科学家研究发现母鸭已经进化出一种避免因遭受具有螺丝锥形状大阴茎而且颇有侵略性的公鸭强行性交而受孕的奇妙方式：那就是母鸭具有顺时针螺旋的阴道，这样就能阻碍具有反向螺旋的雄性生殖器的进入。

关于这种在外生殖器方面发生的性别进化之战的详细内容发表在《皇家学会学报 B》(Proceedings of the Royal Society B) 杂志的第 12 月 23 日号刊物上。

“在强迫性交司空见惯的物种内，雄性通常已经进化出较长的阴茎，但是雌性则共同进化出具有和雄性阴茎相反的回旋方向而且一端不通的回旋状阴道，”该论文的主要作者兼生态学和进化生物学系的博士后研究员 Patricia L.R. Brennan 说。“这种共同进化是在两性的冲突之间由控制受精的一方所产生的。”

这项研究是以 2007 年耶鲁大学首次描述了鸭子性器官奇异生物形态的一份报告为基础的。尽管大多数鸟类并没有阴茎，但是鸭子却具有相对较大而可以弯曲的阴茎 – 它可长达 20 厘米并缩拢在公鸭体内。性交时，公鸭会在母鸭体内伸展或翻转其阴茎。耶鲁大学的 Brennan 及其同事们首次利用高速摄像技术展示了鸭子阴茎的爆发式勃起，整个过程仅耗时半秒钟 – 这被耶鲁的研究小组称之为“爆炸式性交”。

在有很多强迫交配的情况下，这样大的阴茎对于雄性而言本应当是有利于生殖的。不过，耶鲁的研究小组假设雌鸭可能会凭借其复杂的外生殖器而使雄性的性交变得困难。但某些水禽具有重叠螺旋的阴道是否能有助于雌鸟排斥强迫性交呢"

为了证实这一假设，Brennan 及其同事们检查了鸭子的阴茎在一组具有不同形状玻璃管内的翻转情况。笔直的玻璃管，或者和雄鸭的阴茎具有相同逆时针方向旋转的玻璃管都没有减慢阴茎勃起的过程。但是，模拟雌性阴道顺时针螺旋或突然弯曲的玻璃管则完全阻止了阴茎的翻转。这些资料表明雌鸭已经进化出阻碍强迫性交的解剖学机制，同时也为在繁殖方面性别冲突的进化结果提供了全新的见解。

为了控制繁殖而在解剖学方面发生的进化竞赛无疑是自然界性别冲突更为戏剧性的例证之一。

“尽管我们预先知道性别冲突是普遍存在的，而要发现两性之间具有如此戏剧性“军备竞赛”的体系则真是非常难得的。鸭子为我们提供了理解性别冲突进化结果的难以置信的机会，”Brennan 表示。

原始出处：

Proceedings of the Royal Society B December 23, 2009, doi: 10.1098/rspb.2009.2139

Explosive eversion and functional morphology of the duck penis supports sexual conflict in waterfowl genitalia

Patricia L. R. Brennan1,2,*, Christopher J. Clark1,2 and Richard O. Prum1,2

1Department of Ecology and Evolutionary Biology, Yale University, New Haven, CT 06511, USA
2The Peabody Museum of Natural History, Yale University, New Haven, CT 06511, USA

Coevolution of male and female genitalia in waterfowl has been hypothesized to occur through sexual conflict. This hypothesis raises questions about the functional morphology of the waterfowl penis and the mechanics of copulation in waterfowl, which are poorly understood. We used high-speed video of phallus eversion and histology to describe for the first time the functional morphology of the avian penis. Eversion of the 20 cm muscovy duck penis is explosive, taking an average of 0.36 s, and achieving a maximum velocity of 1.6 m s?1. The collagen matrix of the penis is very thin and not arranged in an axial-orthogonal array, resulting in a penis that is flexible when erect. To test the hypothesis that female genital novelties make intromission difficult during forced copulations, we investigated penile eversion into glass tubes that presented different mechanical challenges to eversion. Eversion occurred successfully in a straight tube and a counterclockwise spiral tube that matched the chirality of the waterfowl penis, but eversion was significantly less successful into glass tubes with a clockwise spiral or a 135° bend, which mimicked female vaginal geometry. Our results support the hypothesis that duck vaginal complexity functions to exclude the penis during forced copulations, and coevolved with the waterfowl penis via antagonistic sexual conflict.