Vomeronasal organ

The vomeronasal organ, also called Jacobson's organ, is a pair of small fluid-filled sacs above the roof of the mouth, just behind the upper front teeth. Narrow ducts connect them to the mouth and nose. They give cats a second sense of smell, tuned to chemical messages instead of ordinary odours.

The organ picks up pheromones, the scent signals that animals leave in urine, on skin and in other body fluids. These can tell a cat about the age, sex and breeding condition of another cat that passed by. Signals travel from the organ to parts of the brain that handle social and reproductive behaviour, which is partly why such scents can change how a cat acts.

To use the organ, a cat performs the flehmen response: it parts its lips, holds its breath and draws the scent up to the ducts. You may see this at a scratching post, over a patch of urine or after a cat sniffs a shoe. It is ordinary behaviour. Frequent gaping together with drooling or reluctance to eat is worth a call to your vet.