Wow, I really had a tough time coming-up with a title for this post! There were so many possibilities…
In the December 23rd edition of the journal of the Proceedings of the Royal Society B, several scientists from Yale University detailed their recent experiments into the coevolution of male and female duck genitalia. Specifically, the researchers wanted to evaluate whether or not the physical morphologies exhibited by duck penises and duck vaginas provided evidence of ducks being actively engaged in a sexually antagonistic ‘arms race.’ In hopes of resolving this reproductive riddle, the folks from Yale traveled to a commercial duck farm in California, while equipped with artificial duck vaginas, mineral oil and a high speed video camera.
Sexual conflict arises when the strategies used to optimize reproductive fitness differ between the males and females of a species. In the case of muscovy ducks, like those used in the Yale research, the strategy of the male is to mate as frequently as possible, and with as many different females as he can find – even if sexual coercion is the only means available to achieve these goals. Conversely, the strategy of the female is to select only the healthiest of males as reproductive partners and to outright reject all those deemed unworthy. Because the male and female strategies are in conflict – quantity vs. quality - both sexes have undergone adaptations to out-maneuver the opposite gender. In this back-and-forth battle for reproductive victory the process of sexual selection has discovered an affective tactic - to change the physical shape of the duck’s genitalia.
In addition to being one of the few birds with a penis at all, the phallus of the male muscovy duck is physiologically and morphologically very unique. Unlike the penises of most mammals, which when erect are hydraulically rigid and supported by axial-orthogonal layers of inextensible collagen fibers, the penis of the muscovy has an arrangement of supportive fibers that allow for complete flexibility while erect. Besides being highly limber, the shape of the muscovy’s twenty centimeter long penis is ‘twisted’ forming a distinctive counter-clockwise spiral. These functional traits allow the male to very quickly insert into the female during copulation.
The speed at which copulation occurs is of vital importance to the male, it gives him the ability to inseminate even those females that actively defy his advance. Physical resistance is minimized because the male can insert his penis at a speed of about 1.6 meters per second and complete insemination in just 0.36 seconds – that doesn’t leave much time for fighting. Since the male’s seemingly invincible swiftness eliminates combat as a pragmatic brood planning option, the female line has employed an anatomical counter adaptation to ensure its continued mate choice.
As a defensive escalation in the muscovy’s inter-sexual arms race, the female has undergone adaptation for a counter armament – she’s changed the shape of her vagina. The female duck’s vagina can be found to exist in one of two possible shapes, either with a 135-degree bend, or with a clockwise ‘twist.’ During forced copulations, the morphology of the vagina functionally prevents the male’s penis, or his sperm, from entering sufficiently far into the reproductive tract to result in fertilization. However, when the female muscovy is receptive to a male suitor – when SHE chooses a mate – she actively facilitates the sexual congress by taking on a body posture with an elevated tail. The elevated tail, in combination with muscular contractions, permits the male duck to penetrate further into the reproductive tract and thus improves the likelihood of successful fertilization.
To uncover the fascinating story of co-evolving duck genitalia, the researchers from Yale presented aroused males with replicated duck vaginas. In conducting the experiment, female ducks were placed in a cage with a male, once the male initiated a mounting behavior, the female was quickly removed and the male was filmed with a high speed video camera as he inserted his penis into one of four different shaped molded test-tubes. The mineral oil covered test-tubes either had a straight shape, mimicked the male’s penis with a counterclockwise twist, or they had a female-like clockwise twist or 135-degree bend. After analyzing the film, the scientists discovered that the replicated vaginas that matched the shape and dimensions of real-life female muscovies effectively blocked the male’s penis from reaching very far into the test-tube.
Patricia L. R. Brennan, Christopher J. Clark and Richard O. Prum (2009). Explosive eversion and functional morphology of the duck penis supports sexual conflict in waterfowl genitalia Proc. R. Soc. B
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