Flehmen is a term used to describe a behaviour where a horse extends their neck, raises their head and curls back their upper lip showing their teeth. Flehmening is a mammalian behaviour not only seen in horses but also performed by many mammals including cats, dogs, cattle, goats, giraffes, lions, tigers and rhinoceros.
The flehmen response is used as another form of communication between animals. With the lip curled back the air is sucked through to the roof of the mouth and the palate where the vomeronasal organ (VNO) is located. The VNO is an olfactory-chemosensory organ which detects pheromones and scents in the air. When certain scents are detected, signals are transmitted to the brain which can then trigger various behavioural and/or physiological reactions.
The flehmen response is commonly seen in mares after giving birth. Stallions often do it when around a mare to determine if they are in oestrus and ready to mate.