When your cat sniffs something and then holds their mouth open with a slightly glazed expression, they’re using a second, more powerful sense of smell. This behavior is called the flehmen response, and it lets cats analyze chemical signals that their regular nose can’t fully decode. That frozen, open-mouthed stare isn’t disgust or confusion. It’s deep concentration.
What’s Actually Happening Inside the Mouth
Cats have a specialized sensory structure called the vomeronasal organ (also known as Jacobson’s organ) tucked into the roof of their mouth, just behind the upper front teeth. This organ is separate from the nasal passages and is wired to detect a specific category of chemicals: pheromones and other scent molecules that carry social information.
When a cat catches a whiff of something worth investigating, they’ll sniff the source, then raise their head and hold their mouth slightly open. Their tongue curls into a flicking position that helps push air upward toward the openings of the vomeronasal organ. The result is that signature look: lips pulled back, mouth agape, eyes slightly unfocused. Some people describe it as a grimace or a look of sheer disgust, but the cat is actually running a sophisticated chemical analysis.
What Cats Are “Reading” With This Sense
The vomeronasal organ is tuned to pick up pheromones, which are chemical signals that animals release into their environment. These signals carry a surprising amount of information. A single sniff-and-analyze sequence can tell a cat whether another animal has been in the area, whether that animal is a friend or a stranger, whether a female is in heat, and even how stressed the other animal was when it left the scent behind.
Common triggers include:
- Another cat’s urine or scent marks: These are packed with pheromones that communicate territorial boundaries and reproductive status.
- Unfamiliar animals or people: New shoes, a bag you set on the ground outside, or a visitor’s jacket can all carry scent information worth investigating.
- Other cats’ facial and body rubbing spots: When cats rub their cheeks on furniture or doorframes, they leave pheromones behind. Another cat may flehmen at those exact spots.
- Your own scent: Sweaty socks, worn clothing, and even your skin carry organic compounds that some cats find fascinating enough to analyze this way.
Kittens use this same system early in life to locate their mother’s milk, which means the vomeronasal organ is active from birth and serves different purposes at different life stages.
Why Regular Smelling Isn’t Enough
A cat’s standard sense of smell is already far more sensitive than a human’s. But the nose and the vomeronasal organ process different types of information. The nose handles general odors: food, fresh air, the scent of rain. The vomeronasal organ specializes in social and reproductive chemistry. Think of it as the difference between hearing background noise and listening to a specific conversation. The flehmen response is how cats tune in to that conversation.
This is why you’ll often see the behavior in a two-step sequence. The cat sniffs normally first, picks up something interesting, and then switches to the open-mouthed posture to get a deeper read on the chemical details their nose flagged but couldn’t fully interpret.
Not Just a House Cat Thing
The flehmen response shows up across the entire cat family. Tigers and pumas use it to detect or attract potential mates, recognize related individuals, and even communicate with and soothe their cubs. Wild cats of all sizes rely on pheromone-based communication to mark territory, which allows individuals to avoid each other across large ranges and reduces the chance of dangerous physical confrontations over resources.
Your domestic cat inherited this same system, even if the stakes are lower. Instead of avoiding a rival predator across miles of wilderness, your cat might be investigating whether the neighbor’s cat walked across your porch last night.
When You’ll See It Most Often
Both male and female cats flehmen, though intact (unneutered) males tend to do it more frequently because they’re more attuned to reproductive pheromones. You’re likely to notice it when your cat encounters a spot where another animal has urinated, when you bring something new into the house that carries unfamiliar scents, or after your cat sniffs another cat’s rear end during a social greeting.
Some cats also flehmen at seemingly random things: a particular plant, a piece of fruit, or a plastic bag. This likely means the object is releasing a volatile compound that hits the vomeronasal organ in an interesting way, even if it has nothing to do with another animal. Catnip, for instance, can trigger the response in some cats.
The behavior is completely normal and healthy. If your cat does it frequently, it just means they’re a thorough investigator. If you’ve never noticed it, watch closely next time your cat lingers over a scent. The open-mouthed pause typically lasts only a few seconds before the cat moves on, having gathered whatever information they needed.