I originally wrote this in May 2007. Since mallard necrophilia is a recurring theme in the blogosphere, partly due to the paper's citation as improbable research, there seems to be sufficient interest in mallard sexuality to bring this post back.
Just in time for avian breeding season, PLoS ONE published an interesting article on the evolution of avian genitals. While the males of most bird species lack phalluses, some waterfowl have extremely complex organs. The most complex genitals are owned by members of the genera Anas, Clangula, and Oxytura. Some males have an extraordinarily long phallus (up to 40 cm), which twists counterclockwise and sometimes has spines along its length.
The research team led by Patricia Brennan found that female duck genitals varied to match those of males. When the male of waterfowl species has a long and complex phallus, the female of the species will have a long and complex vagina, but when males have short and simple genitals, females will also. However, unlike phalluses, vaginas spiral clockwise rather than counterclockwise. Some vaginas have pouches that form dead ends for sperm.
(A) Harlequin duck (Histrionicus histrionicus) and (B) African goose (Anser cygnoides), two species with a short phallus and no forced copulations, in which females have simple vaginas as in Fig 1a. (C) Long-tailed duck (Clangula hyemalis), and (D) Mallard Anas platyrhynchos two species with a long phallus and high levels of forced copulations, in which females have very elaborate vaginas (size bars = 2 cm). ] = Phallus, * = Testis, ★ = Muscular base of the male phallus, ▹ = upper and lower limits of the vagina. [Source; click for larger image]Mating among waterfowl sometimes becomes violent. Mallards especially are known for incidents of forced copulation (see here and here). Some have even been known to engage in necrophilia. Forced copulation threatens to negate the careful mate selection that a female has made in the process of pair bonding. The authors of this paper suggest that waterfowl vaginas evolved to become more complex to give a female greater control over which male fertilizes her eggs. By catching the sperm of an aggressor in a pouch and later expelling it, a female can ensure fertilization only by her chosen mate.
As Carl Zimmer notes, apparently no one who observed the genitals of male waterfowl had thought to examine the corresponding female structure. This paper suggests that the evolution of complex genitals may be spurred by a female's defenses and not by competition among males.