DUCKS and geese are not the most gentle of lovers. Males of some species, such as the mallard, have a notorious habit of “raping” females. They and other wildfowl are among the 3 per cent of bird species whose males have phalluses big enough to insert into the vaginas of females whether or not the female is willing.
However, females don’t take it lying down. A study of 16 species shows that females have evolved vaginal features that thwart the males’ advances. The longer and more elaborate the male member of the species, the longer and more elaborate the female counterpart (PLoS ONE, DOI: 10.1371/journal.pone.0000418).
Some birds have vaginas with spiral channels that impede sex by twisting clockwise, the opposite direction to that of the male phallus. Others had as many as eight “cul-de-sacs” in the reproductive tract, pouches that could prevent fertilisation by capturing sperm. These features were only found in species where forced sex occurs, demonstrating the existence of an evolutionary “arms race” between males and females. All other species had simple genitalia.
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“These structures are wonderfully devious to send sperm down the wrong road or impede penetration,” says Tim Birkhead of the University of Sheffield, UK, who co-led the research. “It shows that females are not passive in averting exploitation.”
