It’s long been known than some varieties of duck have absolutely gargantuan reproductive organs, but a new PLoS ONE paper has revealed that females have equally bizarre (and fascinating) wedding tackle. Carl Zimmer initially covered the story and brought it to everyone’s attention, and Bora has graciously collected the various posts that have been made on the topic thus far, but if you’re anything like me you’re probably wondering how did these ducks evolve such elaborate genitals in the first place?
As one comment on the PLoS paper read (hat-tip to Neil);
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?
Such was my initial reaction to hearing the news via this LiveScience article; while female ducks have some control over the fate of sperm in their reproductive tract, they didn’t conspire together to generate more complicated sex organs out of mere will. I’ve mentioned this before, but I do sense that oftentimes adaptations are described as if the animal chose to develop a particular organ in a certain way (i.e. an ancient lobe-finned fish thinking “You know what? I’m sick of the ocean; I need to develop some legs and get out onto land”) rather than being a product of evolution.
Previously I had floated the idea that male-male competition and the frequent occurrence of forced copulations might have produced such complex “lock-and-key” systems, but I had to scuttle it; it doesn’t make sense. If I was right, then females would always be one step behind the males and we would expect to see males develop more elaborate genitals (regardless of the complexity of female genitalia) through competition with each other. This isn’t to say that such competition is not a factor (in addition to being long, many a duck phallus seems to be equipped with barbs or brushes meant to remove the sperm of previous males), but I don’t see it being the driving factor in generation such complexity in females.
Part of the problem with trying to unravel the history of these organs is that we would need to determine the morphological and behavioral traits of the ducks from which the 16 researched species arose from. Did the ancestral ducks experience frequent forced-mating behavior? What was the correlation between the genitals of the males and females? I don’t find it unreasonable to surmise that the ancestral species would follow a pattern similar to the ducks without convolutions; if the females genitals are not complicated, why develop more elaborate genitals (outside of adaptations arising from male-male competition to inseminate the females)? It seems more likely to me that at some point the females, through variation, developed some kind of pouch or dead-end in their cloaca so that when a undesirable male would try to copulate he would be missing his mark (of course assuming that such behavior was established, which may not have been the case). The question is, however, how does such a trait become widespread through the population to allow further increases in complexity? I wouldn’t imagine that such a variation arose in just one female duck and then spread throughout the whole population, especially when 16 different species in 3 lineages and various genera. Indeed, if we could figure out when such traits arose and compare the reproductive tracts of the more complex ducks with relatives that do not such such elaborate equipment, perhaps we could start to get some clues as to how these changes came about.
Overall, I think the female-driven sexual selection hypothesis to be quite reasonable. If female ducks began to develop more complex reproductive tracts only males with a phallus that could successfully navigate such tracts would be able to pass on their genes, further variations continuing the selective pressure for change. I also found it interesting that the authors of the paper mentioned a possible source of pre-adaptation, the spiral-like organs of the female perhaps serving to keep water out of the reproductive tract rather than as a strategy against forced copulation. Personally, I like the idea, but liking an idea doesn’t make it so; we’d need to determine if such an organ was present in females prior to the development of an advanced male phallus. It is curious, however, the such corkscrews in the females go in the opposite direction of the males clockwise phallus, and I would imagine if the males were reacting to a pre-existing structure their reproductive organs would have matched the direction of the females. Again, if it’s possible to determine which came first, the complex phallus or the corkscrew, it would be an important piece of information to this mystery, but for now I have to simply say “I don’t know.”
I guess the biggest question I have at the moment is how did the females develop such complex organs? If it was some sort of pre-adaptation + sexual selection I think that would be reasonable, but that’s merely a hypothesis and I have absolutely nothing to back that up. Like I mentioned above, I am also curious how such traits came to be so widespread (again, perhaps this is pointing to some sort of pre-adaptation) in female ducks, and I think the development of male organs was in reaction to these developments. Also, how does forced copulation fit into all this? Yes, the sexual organs of female ducks may now be useful to prevent themselves from being inseminated by undesirables, but was this always the case, or did it have an effect on genital evolution? I can’t seem to get my head around the notion that the development of advanced reproductive tracts in females would be in response to forced copulations; how is the sperm of the desirable male more likely to spur the evolution of genital complexity than that of a non-desirable male?
I may be wrong on one or all of my ideas listed above, but more study certainly needs to be undertaken if we’re to figure out this mystery. As it stands now I think that male phallus evolution followed the evolution of genital complexity in female ducks, and that pre-adaptation may have had a significant role in allowing this to occur. I could be very wrong about all of this, but it is absolutely fascinating stuff and I hope that more scientists look into sexual selection and the interplay between the sexes. There will likely be more questions than answers for some time, but isn’t that what makes this line of work fun?