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On Evolution and Duck Genitalia

Posted by shade on 02 May 2007 at 16:15 GMT

In the recent New York Times story about the evolution of duck genitalia Dr. Brennan says:

"Once they choose a male, they're making the best possible choice, and that's the male they want siring their offspring," she said. "They don't want the guy flying in from who knows where. It makes sense that they would develop a defense."

I am certainly no expert on evolution but this makes absolutely no sense. As I was taught, evolution is based on mutation and natural selection - not by choice. Humans did not decide to walk upright so they could see over the long grasses and carry stuff in their hands. Giraffes did not think to themselves "Hey, no one's eating those tasty leaves at the top of the tree. Why don't I grow my neck longer so I can?" How is it that female ducks can decide to change their genitalia to lessen the reproductive success of rape? Doesn't it make more sense to say that the vagina of the female duck underwent a strange mutation that only allowed male ducks with a certain bent (as it were) to successfully mate? And then only with the consent of the female? Could it be that the frequency of forced copulation is a result of frustration on the part of male ducks that have not found a willing mate? It simply makes more sense to say that the mutation of the female's anatomy predated that of the male leading, unfortunately, to the rise of duck-on-duck violence. If not, then why haven't the females of other species been able to make the same choice? Ducks aren't the only sexual predators in the wild. And I'm sure there are many women who would argue the merits of vagina dentata.

Dr. McCracken comments on the "male bias" that prevented them from looking at the female duck's anatomy before, but that bias continues in the observation "If a male bird had a long phallus, the female tended to have a more elaborate lower oviduct. And if the male had a small phallus, the female tended to have a simple oviduct." Surely it's the other way around? And what are the rates of forced copulations among the ducks with the simple oviducts and small phalli? Does the relative ease of copulation take the stress out of reproduction? Or are ducks with simple oviducts simply not as picky? Maybe it is the difficulty of mating successfully that leads the other types of ducks to form pair-bonds. Just sayin'

(This commentary was written in response to the NYT story and contains quotes from it. I have read the study in PLoS and I think the questions are still valid)

RE: On Evolution and Duck Genitalia

pbrennan10 replied to shade on 14 May 2007 at 15:37 GMT

Females do indeed choose, and their choices affect evolutionary processes. Half of the science of sexual selection is based on female choice (the other half male-male competition). Of course we did not say that females choose to coil their vagina. The process would indeed be as you describe, a mutation that changed the female morphology in such a way that forced copulating males were less successfull. This would be advantageous for females if forced copulation males are of lesser quality than paired males, or if a pair bond is required in order to succesfully raise chicks.
In order to answer the question of what came first the coiled vagina or the long phallus, we need to look deeper in phylogentic history. If all ancestral ducks have phalluses but no coiling in the vagina, this argues in favor of females developing the defense after the male had the long phallus. We are still collecting specimens.