Why does my cat open his mouth after smelling something?

What it often means

• Another cat’s scent: a 1987 study found that in cats, flehmen is mostly triggered by other cats’ urine.
• A two-step analysis: the nose smells first, then the open mouth lets the molecules travel through two tiny ducts behind the upper front teeth to the vomeronasal organ.
• Information about the other cat: these chemical marks probably tell him who left them and what hormonal state that cat is in.
• Other strong smells: your shoes, a bag brought in from outside, a spot where another animal has been can trigger it too.

Signals to read at the same time

• The duration: a few seconds with the mouth half open, then he goes back to what he was doing. A mouth that stays open longer is something else.
• What he just smelled: a urine mark, your shoes, something brought in from outside, the spot where another cat sat.
• His breathing: flehmen is a frozen pause of a few seconds, not panting. Fast, open-mouth breathing with heaving sides is an emergency.
• What follows: he moves on, rubs his cheeks, or tenses up at the window with a lashing tail. An unfamiliar cat may be marking near your home.
• His mouth: drooling, chattering teeth, pawing at his mouth or trouble chewing are not part of flehmen.

What you can do

• Let him do it without interrupting: he is reading a message, and it is a normal part of being a cat.
• Let him sniff your shoes and bags when you get home: it is information for him. Put them away only if he starts marking them.
• If he does it often by the doors or windows and tenses up afterward, look outside for a neighborhood cat’s marks and clean them with an enzymatic cleaner.
• If a cat outside gets him worked up, cover the bottom of the window with frosted film: fewer face-offs, less tension.
• If you cannot tell flehmen from trouble breathing, film a few seconds of it to show your vet.

When to see a vet

Flehmen is nothing to worry about. Go to the vet right away if your cat breathes with his mouth open at rest or after light activity, pants, or breathes with heaving sides: in cats, that signals respiratory or heart distress. Also see a vet if he drools, keeps his mouth half open without having smelled anything, paws at his mouth or struggles to eat: dental pain, an injury or something stuck in the mouth.