Next time you’re about to grind up against a hot woman on the dance floor, consider this—she might be very fertile. New research shows that men are like fertility detectors, picking up signals that women put off while they dance or walk.

In a study published in Personality and Individual Differences, German researchers asked men to watch videos, and rate the attractiveness of women while they danced and walked.

The women were filmed at two points in their menstrual cycle—when they were non-fertile, and closer to the date of ovulation when they were most fertile. To prevent men from judging the women on visual appearance, like hair and eye color, researchers converted the videos so only the outlines of the women were visible.

Even while watching only a silhouette, men clearly found the women’s movement more attractive when they were most fertile. Other research has found similar effects of a woman’s cycle on men. In one study, lap dancers earned more money when they were close to the date of ovulation.

The German researchers, however, can’t say whether changes in the movement of fertile women is a side effect of the spike in estrogen near ovulation, or if hotter dancing is a clear signal to men that there are fertile women around.

Either way, when you are scoping out women on the dance floor, you might be attracted to more than just their moves.