When a bull elk lifts his head and curls his upper lip into a grimace, he’s not just acting tough for the ladies—he’s trying to figure out if his love life is going to pick up. The snarl is called the flehman response and by curling their upper lips bulls expose their vomeronasal organ (located in their palate) that is used to detect estrus in cow urine, which in turn lets the bulls know when it’s time to go courting. Elk are not alone. Other ungulates and cats, big and small, perform the curl.
Unlike whitetail or mule deer does, cow elk don’t advertise when they’re in heat through altered urination stances or any obvious physical displays. If they did, it would only draw unwanted harassment from the rivals of their herd bull, who already pester them enough.
Often, a herd bull has to approach cows in his harem in a non-threatening mating stance, sometimes repeatedly, to find out if the time is right. Since bulls can’t read minds, the vomeronasal organ and the grimace saves them a little rejection and wasted time.