Forget romantic tales of “the first time I laid eyes on her”—many male animals flare their nostrils when they’re in heat to gauge which females want to knock boots. Animals like hippos and horses use this grimace (called the “flehmen response”) to open glands in the back of the throat that are designed to sense compatible female pheromones. From there, he knows whether to make his move, and the rest is romantic history. Surprisingly, human men have these same glands in the roof of the mouth as well. So next time your man flares his beak? Maybe you’ll have him hooked right then and there.

STUDY: Fertile Women Can Smell Attractive Men