Ovulation is a really useful biological function. Not only does it facilitate pregnancy — though sperm are in no short supply, the ephemeral egg appears just once a month — but new research finds that it also helps a woman select potential partners by enhancing her "gaydar."

All this complex sexual decision-making is going on behind the scenes, according to a study published online this week in the journal Psychological Science that found that straight women at their peak period of fertility are far more accurate than non-ovulaters at sussing out who's gay and who's not just by looking at a man's face.

"We consistently find that people have no idea they are able to do this," says Nicholas Rule, an assistant professor of psychology at the University of Toronto and the study's lead author. "They come out of the experiment completely frustrated and say, This is so hard, no one can do this, and then we look at the data and they're doing amazingly well."

Rule and colleagues at Tufts University put 40 Tufts undergraduate women — all of whom were heterosexual and ovulating — through three experiments designed to test their hypothesis that women pay more attention to men's sexual orientation when they're extremely fertile.

First, the participants were asked to look at 80 images of men's faces; half the photos — which were similar in terms of expression and attractiveness — belonged to gay men, while the other half featured straight men. A participant's ability to determine the sexual orientation of the men in the photos was closely associated with how close she was to peak ovulation.

"The closer you get to peak ovulation, accuracy goes up, up, up, peaks at ovulation, then starts to go back down again," says Rule. "There is a linear effect."

Then, researchers substituted 100 female faces — half straight, half lesbian — and performed the experiment again. This time, they found no association between fertility and so-called gaydar, an informal term referring to the ability to intuit a person's sexual orientation.

"It's not just that women are more attentive to nonverbal cues around ovulation," says Rule. "It's really something specific about paying attention to men's sexual orientation."