Why Do Cats Leave Their Mouths Open After Smelling Something?

When a cat smells something a bit funky, it may keep its mouth open for what seems like an abnormally long amount of time. It’s called the flehmen response, a.k.a. “stinky face.” As kitten rescuer Hannah Shaw wrote on Instagram, it’s “a cat’s way of analyzing an unfamiliar and interesting scent. The flehmen response allows the scent to travel to the vomeronasal organ on the roof of the mouth.” The vomeronasal organ—also called Jacobson’s organ—is a region of sensory cells within the olfactory system of mammals, amphibians, and reptiles.

Though the cat is contorting its furry face into a strange expression, it’s just actually pulling back its upper lip. The cat uses its mouth, not nose, to suck in air and filter it into the vomeronasal area. Scientists think the sensory information moving through the vomeronasal organ falls somewhere between the sense of smell and taste. Dogs have these receptors, too, but the average cat has 30 kinds of receptors while a dog has only nine. Big cats like lions and tigers also exhibit the flehmen response, as do other animals such as horses and hedgehogs.

Similar to the flehman response is the blep—a cat’s adorable habit of not quite retracting its tongue all the way. Here, the cat is “smelling” its environment by collecting pheromones that are passed to the vomeronasal organ. It might look silly and cute like a flehman response, but cats do it for a reason.

Male felines are more likely to make the face than females, but the latter uses the method to keep track of their kittens. “Male cats use the flehmen response in relation to mating,” veterinarian Sasha Gibbons told Catster. “Scents can help indicate compatibility and if timing is right.” Neutered male cats are known to exhibit the flehmen response without having mating in mind.

The next time Fluffy makes that face, snap a photo or video and post it to Instagram, and try to figure out what she’s sniffing.

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A version of this story originally ran in 2019; it has been updated for 2023.