That open-mouthed, squinty grimace your cat makes after sniffing something isn’t disgust. It’s called the flehmen response, and it activates a second smell system most people don’t know cats have. When your cat curls back its upper lip, holds its mouth open, and seems to stare into the middle distance, it’s pulling scent molecules into a specialized organ on the roof of its mouth to analyze chemical signals that regular sniffing can’t fully decode.
The Flehmen Response Explained
The word “flehmen” comes from a German term for curling the lip, and it describes a behavior seen across many mammals, from horses to big cats. During the flehmen response, a cat lifts its head slightly, parts its lips, and sometimes protrudes its tongue. The whole expression can look like shock, confusion, or even revulsion, but the cat is actually concentrating. It’s drawing air (and the scent molecules in it) across a small sensory organ located just behind the upper front teeth.
This organ, called Jacobson’s organ or the vomeronasal organ, sits in a pair of tiny fluid-filled sacs connected to the roof of the mouth through narrow ducts. Heavy, nonvolatile molecules land on the tongue and travel up through these openings. Once inside, the molecules contact specialized receptor cells lining the organ’s walls. These receptors connect to nerve pathways that lead directly into the brain’s limbic system, the area that processes emotions, memory, and social behavior. That’s a different route than normal smell takes, which is why the flehmen response gives cats access to a layer of information their nose alone can’t provide.
Two Separate Smell Systems
Cats essentially have two distinct chemical-detection systems working in parallel. The main olfactory system, their nose, handles everyday airborne scents: food, predators, the outdoors. Molecules contact the olfactory tissue inside the nasal cavity and send signals through a pathway that reaches the brain’s cortex, where conscious identification happens.
The vomeronasal system works differently. It’s specialized for detecting pheromones, which are chemical signals animals release to communicate with members of their own species, along with other biologically meaningful molecules. Instead of routing through the same brain areas as regular smell, signals from the vomeronasal organ bypass that pathway entirely and project into limbic structures like the amygdala. This means the information processed through the flehmen response connects more directly to instinct and emotion than to conscious thought. Your cat isn’t thinking about what it smells so much as feeling it.
What Triggers the Face
The flehmen response is most commonly triggered by pheromones and other chemically rich scents. In the wild and at home, these include:
- Urine from other cats: loaded with information about territory, reproductive status, and identity
- Facial pheromones: left behind when cats rub their cheeks on objects or other animals
- Anal gland secretions: another source of individual chemical signatures
- Dirty laundry: your worn clothes carry concentrated body oils and sweat that cats find chemically interesting
- Any unfamiliar or novel scent: new objects, bags of groceries, or a visitor’s shoes can all prompt investigation
Your cat isn’t reacting because the smell is bad. It’s reacting because the smell is information-dense. The flehmen response is the equivalent of leaning in and reading the fine print.
Why It Evolved
Pheromones carry survival-critical data. They mark territorial boundaries, help kittens locate their mother’s milk, signal whether another animal is a friend or a threat, and broadcast reproductive readiness. A cat that can read these signals accurately has a significant advantage. It knows whether a rival has claimed an area, whether a potential mate is in heat, and whether a nearby animal is stressed or relaxed.
The flehmen response essentially gives cats a way to “taste” the air for this information. Because the vomeronasal organ is tuned to heavier molecules that don’t float easily through the nasal passages, the open-mouthed posture helps funnel those compounds where they need to go. It’s not a passive sniff; it’s an active, deliberate sampling technique.
Male Cats Do It More Often
Research on flehmen behavior in cats has found a clear sex difference, though it’s more nuanced than it first appears. During encounters with other cats, males perform the flehmen response far more often than females. This makes sense given that males are more driven to assess reproductive cues from females in heat.
However, the difference is situation-specific. When researchers let cats explore a room marked with urine (without another cat present), females also performed flehmen, just less frequently than males. And when urine was applied directly near a cat’s nose and mouth, both males and females responded at the same rate. Hormone levels play a role too: in one study, giving testosterone to spayed females significantly increased their tendency to investigate other cats’ genital areas and then perform flehmen. So the behavior isn’t exclusive to males. It’s influenced by context and hormones, not hardwired to one sex.
When to Pay Attention
The flehmen response is completely normal and healthy. Some cats do it multiple times a day, others rarely, depending on personality and environment. A cat in a multi-cat household will likely do it more often because there are more pheromone signals to process. A single indoor cat might only do it when you bring home something new or when it catches a whiff of an outdoor cat through a window screen.
The only time the open-mouthed posture warrants a closer look is if it seems paired with other signs of respiratory distress, like labored breathing, drooling, or pawing at the mouth. In those cases, the cat may not be performing flehmen at all but rather struggling with a dental issue, something stuck in its throat, or a breathing problem. The distinction is usually obvious: a flehmen response lasts a few seconds, the cat looks alert and curious, and it goes right back to normal afterward.