Sexual Signaling

This doesn't surprise me at all:

Bennett Galef's team at McMaster University in Hamilton, Ontario, Canada, placed two male rats behind mesh screens at either end of a tank, one of which had recently copulated with a female. A number of different female rats were then placed, separately, in the middle of the tank. Galef's team found that these females typically moved over and lingered longer near the stud. When trials were repeated using female rats with no sense of smell, they showed no preference for either male...

Or, in short, females use the scent of recent sex as a signal of mate quality. Only the males actually in high demand are able to send that signal at low cost.

Signaling in the mating/dating market has been a popular topic in the pop-econ literature. In The Economic Naturalist, for instance, Robert Frank addresses the question of why it is easier to find a partner when you already have one. He relates the story of a young man who got much more attention from the ladies in a bar when accompanied by a female friend. The explanation?

"It's hard to tell whether someone's okay just by looking at him," she explained. But because other women in the bar understood that attractive women are generally in demand by men, the fact that she, an attractive woman, was paying such close attention to a man she obviously knew well was a credible signal that he was okay.

This topic also comes up, at least tangentially, in Tyler Cowen's Discover Your Inner Economist, and it might also make an appearance in Tim Harford's The Logic of Life, though I can't recall with certainty.