Female Ducks Evolve To Protect Them From Rape (5/3/2007)

A team of scientists from the University of Sheffield and Yale University in the US have discovered that the female reproductive systems in some ducks and geese have evolved in order to keep unwanted male attention at bay.

In their study of the female reproductive tract (oviduct) of waterfowl, the scientists found two unexpectedly complex and entirely novel structures that all seem designed for one purpose — to selectively exclude the male reproductive organ, phallus or penis.

In most birds, the oviduct is a simple tube, but in some waterfowl, the oviduct has two unique structural novelties: sacs and spirals. The sacs are out-pocketings of the tube that are just inside the opening of the oviduct. The scientists have found that they appear to function as `dead-ends,´ or false passages,´ and that if the phallus were to enter one of these sacs, it would not progress further into the oviduct where it would deposit sperm more effectively.

The second unusual feature of the oviduct is a series of tight, clock-wise spirals. Interestingly, the male phallus is also spiral-shaped, but it twists in the opposite, counter-clockwise, direction. The scientists suggest that the twists in the oviduct appear designed to exclude the opposing twists of the male phallus in an exquisite anti-lock-and-key system.

Male waterfowl are also distinct in having great diversity among different species in the length and shape of their phallus.

This diversity in waterfowl phallus size and shape has been previously explained as a result of sperm competition. Other scientists speculated that sperm from males with a longer phallus had a competitive edge over sperm from those less well-endowed. This latest research though, published in the online journal PLoS ONE shows that there is a lot more to the story.

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 the 14 different species of ducks and geese, the authors show that the genitalia of males and females have dynamically co-evolved with one another.

Patricia Brennan, Lead Author, and a joint post-doctoral researcher with Tim Birkhead in the department of Animal and Plant Sciences at the University of Sheffield and Ecology and Evolutionary Biology at Yale University, said: "I think 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.

"In response to male attempts to force their paternity on females, female waterfowl may be able to assert their own behavioural and anatomical means of controlling who fathers their offspring,"

Professor Tim Birkhead, from the department of Animal and Plant Sciences at the University of Sheffield, said:" We believe that elaborate phalluses and female oviducts have co-evolved in a dynamic response to one another. Males evolve longer phalluses to increase their chances of fertilisation; in response females evolve more complex oviducts to keeps males at arms length."

He added: "We speculate that these physical barriers are easily overcome when females cooperate during copulation, and that they only function to exclude unwanted advances."

Note: This story has been adapted from a news release issued by The University of Sheffield

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