Cats, like many other animals, have an extra olfactory organ, called the vomeronasal organ or Jacobson’s organ, that helps them sense scents and excretions from both other cats and their prey.
The organ is very well developed in some animals, like reptiles, and almost vestigial in humans.
In cats, it is a pouch above the hard palate that communicates with the nasal and oral cavities through a duct that opens behind the incisor teeth.
The organ is believed to perceive pheromones, the chemical odors in urine and glandular secretions, and possibly the secretions of other organs, as well as the residual scent marks that cats and other animals leave in the environment.
At mating time, males may show an open-mouth response to the urine of a receptive female.
Called the Flehmen response, it involves exposing the teeth by retracting the upper lip and sniffing.
Many cats, both male and female, exhibit this response when the vomeronasal organ picks up the scent of a chemical in catnip called nepetalactone.
Some may also show the response as they pick up the scent of animals they are pursuing.