Twisty maze of duck oviducts


I'm sure you've already heard about it, so I'm a little redundant to bring it up — Carl Zimmer has a spiffy article in the NY Times about duck phalluses. No, that's not quite right; the most interesting part of the story was the bit about duck oviducts. Female ducks have been evolving increasingly convoluted oviducts to baffle the efforts of duck rapists to inseminate them, and male ducks have been evolving concomitantly long phalluses to thread the maze and deliver sperm to the ovaries.

I'd heard about these huge intromittent organs in ducks before, but this is another fascinating revelation: it took a woman scientist to suggest that maybe, just maybe, they also ought to look at what's going on in the female ducks, and then the whole wonderful story of coevolution of these structures emerged. It's actually a rather embarrassing instance of a scientific blind spot, where the biases of the investigators led them to overlook an important component of the story.


Of course, they don’t evolve to baffle the efforts of duck rapists to inseminate them (that’s just the result of selection), they are selected if that’s a better strategy for successful reproduction. What I don’t understand is why it’s a better strategy. Are duck rapists less virile? In other words, do they produce fewer or less successful offspring than the mate that the females bond to?

Are duck rapists less virile? In other words, do they produce fewer or less successful offspring than the mate that the females bond to?

My (totally uninformed) guess is that their offspring suffer from this lack of bonding, and their female victims suffer from the burden as well, and are thus disadvantaged relative to the female who is better able to avoid non-consensual insemination.

Then again, I don’t know quack about ducks, so maybe the problem arises from the duck rapists’ failure to pay the AFLAC premiums on time…

Well, one way or another, that’s true, since the ducks seem to be evolving towards biologically enforced consensual mating. But how do they “suffer”?

I’m imagining big enforcer ducks from AFLAC beating on the rapists.… Maybe swans, as they really can be nasty.

Maybe the lack of bonding means that single moms can’t defend their turf against cooperating couples? Or that the young don’t get properly socialized, and thus can’t go on to start families of their own? Or maybe a female who is raped is injured in the process and thus disadvantaged in doing…whatever female ducks do to help their young grow up?

Geese, too.

You guys are probably right and pair bonded ducks probably do have an advantage when rearing young. However, if paternity is uncertain than the bonded pair might raise the young regardless of who the father is.

It is also possible that female choice might play a role. That is to say that if the female is able to discriminate and mate preferentially with males of superior phenotype (and genotype) then her offspring might have a selective advantage. Thus, being able to prevent forced copulation might confer an advantage and be selected for.

In any event, it is certainly not uncommon for different kinds of sexual selection to produce results this spectacular. Still, the pictures could make some male humans develop feelings of inadequacy.

I’ve got an alternative idea. Foot size in ducks would certainly be selected for (for swimming ability).

You’ve all heard about the supposed correlation in human males between foot size and…, well, you know. Suppose it’s really true for ducks. Then there would certainly be an advantage for the female ducks to select for male ducks with the largest… foot size.


Now the literature and art world have to retract all those faulty depictions of Leda and the Swan based on insufficient background research; it was probably a duck.

GvlGeologist Wrote:

Suppose it’s really true for ducks.

But that is the catch, isn’t it? As I understand it, the mammalian penis is basically grown by the same growth controls as for “other extremities”, and individually their appendages, in turn basically being the same as for the body plan, thus possibly having some correlations here. (Which, I think, with no one among the various possible ones established outside folk belief. Or have I missed some research here?)

But the duck “intromittent” (sp?) organ, how is it grown? And why do the ducks loose it between seasons? (And how does that feel? :-| ) Questions abound.

GvlGeologist, FCD Wrote:

What I don’t understand is why it’s a better strategy. Are duck rapists less virile? In other words, do they produce fewer or less successful offspring than the mate that the females bond to?

Probably. Apparently a lot of female ducks, mallards included, have an “inciting display…” basically, behavior that tells their bonded male to go fight whoever’s around. They use it, for instance, when a strange male approaches. So the bonded males have already proved themselves, by winning a bunch of fights in front of the female, and their genes are likely to be superior to a stranger’s.

Normally you’d also raise the question of paternal care, as others have in this thread. But mallard males don’t care for their kids anyway; they leave after incubating the eggs for a week. So it seems unlikely to me that they could effectively “punish” the female for extrapair copulation by neglecting her offspring.

(From what I can glean from Google, there’s one more bit of evidence in favor of “good genes” and against “good parenting” in this particular case: Female ducks are also unusually uninterested in any kind of extrapair copulation; unlike, for instance, most female songbirds, who willingly mate with a large/bright/loud/otherwise sexy male when their own male’s out of sight. That suggests that female ducks are already bonded to the best male available, genes-wise.)

It’s a lot like the Sexually Antagonistic evolution in Drosophila, where the males are constantly evolving new toxins in their semen that make the females lethargic after mating in order to insure the offspring are the males, females are selected for resistance so they can mate with lots of males.

I’m no expert on duck mating rituals, but I do recall that in most birds, the female chooses a mate based on perceived genetic fitness (as keyed by some trait, such as feather color, song, vigorous display, etc.) If that’s the case in ducks, then a “male duck rapist” would be a male not chosen by a given female because he wasn’t perceived as genetically fit. The selective advantage in confounding involuntary matings, then, would be that a less-fit male would be less likely to mate successfully (if forcefully) with a given female, and the offspring would likely be more fit. One could think of the “rapist” male in ducks as not terribly different from, say, “interloper” males in frogs. In this case, though, it’s the females evolving a manner of preventing the interloping, thus helping to maximize the chance of successful insemination by a more-fit male.

Just a hypothesis, of course.

Yea, I was really curious as to why this would be an advantage. If the female duck made it harder for rapist, this means that the offspring would have to be at a disadvantage, and the female ducks with the more complicated oviducts produced more successfully by mating with a bonding male that stuck around to help out with the eggs. But then what explains the pressure of male rapist ducks having longer phalluses since their offspring would be at a disadvantage?

Maybe it’s the bonding males that successfully breed that pressue the length of the phalluses. In fact, the more I think about it, that makes sense.

Speaking of which, there’s a couple of Canadian geese outside my work window who have been guarding an egg for the past several weeks. It’s been fun to watch. The male is pretty aggressive if you come near the nest. It’s really sweet to watch them work so hard to protect the one egg. I’ve also witnessed quite a few duck rapes before and it’s awful to watch.

Wow, the square peg in round hole’ idea has nothing on this opposite handedness in duck genitalia. Almost ‘ducks are from Venus; drakes are from a mirror universe’. Intelligent design, my aunt Fanny. Really clever, these ducks.

Maybe it’s the bonding males that successfully breed that pressue the length of the phalluses.

I suppose that the bonded male would still have to be able to get through the obstacle course (so to speak).


Why do female ducks choose lighter colored ducks for mating?

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This page contains a single entry by PZ Myers published on May 1, 2007 3:04 PM.

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