A Corkscrew By Any Other Name is Still Not Romantic

For most of the behaviors I’ve discussed so far there has always been some effort and choice in the matter of individuals mating. A male develops large plumage to entice a female to choose them, both sexes evaluate one another for health and have extra-pair copulations accordingly, and so on. However, nature isn’t all that nice most of the time and the avian world is not exception to that. Some species of bird have forces copulations, wherein the females have absolutely no say in the matter and are forced to mate with whichever male is strong enough to force the copulation. Ducks, as well as several other species of waterfowl, are well known for this forced copulation mating system and as such have developed some outright WEIRD strategies for both sperm competition as well as insemination avoidance.

Now at this point some of you will know what I’m talking about, but for the rest prepared to be baffled and be warned: there are graphic duck genitalia images ahead. But they’re really cool. Because male ducks have adapted to sperm competition by having corkscrew penises. Yes, you can go back and re-read that. In a mating system where force is the only factor is achieving a mating males are in essence taking a chance that any one female that have sex with will actually become pregnant with their offspring, as every other male in the pond is also copulating with as many females as they can find. As such, many males ended up with a corkscrew penis which, in the case of the Muscovy duck, is able to evert and become fully erect in 0.36s (Brennan, Clark & Prum 2010). The high explosive rate actually helps force the penis further into the vagina, and some species of duck even have brush like appendages along their penis in order to collect and in a sense, “scrub away” competitors sperm. 

 Male Duck shown with fully extended penis. They can reach up to 40cm in length.

Now this is all very impressive in itself, but it gets even more interesting when you look at how the females of these species have adapted to thwart the males. These species of waterfowl essentially have antagonistic sexual conflict, in that the females do not wish to copulate (I don’t think I need to elaborate on why) yet are continually forced into it by aggressive males. Well, the corkscrew shape of the penis didn’t just emerge from thin air. Female duck vaginas are actually also corkscrewed in shape, just in the OPPOSITE direction. At some point in more recent evolutionary time the direction of the vaginal geometry flipped, making it more difficult for the males to successfully fertilize the females. The same study done by Brennan et al on the explosive time also looked into the effectiveness of the penetration of those males into different shapes. They used glass tubes to see how far the penis was able to reach in what must have been the oddest experiment done to date.

They tested a straight tube, a counterclockwise spirals which matched the male penis chirality, and finally they tested a clockwise spiral and a 135˚ bend. The last two mimicked actual vaginal geometry found in Muscovy ducks, and they found that the penis was far less successful in reaching any great depth into those tubes compared with the straight and counterclockwise spirals. This supports the idea that difference in corkscrew direction coevolved between the two sexes of many waterfowl species. The rather abused ladies had found a way to fight back. 
 

Example of oppositely spiraled genitals as well as the tubes used.

What’s more, in some species of duck females have even been found to have several “dead ends” in their vaginal tracks. Enfolds of the vaginal lining provide a sort of diversion to the penis, which can get re-directed into pockets that lead nowhere and store the sperm away from any of the actual eggs. This prevents fertilization while allowing the male to believe that he has successfully fertilized that particular female and move along.

So there you have it. Nature is not always pretty, but when organisms are faced with extreme challenges they will adapt, even if it’s to defend against their own species. And some of those adaptations can be pretty screwy!

References

Brennan, PLR, Clark, CJ & Prum, RO 2010, ‘Explosive eversion and functional morphology of the duck penis supports sexual conflict in waterfowl genitalia’, Proceedings of the Royal Society B: Biological Sciences, vol. 277, no. 1686, pp. 1309–1314.

http://www.kiwiblog.co.nz/2012/10/the_ducks_will_get_you_in_the_end.html

http://blogs.discovermagazine.com/notrocketscience/2009/12/22/ballistic-penises-and-corkscrew-vaginas-the-sexual-battles-of-ducks/