Why Do Cats Open Their Mouth When Smelling?

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Cats open their mouth as part of the flehmen response/reaction. The flehmen response is a behavior where the cat elevates their upper lip and opens its mouth for about 5-10 seconds allowing the transfer of pheromones and other scents to the Jacobson’s organ/vomeronasal sensory organ (VNO) located just above the roof of the mouth, behind the teeth in the nasal septum.

The VNO triggers an appropriate behavioral response from the cat when it comes into contact with specific pheromones and scents, like the scent of another cat.

image of black and white cat with yellow eyes picking up scent and exhibiting the flehmen response
close up of a black and white cat picking up a scent on a grey background

When do cats perform the flehmen response?

A cat performs the flehmen response when investigating sites of particular interest to it, odors or tastes. This can help the cat identify the presence of another cat or food.  It also has a variety of other uses: Male cats use it to identify the presence of a female cat in heat, whilst female cats use it to track the whereabouts of their children to name a few examples.

Cats also use the VNO when performing scent rubbing to help distinguish between similar smelling substances before performing the rubbing.

The Flehmen response in other animals

The Flehmen response is also seen in other animals such as dogs, horses, cattle, pigs, goats, big cats, snakes and lizards to name some..Differently from cats, horses and other hoofed animals curl their upper lip and reveals their teeth rather than opening their mouth as cats do.

Snakes also has a similar response when they stick their tongue out to facilitate the transfer of scents by gathering them with the tongue and touching the tongue to the opening of the organ.

What is the Jacobson’s organ / vomeronasal organ?

The Jacobson’s organ was named after the Danish surgeon Ludwig Lewin Jacobson when he rediscovered the organ in the human nose. The organ had first been discovered by Frederick Ruysch prior to 1732. Jacobson announced his findings to the world in 1809. 

Although present in humans, the organ is non-functional. 
The organ got its other name, the vomeronasal organ (VNO for short) from it’s adjacent unpaired bone (the vomer bone in latin) and its location in the nasal septum.