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The Lake Duck (Oxyura vittata) is a small South American stiff-tailed duck. It is also called the or Argentine Blue-bill, Argentine Lake Duck, or Argentine Ruddy Duck.

The Lake Duck occurs in Argentina, Chile, Paraguay, and Brazil in southern South America.[1]

It is notable for possessing, in relation to body length, the longest penis of all vertebrates; the penis, which is coiled up in flaccid state, can reach about the same length as the animal itself when fully erect, but more commonly is about half the bird's length.[2][3] It is theorized that the remarkable size of the spiny penis with its bristled tip may have evolved in response to competitive pressure in these highly promiscuous birds, removing sperm from previous matings in the manner of a bottle brush.

Although most male birds have no penis,[4] ducks have a long corkscrew penis, and the females have a long corkscrew vagina, which spirals in the opposite direction.[5] The males often try to force copulation, but the complex mating geometry allows the females to retain control—most of the forced copulations do not result in successful fertilisation.[6]