When you see a bull curl his upper lip back and seemingly try to smell something in the air, it's an example of the flehmen response (German word, meaning to “curl the upper lip”). The flehmen response is a type of curling of the upper lip in ungulates and other mammals (and even reptiles), which helps to faciliate the transfer of pheromones and other scents into an organ called the vomeronasal organ (also called the "Jacobson’s Organ"). When an elk uses the flehmen response, they draw back upper lip which curls back toward the nostrils. They do this when examining scents left by other animals. The action of curling the upper lip back helps to to expose the vomeronasal organ (Jacobson's organ) and draws scent molecules in and towards the organ. This behavior allows the elk to detect airborne scents, like from urine, of other elk or even the presence of predators. The flehmen response also allows elk, particularly bulls, to determine the presence or absence of estrus. Anybody have anything else to add to my dry explanation
