Flehmen response

The Flehmen response is a particular type of curling of the lips in ungulates, felids, and many other mammals, which facilitates the transfer of chemicals into the vomeronasal organ. This behavior allows animals to smell the urine of others in order to determine several factors. These factors can be the presence or absence of estrus, the physiological state of the animal, and how long ago the animal passed by. This particular response is most recognizable in stallions when smelling the urine of a mare in heat.