What Is the Flehmen Response? Why Cats Make That Weird "Stinky Face"
Every so often a cat will freeze mid-sniff, mouth slightly open, upper lip curled back like they've just smelled something offensive. It looks like disgust, but it's the opposite: this is a cat concentrating as hard as it can on a smell, using a sense organ humans don't have.
What the flehmen response actually is
That open-mouthed grimace is called the flehmen response, and it isn't unique to cats. Goats, horses, and big cats all do a version of it. PetMD describes it as a cat holding its mouth slightly open and curling its upper lip after investigating an interesting scent, most often a scent left by another cat.
The organ behind the face
The reason for the odd expression is a second scent-processing organ cats have in addition to their nose, called the vomeronasal organ or Jacobson's organ, located in the roof of the mouth just behind the front teeth. Cats.com explains that holding the mouth open like this routes air directly over that organ, letting the cat pick up chemical information, particularly pheromones, that its regular sense of smell doesn't fully process. The mechanism was described in detail in a classic study on chemocommunication in domestic cats published in 1976, which is still the foundational reference on how Jacobson's organ and the flehmen posture work together.
What actually triggers it
The flehmen response shows up most often around urine, particularly the scent mark of an unfamiliar cat, and it's more common in male cats investigating signs of a female in heat. Catster notes that any cat can do it toward any sufficiently interesting scent, not just reproductive ones, and it isn't limited to intact cats. A cat sniffing a stranger's shoe, another cat's bedding, or a spot on the carpet may pause into the same open-mouthed pose.
Is it ever a cause for concern?
On its own, the flehmen response is entirely normal and not a sign of pain, illness, or distress, despite how uncomfortable the face looks to a person. It typically lasts only a few seconds before the cat's expression returns to normal. It's worth a vet visit only if it's paired with other changes, like frequent gagging, drooling far beyond the brief flehmen pause, or signs of dental or oral pain, since those would point to something separate from ordinary scent investigation.
Frequently asked questions
Do kittens do the flehmen response?
Yes, though it's seen less often in very young kittens and tends to become more frequent as a cat matures and starts investigating scent marks more purposefully.
Why does my cat only do this with certain smells?
It seems to be reserved for scents the cat finds unusually significant, most often urine or strong pheromone-carrying scents, rather than being triggered by every smell a cat encounters.
Is the flehmen response the same as my cat's reaction to catnip?
No, they're different. The catnip response (rolling, drooling, rubbing) is a separate reaction to the compound nepetalactone, while the flehmen response is specifically about routing scent to the vomeronasal organ.
Sources
- Verberne, G., "Chemocommunication among domestic cats, mediated by the olfactory and vomeronasal senses. II.", Zeitschrift für Tierpsychologie, 42(2) (1976): 113-128
- PetMD, "What Is the Flehmen Response in Cats?"
- Cats.com, "What Is the Cat Flehmen Response?"
- Catster, "What Is the Flehmen Response in Cats? Our Vet Answers"
This article is for general information and isn't a substitute for advice from your own veterinarian, who can evaluate any unusual facial behavior alongside your cat's full health picture.
