic example of a sexual coevolutionary arms race.