Sci is going to so some selfish Friday Weird Science today. Selfish, because this article isn't new, and was reported on by one of the GREATS. This guy. He (of course) did a completely brilliant job, and when he talked about it at SciOnline this past weekend, Sci was compelled to go and see the material for herself. And it's something to SEE. And so see it you shall.
The reason this is selfish is because Sci knows it's been reported on before. She wants to do it her ownself, as an excuse to read and understand the paper. And an excuse to giggle about it while she blogs it. There's that, too.
Images below the cut NSFW, especially if you work with ducks.
Brennan, Clark, and Prum. "Explosive eversion and functional morphology of the duck penis supports sexual conflict in waterfowl genitalia" Proceedings of the Royal Society B, 2009.
There were a couple of things that Sci learned from this paper:
1) ducks are one of the few types of birds to have a penis (it's a typical formation in waterfowl). This is because, in most species, semen is ejected from the cloaca of a male into the cloaca of a female (they touch cloacae to do this, in what's called a "cloacal kiss").
A bird cloaca. It's a little hard to see, but the tail feathers are stick up at the top, feet are at the bottom. And in the middle is the hole.
In male ducks, however, it's a little different. They have a penis which enters the cloaca and oviduct of the female, and the semen comes out the tip. Like ya do.
2) the penises of waterfowl are not made erect by blood flow, but by LYMPH. During erection, lymph fluid accumulates in the penis and makes it extend outward. Crazy, huh? Lovin' that convergent evolution.
So why, you might ask, do ducks have a penis? If other birds don't really seem to need it, why do ducks? The hypothesized answer is that a lot of copulation (that's sex) in ducks is actually FORCED. The girls are not enthusiastic, and the males have to find a way to hold them down.
And this is where evolution comes in. It's not a good idea for the females to just be having any dude's babies, she wants to make sure she gets the fittest duck of the bunch. So she wants to make her oviduct access more difficult. In the case of ducks, it means that, while the males penis comes out in a spiral in ONE direction, the female's oviduct has evolved to spiral THE OTHER WAY, as you can see here:
The spiral that comes out for the males can be almost the length of the entire duck!
Not only that, the female's oviduct has lots of little blind ends and pouches off the end of it, making it easy for the male's penis to get stuck in blind ends, or to not penetrate successfully at all. In return, the male's penis is flexible, allowing him to bend around and try to get through the backwards shape. This phenomenon of sex one-upmanship is called antagonistic coevolution. To prove that this does happen in ducks, the authors of this study did slow motion video capture of duck penises. Not only that, they did slow motion capture of duck penises going into tubes of various shapes, so show that the antagonistic evolution of the female's oviduct presented a mechanical barrier to the male duck.
Their data is self-evident:
(I think that would be SO much better if there was a hardcore electric guitar riff going on in the background).
Note the semen coming out at the tip of the penis (there's some fluid at the base that I mistook for semen at first). This process is slowed down in the video, but in reality takes 1/3 of a SECOND. EXPLOSIVE. *BAM*!!!
They they had the ducks do their thing into a series of tubes. They started with silicone tubes, but ended up switching to glass, because the duck's penises were so powerful they BROKE the silicone tubes:
(This one I think needs more heroic music. Something with trumpets.)
So why would female ducks have evolved so many barriers? This way, when they are forced into copulation, the males forcing them won't get as far in, and will be delivering sperm nearer the cloaca. When the female pair bonds to the mate of her choice, on the other hand, she'll be more cooperative (or at least not trying to dunk him or fly off), and he'll get further up the oviduct and have a higher chance of making sure the babies are his.
There is also a possibility that the females can store sperm from the other males in the little pouches off their oviducts, but this seems kind of unlikely. Females bond to the men of their choice long before the breeding season begins, and so it is not in their interest to get some other dude's babies in there. But this still should be tested (and given the depth of Brennan's work in the duck, it might well be tested).
So there you have it, the twisted evolution of the duck penis.
Brennan, P., Clark, C., & Prum, R. (2009). Explosive eversion and functional morphology of the duck penis supports sexual conflict in waterfowl genitalia Proceedings of the Royal Society B: Biological Sciences DOI: 10.1098/rspb.2009.2139