The second structural novelty of the female is a series of tight, clock-wise spirals in the tubular oviduct. "Interestingly, the male phallus is also a spiral, but it twists in the opposite, counter-clockwise, direction," says Yale ornithologist and co-author Richard Prum. "So, the twists in the oviduct appear designed to exclude the opposing twists of the male phallus. It's an exquisite anti-lock-and-key system."

The number of sacs and spirals in the reproductive tract of various female waterfowl correlates strongly with the length of the male phallus. Comparing the phallus size and oviduct shape in 14 different species of ducks and geese, the authors show that the genitalia of males and females have dynamically co-evolved with one another.

According to the study, in various independent lineages of ducks females developed more elaborate oviducts as males evolved longer phalluses. In other lineages females lost oviduct complexity as the phallus evolved toward smaller size.

Why all this dynamic evolution? Brennan hypothesizes that the female waterfowl have evolved these unique anatomical features as physical counter-measures to evade male attempts to assert control over reproduction. "Despite the fact that most waterfowl form monogamous pairs, forced copulations by other males the avian equivalent of rape are common in many waterfowl," said Prum. "The length of the phallus of a species is strongly correlated with the frequency of forced copulations."

"In response to male attempts to force their paternity on females, female waterfowl may be able to assert their own behavioral and anatomical mean
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Contact: Janet Rettig Emanuel
janet.emanuel@yale.edu
203-432-2157
Yale University
30-Apr-2007