The mallard Anas platyrhynchos: a species with high levels of forced copulations. Females have very elaborate vaginas (left) to counter males’ phalluses (right). (Image: Patricia Brennan et al.)

Some female ducks and geese have evolved complex genitalia to thwart unwelcome mating attempts, according to a new study.

Males of some species, such as mallard, have a notorious habit of “raping” females. They and other wildfowl are among the 3% of bird species whose males have phalluses big enough to insert into the vaginas of females, whether or not the female consents.

Now, in the most detailed analysis yet of duck and goose vaginas, researchers have established that females of these species have evolved vaginal features to thwart unwelcome males.


Tim Birkhead at the University of Sheffield in the UK and colleagues examined vaginas and the corresponding phalluses from 16 wildfowl species. They discovered that the longer and more elaborate the male member, the longer and more elaborate its female recipient was.

No-entry signs

Some vaginas had spiral channels that would impede sex by twisting in the opposite direction to that of the male phallus. Others had as many as eight cul-de-sac pouches en route, that could prevent fertilisation by capturing unwelcome sperm. Moreover, these features were only found in species renowned for forced sex. All other species had simple male and female genitalia.

“These structures are wonderfully devious, sending sperm down the wrong road or impeding penetration,” says Birkhead.

He says that the features demonstrate an evolutionary “arms race” in which control over reproduction alternates between the sexes. If the male develops a longer, more elaborate phallus to force copulation, females wrest back control by developing features to thwart males who rape.

“It shows that females are not passive in averting exploitation by males with large phalluses,” says Birkhead.

The study appears in the online version of the journal PLoS ONE.