Bizarre Sex Organs II

Since
the last one of these I did was kind of scary, I decided to do
one that is not so scary.  It is just plain weird.
 But there is an interesting story to it.







The
female is on the left; the male is on the right.  These are
the genitalia of mallards: href="http://animaldiversity.ummz.umich.edu/site/accounts/information/Anas_platyrhynchos.html"
rel="tag">Anas platyrhynchos.
 The photo is from href="http://www.livescience.com/php/multimedia/imagedisplay/img_display.php?pic=070430_duckgenital_evolu_02.jpg&cap=The+genitals+of+the+mallard+%28Anas+platyrhynchos%29%2C+female+vagina+on+the+left%2C+male+phallus+on+the+right+%28size+bar+%3D+2+centimeters%29.+An+escalating+sexual+arms+race+between+male+and+female+mallards+over+which+sex+gets+to+control+reproduction+has+resulted+in+increasingly+longer+phalluses+and+ever+more+elaborate+vaginas.+Credit%3A+Patricia+Brennan&title=Ducks+Wage+Genital+Warfare&title=Ducks%20Wage%20Genital%20Warfare">LiveScience.
 The corresponding article is href="http://www.livescience.com/animals/070430_duckgenital_evolution.html">here.
 Hat tip goes to href="http://laelaps.wordpress.com/2007/05/01/scary-sex-organs-ducks-turtles/">Brian
Switek of Laelaps.



What is the reason for this bizarre configuration?  



Ducks are
among the few types of bird that retain the type of phallus seen it
reptiles.  Even among experts, though, it is not immediately
apparent as to why this anatomy would evolve:

After
looking at the genitalia of a male duck, "I became immediately
intrigued by what the female anatomy would look like to accommodate
such a bizarre organ," said behavioral ecologist Patricia Brennan, who
researches at both Yale University and the University of Sheffield in
Britain.

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The results of her subsequent research are detailed in everyone's
favorite journal, PLoS
One.



href="http://www.plosone.org/article/fetchArticle.action?articleURI=info%3Adoi%2F10.1371%2Fjournal.pone.0000418">

href="http://www.plosone.org/article/fetchArticle.action?articleURI=info%3Adoi%2F10.1371%2Fjournal.pone.0000418">Coevolution
of Male and Female Genital Morphology in Waterfowl

Brennan PL, Prum RO, McCracken KG, Sorenson MD, Wilson RE, et al.
(2007) Coevolution of Male and Female Genital Morphology in Waterfowl.
PLoS ONE 2(5): e418. doi:10.1371/journal.pone.0000418

Most
birds have simple genitalia; males lack external genitalia and
females have simple vaginas. However, male waterfowl have a phallus
whose length (1.5->40 cm) and morphological elaborations vary
among species and are positively correlated with the frequency of
forced extra-pair copulations among waterfowl species. Here we report
morphological complexity in female genital morphology in waterfowl and
describe variation vaginal morphology that is unprecedented in birds.
This variation comprises two anatomical novelties: (i) dead end sacs,
and (ii) clockwise coils. These vaginal structures appear to function
to exclude the intromission of the counter-clockwise spiralling male
phallus without female cooperation. A phylogenetically controlled
comparative analysis of 16 waterfowl species shows that the degree of
vaginal elaboration is positively correlated with phallus length,
demonstrating that female morphological complexity has co-evolved with
male phallus length. Intersexual selection is most likely responsible
for the observed coevolution, although identifying the specific
mechanism is difficult. Our results suggest that females have evolved a
cryptic anatomical mechanism of choice in response to forced extra-pair
copulations.

In other words, the females have developed a way to make it difficult
for males to impregnate them without the female's cooperation.