The Evolutionary Dynamics of Sexual Selection

In the Economist:

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.