The cat flehmen response is a distinctive facial expression where a cat opens its mouth slightly, curls back its upper lip, and often appears to be “grimacing” or almost smiling for several seconds after sniffing an interesting scent. It looks unusual enough that many owners assume something is wrong, but it’s actually a normal, purely sensory behavior tied to a specialized scent organ unique to cats and several other animal species.
What Is the Flehmen Response?
“Flehmen” comes from a German word roughly meaning “to bare the upper teeth,” and the term describes the same core behavior across the many animal species that display it, including cats, dogs, horses, and big cats like lions and tigers. During the response, a cat opens its mouth partway, curls the upper lip back, and often holds still with a slightly glazed, unfocused expression for a few seconds to around half a minute before returning to normal.
The behavior is triggered almost exclusively after a cat sniffs a particular scent — commonly urine markings from other cats, but also sometimes unfamiliar objects, people’s clothing, or strong environmental odors. It’s a brief, self-contained response rather than an ongoing state, and cats typically resume normal behavior immediately afterward with no lingering effects.
The Vomeronasal Organ and Why Flehmen Happens
The flehmen response exists to give a scent extra access to the vomeronasal organ (also called Jacobson’s organ), a specialized scent-detection structure located in the roof of a cat’s mouth, connected by a duct just behind the upper front teeth. Unlike the regular nose, which processes everyday smells, the vomeronasal organ is specifically tuned to detect pheromones — chemical signals other cats leave behind that carry detailed social information.
By curling the lip and opening the mouth, a cat draws air more directly across the ducts leading to this organ, essentially “tasting” the scent in addition to smelling it through the nose. This dual system — regular nose plus vomeronasal organ — gives cats a far richer read on scent-based information than smell alone could provide, particularly for the specific pheromones layered into urine markings and glandular scent deposits.
What Information Cats Gather Through Flehmen
- Reproductive status: Unneutered male cats frequently use flehmen after encountering urine from a female in heat, gathering detailed pheromone information relevant to mating readiness.
- Territorial information: Scent marks left by other cats — through urine spraying or facial rubbing — carry identity and status information that flehmen helps a cat process in more detail.
- Identifying unfamiliar cats: The response helps cats build a more complete “profile” of an unfamiliar cat’s scent, useful information in multi-cat households or when a new cat’s scent appears in a shared territory.
- General novel scent investigation: Cats sometimes flehmen at other strong or unusual smells, including specific fabrics, foods, or products, even without another cat’s pheromones directly involved.
Is the Flehmen Response Normal?
Yes — the flehmen response is entirely normal, healthy feline behavior and isn’t associated with pain, illness, or distress. It’s a brief, voluntary sensory action, not an involuntary muscle spasm or neurological issue, and cats display it throughout their lives without it indicating any underlying problem.
Owners sometimes mistake the expression for a sign of nausea, pain, or a dental problem because of the slightly grimacing appearance, but a genuine flehmen response is brief, self-resolving, and directly tied to a specific scent-triggering moment. If a cat’s mouth appears to be held open or contorted for an extended period without an obvious scent trigger, or if it’s accompanied by drooling, pawing at the mouth, or apparent discomfort, that pattern is different from flehmen and warrants a veterinary check to rule out dental disease or an oral injury.