The Argentine Blue-bill or Argentine Lake Duck (Oxyura vittata) is a small South American stiff-tailed duck.
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.
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.
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