The Flehmen response (also called Flehmening or Flehming) occurs when an animal senses the urine of another. The animal curls back its upper lip exposing its front teeth and inhales, often with the nostrils closed. Although it looks somewhat menacing, it is not to be confused with a threat display. Animals use it to gain information about the other animal that excreted the urine.
These two lion brothers were "hot on the trail" of a pride to which they did not belong. The pride had moved away, perhaps trying to avoid a confrontation. Male lions from outside of the pride are likely to kill any young cubs they did not father. The death of the cubs brings the females of the pride into estrus, with the possibility of subsequent mating. Photo © copyright by Dr. Edward Mikol.