보통 sex에 관한 battle이라면 자신의 씨를 뿌리기 위한 수컷들끼리의 경쟁을 생각하게 되는데 그러한 상식을 깨는 흥미로운(조금은 민망스러운) 기사이다.

과학 전문지가 아니라 조금 미심쩍기는 하지만 이 정도 수준의 잡지라면 검증되지 않은 내용을 기재하지는 않았을 테니…

Sexual selection | A game of ducks and drakes
May 3rd 2007
From The Economist print edition 

The evolutionary battle of the sexes has some curious consequences

SEX, in most species of bird, is a consensual activity. It has to be. Males have no penises and are armed with a genital opening which looks little different from that of a female. Intercourse happens when these two openings are brought together in what ornithologists refer to as a cloacal kiss. In these circumstances, rape is a difficult option.

Drakes, however, are notorious rapists—forcing their attentions on ducks indiscriminately—and it is surely no coincidence that they are among the 3% of male birds that do have a penis. In fact, drake penises come in a wide variety of shapes and sizes that are thought by students of the subject to be part of an arms race to ensure that it is the owner’s sperm that fertilise the next generation of ducklings, rather than anybody else’s.

The question is, an arms race against whom? The males of many species of insect have similarly elaborate genitalia. These seem designed to compete directly against other males—for example by scraping out the sperm of previous suitors or breaking off and blocking the female’s genital opening. But Patricia Brennan, of Yale University, and her colleagues suspected that in ducks and drakes the arms race might be between the sexes rather than between members of the same sex. Females, in other words, would rather choose which males inseminate them. And if rape is inevitable, evolution might provide them with other ways of making this choice.

Surprisingly (or perhaps not, given the male-dominated nature of science) the elaborate cataloguing of drake genitalia had not been extended to ducks. So Dr Brennan got out her scalpel and started looking. As she reports in the Public Library of Science, she discovered that the sexual passages of ducks are every bit as elaborate as the penises of drakes are. Indeed, for every species, the more elaborate the latter were, the more elaborate were the former. In particular, duck vaginas are often equipped with additional passages that have blind endings, and frequently corkscrew in a clockwise direction, in contradistinction to the anticlockwise thread of a drake’s penis.

Dr Brennan interprets these anatomical flourishes as anti-insemination mechanisms. The contrary threads of the genitalia make successful penetration much harder if the female is not co-operating. The blind passages make it likely that sperm will be ejaculated into the wrong place and thus fail to fertilise the duck’s eggs. Conversely, a willing female should be more easily inseminated.

That interpretation is backed up by the fact that ducks with the most elaborate genitalia are those from species where the drakes have the largest penises. These species are also the ones where rape is most often observed. In ducks and drakes, it seems, size matters. But not in a way that is good for females.

http://www.economist.com/science/displaystory.cfm?story_id=9111447